Cleaner Aluminum Melts in Foundries: A Critical Review and Update

Cleaner Aluminum Melts in Foundries: A Critical Review and Update

R. Gallo Foseco Metallurgical Inc., Cleveland, Ohio

Copyright 2008 American Foundry Society

ABSTRACT During a period of almost 4 years, hundreds of degassing and fluxing trials and evaluations of molten metal cleanliness were

conducted in green and dry sand, low pressure, permanent mold (tilting and gravity), and semi-permanent mold (gravity)

foundries.

Metal cleanliness evaluations were made by using techniques such as Prefil, Metalvision MV 20/20, and PoDFA for

inclusion contents, Prefil and ALSPEK H for hydrogen levels, and reduce pressure test (RPT) for molten metal quality index.

Molten metal degassing/cleaning techniques involved rotary degassing, rotary flux injection, different rotors and fluxes, and

different techniques to apply the flux.

The intention of this article is two-fold. First of all, to offer an interpretation of the performance of present day

degassing/cleaning techniques, and secondly, to provide a different approach to better understand how present day molten

aluminum alloy cleanliness evaluation techniques can provide qualitative, quantitative and/or a combination of qualitative

and semi-quantitative understanding of the process.

The results and discussions presented in this article are based on combining and grouping together similarities and

dissimilarities of the results obtained during the cleanliness assessment of the molten A356 aluminum alloy used by the

foundries, rather than by correlating the assessment to a single melting practice.

Thus, this article presents the effects of different rotary degassing and fluxing techniques, using different rotors design and

fluxes upon the quality of the molten metal in crucible furnaces and transfer ladles after metal treatment, as well as on the

quality of a finished casting product. In addition, the effects of different ingot/scrap raw material charging ratios on the initial

molten quality are also discussed. Finally, the amount of dross and constituents produced by the metal treatment processes

are presented.

INTRODUCTION Sound principles of foundry practice are essential for reducing foundry scrap, improving foundry profitability, minimizing

casting process variation, and for producing quality castings meeting customer specifications. The extent to which foundries

depart from such sound principles will have a negative impact on the overall performance of the foundry. Sound foundry

practices consist of the proper melting and casting technologies along with practical and theoretical understanding of five

physical and chemical characteristics of the melt being cast, and the relationship of the melt with the surface phenomena

during the casting process.

The five characteristics are: 1) the propensity of the molten bath to absorb hydrogen, 2) the easiness with which a continuous

film of aluminum oxide will form on the nascent aluminum surface when is exposed to oxygen and water vapor, 3) the

beneficial protection against gas absorption and oxidation provided by the aluminum oxide film, given that such oxide film is

not broken during the casting process, 4) the deleterious effects of the oxide film if it is broken during the casting operation,

and 5) the logarithmic increase in aluminum oxidation rate as the temperature of the melt increases.

Hydrogen is made available at the surface of molten aluminum alloys through the reaction of the molten bath with the water

vapor present in the melting environment.1, 2, 3

Sources of water vapor include moisture content from: ambient air, charging

materials, melting practices, combustion by-products, lubricants on scrap returns, treatment tools, and fluxes.

The reaction between water vapor and molten aluminum yields not only hydrogen gas but also the formation (within

milliseconds) of an amorphous aluminum oxide (Al2O3) film on the surface bath, which acts as a protective oxidation barrier

for the molten metal underneath the film. These amorphous films have been referred as young films.4 The hydrogen

produced through the reduction of water in turn dissociates into its atomic form at the melt surface and then diffuses through

the amorphous aluminum oxide film from which it is quickly dissolved by the molten bath.

In the process of melting and handling liquid aluminum alloys, the molten metal is exposed not only to the hostile gaseous

environments of the products of the combustion but also the surface of the molten bath is constantly disturbed due to one or

more of the following melting practices: 1) charging, 2) skimming, 3) cleaning, 4) degassing, 5) transferring, and 6) ladling.

Any of these melting practices causes the thin alumina films to crumble, to break, to fold, to re-oxidize, and to encapsulate

un-oxidized molten aluminum generating wet dross and or bifilms 4 as well as causing rapid alumina film thickening (oxide

build-up).

Moreover, another difficulty that aluminum foundries face in the molten metal bath is the load of non-metallic and the

metallic impurities that are suspended and floating in the bath. Non-metallic and metallic impurities are introduced into the

melt during the charging process, as well as during the treatment, and handling operations of the molten metal. Even when

melting primary ingot, non-metallic impurities like hydrogen and aluminum oxide are introduced. However, as would be

expected, the majority of the non-metallic and metallic impurities come from charging “returns.” The term “returns” means

scrap castings, gates, risers, trimmings, etc. According to the type and origin of the return metal, and to the conditions of raw

material storage, the metal can contain considerable amounts of both non-metallic and metallic impurities.

Hydrogen absorption, formation of dross, generation of bifilms, metallic and non-metallic inclusions and oxide build up are

inherent characteristics when melting and handling molten aluminum, regardless of the melting and/or holding furnace

design, or the energy used (gas or electricity). Therefore, it is of vital importance to understand and control such inherent

characteristics because they will greatly affect the quality of the molten metal, which in turns impact the final casting quality

with respect to porosity, shrinkage, oxides and inclusions.

While the existing types of inclusion material that would be present in melting and holding furnaces and or transfer ladles

will vary from foundry to foundry, their removal is essential for proper molten metal cleanliness. A number of commercially

accepted melt treatment techniques are being used by aluminum foundries to remove and separate inclusions from the molten

aluminum alloy prior to casting. These include various different methods of fluxing, degassing, and filtration in the furnaces

and in the gating system 5, 6, 7, 8, 9 10, 11

Any of these techniques will have an impact on the melt cleanliness of the molten

aluminum alloy. However, the effectiveness to evaluate their removal would rely on the melt cleanliness measurement

technique being used.

Over the last 40 years, several techniques have been developed and used for assessing the cleanliness of molten aluminum

casting alloys. These include qualitative, quantitative, and analytical laboratory procedures, as well as on-line and off-line

techniques such as: Reynolds 4MTM (The Mansfield Molten Metal Monitor), Porous Disc Filtration Analysis (PoDFA),

Liquid Aluminum Inclusions Sampler (LAIS), Liquid Metal Cleanliness Analyzer (LIMCA), Pressure Filtration Technique

(Prefil-Footprinter), Qualiflash, Reduced Pressure Test (RPT), Metalvision MV 20/20, TYK, Alscan, ALSPEK H, and the K

Fracture Mold. The strengths and weaknesses of these methods with regards to equipment requirements, sampling,

sensitivity, timing, and practical means of assessing inclusion levels in the foundry floor have been fully discussed and

published in the literature. 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28

OVERALL TRIALS RATIONAL In a very extensive molten metal quality assessment during a period of almost 4 years, hundreds of degassing and fluxing

trials and evaluations of molten metal cleanliness were conducted in 10 major aluminum foundries in the United States using

A356 aluminum alloy. The assessment might have included just their standard operating production practices (charging,

melting, alloying, handling, metal temperature, degassing cycle, etc.) or a comparison against a different degassing and/or

fluxing practice. In any event, several assessments were repeated in different occasions. The assessments were conducted in

green and dry sand, low pressure, permanent mold (tilting and gravity), and semi-permanent mold (gravity) foundries

operating a variety of melting and holding furnaces including reverberatory furnaces of different holding capacities, and

crucible furnaces and transfer ladles with molten metal capacities between 1,000 to 2,000 pounds.

Metal cleanliness evaluations were performed by using Prefil, Metalvision MV 20/20, and PoDFA techniques for inclusion

content, Alscan and ALSPEK H for hydrogen level, and RPT (via specific gravity) for molten metal hydrogen and molten

metal quality index respectively. Molten metal samples for PoDFA, Prefil and RPT evaluations were taken only after careful

skimming of the dross from the top molten surface layer once the metal treatment was finished. Similarly, careful skimming

was done before submerging the probes for the Alscan, the Metalvision MV 20/20 and the ALSPEK H analyzers. Based on

economical and practical reasons, only certain Prefil and PoDFA samples were selected for examining the filter cross-section

for impurities. The molten metal degassing and fluxing treatment techniques that were utilized in the assessments included

combinations of rotary degassing and/or rotary flux injection equipment with a variety of rotors, fluxes, and flux application

techniques. The results from the different analytical tools that were used in specific sections of the assessments will be

covered as needed throughout the article. The equipment and products used for the molten metal degassing and fluxing

treatments included the most commercially accepted technologies in combination with proven effective products being used

in foundries.

It is not the intention of this article to describe or correlate specific alloying constituents (spectrographic analysis) with an

assessment of the molten metal treatment but rather to A) offer an interpretation of the performance of commercial degassing

and fluxing techniques, and B) to provide a different approach to better understand how present day molten aluminum alloy

cleanliness evaluation techniques can provide qualitative, quantitative and/or a combination of qualitative and semi-

quantitative understanding of the process.

The presentation of the results and discussions that follow are based on combining and grouping together molten cleanliness

results achieved during similar metal treatment practices rather than by relating them to a specific foundry practice or

location. By presenting the results in groups based on similarities and dissimilarities, the results can be used to expose a

broader field that could be used as starting point to implement tighter process controls. The order of the evaluations does not

necessarily correspond to the sequence of the actual trials.

As to be anticipated, the challenges of such assessments were related to foundry operating requirements and analytical tools

limitations. With respect to the foundries, the challenge was in being able to use the analytical tools around the restricted

layouts, the stringent schedules and within the operating limitations. In many instances such restrictions did not allow for the

completion of hydrogen measurements because of the very short time that was allowed in the process (or degassing cycle) for

using the molten metal once it was treated. Limitations of the analytical tools included: samples per hour, interpretation cost,

delay cost, response time, interpretation time, etc. In addition, another challenge was to collect the proper information for the

reduced pressure test and being allowed to use a baseline (between foundries) vacuum pressure of 27.5 +/- 5 inches of

mercury. Nevertheless, once the data was collected and analyzed, it was noticeable that even with the present limitations,

current analytical tools and proper use of equipment could help in better process understanding, which eventually would lead

into a better process control.

In the first phase of the study, the effects of different rotary degassing and fluxing techniques, using different rotor designs

and fluxes, were assessed with respect to the resulting metal quality in transfer ladles. The second phase of the study

consisted of evaluating the effects of different ingot/scrap return charging ratios upon the quality of the molten metal in

crucible furnaces after similar rotary degassing and fluxing techniques. The third phase consisted in evaluating the combined

effects of different metal charging combinations upon the molten metal quality as a result of rotary degassing or rotary flux

injection upon the quality of the finished castings. In the fourth phase, the effects of different techniques of using fluxes in

the metal treatment of transfer ladles and crucibles upon the amount of dross and its constituents were evaluated.

FIRST PHASE In the first phase, five different rotary degassing and fluxing techniques were assessed with respect to the resulting metal

quality in transfer ladles holding about 2,000 pounds of liquid alloy. The molten metal quality was evaluated before and after

rotary degassing, and rotary flux injection. The five processes that were evaluated were:

Process 1: Degassing with a rotary degassing unit fitted with rotor design “A” but without using any flux in the operation.

Process 2: Fluxing and dross skimming before the degassing operation, which was conducted with a rotary degassing unit

fitted with rotor design “A”. In this particular trial, 5.5 pounds of flux “X” were manually added on the top metal surface

before the degassing operation. The manual stirring and skimming operation was accomplished in three minutes.

Process 3: Fluxing and dross skimming before the degassing operation, which was conducted with a rotary degassing unit

fitted with rotor design “B”. In this particular trial, 5.5 pounds of flux “X” were manually added on the top metal surface

before the degassing operation. The manual stirring and skimming operation was accomplished in three minutes.

Process 4: Degassing, and flux injection with a rotary flux injection unit fitted with rotor design “B”. In this case, flux “Y”

was injected at a rate of 0.4 pounds per minute, for 3 minutes.

Process 5: Degassing, and flux injection with a rotary flux injection unit fitted with rotor design “C”. In this case, flux “Y”

was injected at a rate of 0.4 pounds per minute, for 3 minutes.

For each of these processes (1 through 5) the same melting furnace, which functioned under the process parameters

established in the respective standard operating procedure (SOP), was used to tap out molten metal into the transfer ladle.

Although the charging mixing ratio was kept the same during the evaluations, neither the mixing ratio nor the quality in the

melting furnace were monitored since they were considered to be irrelevant to the intent of the assessments. The only

emphasis was in keeping a uniform molten metal temperature of 1360ºF +/- 20ºF (738ºC +/- 11ºC) at the starting of the

degassing process.

While the corresponding parameters for each of the five processes are summarized in Table 1, the different rotors designs that

were evaluated are shown in Figure 1.

Table 1. Summary of conditions and parameters used per corresponding scenario.

Process Equipment Rotor

Flux Type

Flux Method of Addition

Flux Added

Fluxing Time (Min)

Degassing Time (Min)

Total Cycle (Min)

1 Rotary degassing A None N/A 0 N/A 12 12

2 Rotary degassing A X Manual 5.5 lbs N/A 12 12

3 Rotary degassing B X Manual 5.5 lbs N/A 12 12

4 Rotary flux injection B Y Injected 0.4 lbs/min 3 9 12

5 Rotary flux injection C Y Injected 0.4 lbs/min 3 9 12

Figure 1. The three different designs of rotors evaluated.

The powder fluxes “X” and “Y” were used during the evaluation of processes 1 to 5 because in addition to being the

preferred choices for the manual and injection processes respectively, they were also commonly used in the foundries

evaluated. In addition to the NaCl and KCl base formulation, the difference between flux “X” and flux “Y” was in the

proportion and different main reactive elements. While in flux “X” the main reactive elements were NaF, NaNO3, Na2SO4

and Na2CO3, the main reactive elements in flux “Y” were Na2SiF6, Na2SO4 and Na2CO3. Flux “F”, with a different make up

of NaF, NaNO3, Na2SO4 and Na2CO3, was also used for process 8, as it will be described later.

For each of the above five processes, at least three Prefil, and three Alscan readings, and three specific gravity measurements

were conducted. In addition, cleanliness measurements using Metal Vision 20-20 and hydrogen readings using ALSPEK H

were also conducted. For each process at least three similar trials were performed to determine the metal cleanliness.

Typical Prefil curves representing the trials from phase one are depicted from Figures 2 to 7. Figure 2, which groups together

all the resulting typical slopes obtained during the evaluations, summarizes the five processes simultaneously. Explicit slopes

representing corresponding processes are shown from Figures 3 to 6. Figure 7 depicts the best slopes from each process.

The comparison of the Prefil curves in Figures 3 to 6, indicated that:

A. Rotary degassing without any usage of flux provides the lowest molten metal quality (Ladles 5 and 7 in Figure 6).

B. Rotor designs “A” and “B” provided similar cleanliness conditions. Perhaps the “B” rotor provided a slightly tighter

consistency (Figure 4 versus Figure 6).

A B C

C. Neither of rotors designs “A” nor “B”, when used with manual flux additions, delivered the same molten cleanliness

quality as the one obtained when the rotary flux injection unit was fitted with rotor design “C” while using flux “Y”

(Figures 4 and 6 versus Figure 3).

D. The rotary flux injection provided cleaner molten metal than the one obtained after only straight degassing after manual

flux addition (Figures 3 and 5 versus Figures 4 and 6).

E. Flux “Y” provided cleaner molten metal.

F. The rotary flux injection process provided the cleanest molten metal regardless of which type of rotor was used (Figures

3 and 5 versus Figures 4 and 6).

Despite the fact that rotors “B” and “C” provided the cleanest (and similar among them) molten metal quality after rotary flux

injection with flux “Y”, from the operational perspective rotor “C” was better since the two flanges of rotor “B” made the

rotor prone to need careful and constant cleaning after every cycle because the flux tended to stick between the flanges. Any

flux sticking between openings caused more and erratic splashing during the metal treatment.

By comparing the amount of flux used during rotary flux injection, 1.2 pounds per cycle, versus the amount used during

manual addition with rotary degassing, 5.5 pounds, and taking into consideration that rotary flux injection yielded the

cleanest molten metal, it is obvious that rotary flux injection is a more economical process that could reduce the manual

addition of flux up to 78%.

Figure 2. Typical Prefil curves representing the ranges obtained for the 5 processes that were evaluated in phase 1.

Figure 3. Prefil curves for process 5, rotary flux injection with rotor “C” and flux “Y”.

5

8

4

7

17

3

16

15 9

13 10

2

14

12

11

Figure 4. Prefil curves for process 3, rotary degas with rotor “B” and manual addition of flux “X”.

Figure 5. Prefil curves for process 4, rotary flux injection with rotor “B” and flux “Y”.

Figure 6. Prefil curves for rotary degassing process with rotor “A”. Process 1 (no flux used in ladles 5, and 7) and process 2

(manual addition of flux “X” in ladles 2, 3, and 4).

Ladle 14

Ladle 11

Ladle 8 Ladle 9

Ladle 10

Figure 7, which depicts the best Prefil slopes obtained respectively in each process, shows that process 4 and 5 yielded the

best metal cleaning as compared to the other processes. While a comparison of the slopes would indicate that process 4 (ladle

8) was better by a narrow margin than process 5 (ladle 16), the evaluation of hydrogen gas content and specific gravity

indicated the opposite. Hydrogen gas content for process 5 was 0.08 ml/100 g Al, and for process 4 was 0.19 ml/100 g Al.

Specific gravity for process 5 was 2.63 and for process 4 was 2.50. This difference is very significant to disregard. The

molten metal quality among similar melts with respect to inclusions might be the same regardless of a very dissimilar

hydrogen gas content. Therefore for total metal quality, inclusions and hydrogen gas content should both be evaluated.

Figure 7. Best Prefil curves from each process.

Molten metal hydrogen gas content measurements and specific gravity tests were obtained for each Prefil analysis conducted.

Hydrogen measurements were taken with Alscan and ALSPEK H units. Because of the different technology among these

instruments, hydrogen gas evaluations during the complete degassing and fluxing treatments were possible only with

ALSPEK H. Since the evaluations with Alscan could only be performed after the degassing treatment process, the resulting

Alscan results had to be interpreted with caution due to the fact that, in the majority of the cases, such tests were aborted

before the unit reached the minimum required stabilization time of 10 minutes. Alscan tests had to be aborted because of the

stringent limited time that was available for holding the transfer ladle for molten metal testing after the molten metal

degassing cycle. The limiting factor was a combination of molten metal temperatures, and low metal level in the holding

furnaces.

The Alscan measurements that were monitored ranged anywhere from 200 seconds to 900 seconds. As the Alscan test was

aborted, a warning notice came indicating that the equipment did not reach equilibrium. Readings lower than 600 seconds

were not considered at all. Thus, the hydrogen readings that were present when the Alscan test was aborted were not

considered in the evaluations when trying to correlate with specific gravities and hydrogen levels given by ALSPEK H.

From the well-known accuracy of Alscan, it would have been misleading to compare the readings knowing that the tests were

aborted. Eventually the Alscan readings, time permitted, would have reached similar levels as the ones given by ALSPEK H.

The advantage of ALSPEK H was in regards to the operational capability to read hydrogen levels during the metal treatment

cycle. Table 2 shows the specific gravity values as well as the hydrogen levels obtained with Alscan and ALSPEK H that

corresponded to the best Prefil curves for each of the five processes. The lowest hydrogen level of 0.08 ml/100 g Al

correlated well with the highest specific gravity of 2.63. Also the lowest hydrogen reading of 0.19 ml/100 g Al correlated

well with the lowest specific gravity of 2.50. By evaluating the hydrogen levels given by ALSPEK H and the corresponding

specific gravities, it was obvious that the process 5, rotary flux injection using rotor “C” and flux “Y”, provided the best

molten metal treatment process.

Ladle 4

Ladle 8

Ladle 14

Ladle 16

Ladle 7

Table 2. Hydrogen levels and specific gravities as a function of the best Prefil curve obtained in each process. Alscan test aborted before equilibrium

By evaluating the worst Prefil curves from each process in Figures 3 to 6, it became obvious that the worst slopes obtained

respectively in each process were from ladles 13, 10, 12, 2, and 5, which correspond to processes 5, 4, 3, 2, and 1

respectively. Again, the evaluation showed that processes 4 and 5 yielded better metal cleaning than the other three remaining

processes. By comparing just the Prefil slopes of processes 4 and 5 it can be arguable which one provided the cleaner metal.

However, when evaluating and taking into account the specific gravity as well as the hydrogen gas content given by

ALSPEK H, process 5 was again better, although only slightly, with a hydrogen gas content of 0.15 ml/100 g Al against a

hydrogen gas content of 0.16 ml/100 g Al for process 4.

The values of hydrogen levels and specific gravities corresponding to the worst Prefil curves obtained in each process are

given in Table 3. The same considerations described before applied to the Alscan readings. From the outlook of hydrogen

levels, Table 3 shows that any type of rotary flux injection is better than just rotary degassing with or without flux. It was

interesting to notice that the best Prefil curve in process 1, including its respective specific gravity and hydrogen gas content,

was not better than the worst Prefil curve for rotary flux injection.

Table 3. Hydrogen levels and specific gravities as a function of the worst Prefil curves obtained in each process.

Alscan tests aborted before equilibrium

.

A typical curve of hydrogen gas content versus time that was obtained during the evaluations conducted with ALSPEK H is

shown in Figure 8. This figure in particular depicts the hydrogen changes in the molten metal during the treatment performed

for process 5 and ladle 16 (the one that yielded the best Prefil curve and lowest hydrogen gas content). Typically, four

different regions were clearly identifiable during any of the molten metal treatments evaluated: sensor stabilization, flux

injection, degassing, and molten metal holding after degassing. The hydrogen gas levels that existed at the end of each

corresponding degassing operation were the levels assigned to that process. As would be expected, the length and the slope of

each region within the hydrogen curve varied according to the molten metal treatment performed.

Process Equipment Rotor Flux Alscan Alspek H

Specific

Gravity

4 Rotary flux Injection B Y 0.276 0.19 2.50

5 Rotary flux Injection C Y 0.192 0.08 2.63

2 Rotary degasser A X 0.259 0.14 2.58

3 Rotary degasser B X 0.222 0.13 2.60

1 Rotary degasser A none 0.284 0.15 2.57

Hydrogen in ml/100 g Al

Process Equipment Rotor Flux Alscan Alspek H

Specific

Gravity

5 Rotary flux Injection C Y 0.238 0.15 2.57

4 Rotary flux Injection B Y 0.221 0.16 2.56

3 Rotary degasser B X 0.240 0.19 2.51

2 Rotary degasser A X N/A N/A N/A

1 Rotary degasser A none 0.284 0.21 2.47

Hydrogen in ml/100 g Al

Figure 8. Typical graph given by ALSPEK H showing hydrogen gas content and metal temperature as a function of time and metal treatment cycle.

Based on the Prefil slopes it was decided to evaluate, under the microscope, the best and the worst Prefil samples from each

process. As is customary, a detailed examination of the corresponding Prefil filter cross-section was conducted to evaluate

the inclusions trapped in the metal. Table 4 shows a typical summary of the metallographic analysis report obtained in

correlation with the Prefil evaluations. Typically, such report illustrates the analysis by three factors: 1) overall metal

cleanliness by inclusion content given in mm2/Kg, 2) oxide films given by number/Kg, and 3) the inclusion type given by

mm2/Kg and in percentage of the respective total content.

Thus, table 4, depicts that the worst Prefil curves generated higher levels of inclusion content, from 0.241 to 1.325 mm2/Kg,

as compared to the low levels of inclusion content resulted from the best Prefil curves (0.069 to 1.125 mm2/Kg). This table

also shows that the number of oxide films and their distribution range was higher and larger in the worst Prefil curves than in

the best Prefil curves; 7 to 64 per Kg versus 4 to 19 per Kg. Besides, higher levels contents of TiB2 inclusion type particles

entrapped within the filter were found in the worst Prefil curves than in the best Prefil curves. The difference was between

0.229 - 1.108 to trace – 01.080 mm2/Kg respectively. The majority of better results seen in the best Prefil curves may be

attributed not only to the use of flux but also to the stirring action of the rotary flux injection process.

The optical microphotographs shown from Figures 9 to 12 are being used to only illustrate the types of oxides and inclusions

observed in the samples. As expected, each sample has a different level of inclusions. Figure 9 shows the presence of TiB2

obtained during the evaluation of the Prefil curve 7. Figure 10 shows the presence of spinel (MgAl2O4) obtained during the

evaluation of the Prefil curve 7. Figure 11 provides the example of carbides in the form of Al4C3 that were obtained during

the evaluation of the Prefil curve 16. Figure 12 shows the presence of magnesium oxides (MgO) and spinel (MgAl2O4)

obtained during the evaluation of the Prefil curve 16.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 100 200 300 400 500 600 700 800 900 1000

Time in seconds

Hydro

gen m

l/100 g

Al

600

620

640

660

680

700

720

740

760

780

Molten m

eta

l te

mpera

ture

C

Hydrogen Temperature

Fluxing & Degassing

Degassing

Start of 12 minutes

treatment cycle

Probe

stabilization

Table 4. Summary of metallographic analysis report for best and worst Prefil curves from processes 1 through 5.

8 16 4 14 7 13 10 12 2 5

Overall Metal

Cleanliness

Inclusion Content

(mm2/Kg) 0.076 0.109 0.166 0.069 1.125 0.241 0.355 0.505 0.955 1.325

Oxide Film

Content (No/Kg) 19 4 4 5 16 7 33 37 64 26

Oxide Films

(Gamma-Al2O3)

Number/Kg 19 4 4 5 16 7 33 37 64 26

Length Short Short Short Short Short Short Short Short Short Short

Thickness Thin Thin Thin Thin Thin Thin Thin Thin Thin Thin

Inclusion Type in

mm2/Kg and in

percentage

TiB2

0.016

21% trace

0.138

83% trace

1.080

96%

0.229

95%

0.480

95%

0.323

91%

0.927

97%

1.108

97%

(Ti-V)B2

Al4C3 < 3 μm trace

0.081

74% trace

0.032

46%

0.011

1% trace trace trace trace trace

Al4C3 > 3 μm

Borocarbides

MgO

0.032

42%

0.015

9%

0.030

43% trace trace trace trace

0.019

2% trace

MgO-cuboids

MgAl2O4 - spinel

0.028

37%

0.028

26%

0.013

8%

0.008

11%

0.034

3%

0.012

5%

0.025

5%

0.032

9%

0.010

1% 0.034 3%

Dispersed Al2O3

Alumina needles

Nitride graphite

Salt/fluoride

Refractory

Filtered weight (kg) 1.40 1.40 1.40 1.40 0.67 1.21 1.14 1.08 0.91 0.67

Ladle Number

Best slopes Worst slopes

MgAl2O4

TiB2

Figure 9. Optical microphotograph of sample # 7 showing the presence of TiB2 inclusions at the filter.

Figure 10. Optical microphotograph of sample # 7 showing the presence of spinel (MgAl2O4) inclusions at the filter.

SECOND PHASE The second phase of the study consisted in evaluating the effects of different ingot/scrap return charging ratios on the quality

of the molten metal before degassing, after degassing and/or fluxing, and during holding periods of up to 2 hours after

degassing/fluxing. To evaluate the metal quality during the holding periods, samples were taken after removing 3 inches of

metal off of the top surface after the molten metal had settled for 2 hours (e.g. ladling out operation). For these series of

assessments, three 1,200 pound gas fired crucibles were charged differently; with 100% ingot, with 100% return scrap, and

with an ingot/scrap ratio of 60%/40%. For each of the charging conditions three different rotary degassing and fluxing

techniques were evaluated in several occasions.

Process 6: Degassing with a rotary degassing unit fitted with rotor design “A” but without using any flux in the operation.

Although this process was similar to process 1 with respect to the degassing technique, the difference was related to the

application of the degassing process for three different charging conditions as described in the above paragraph.

Process 7: Degassing, and flux injection with a rotary flux injection unit fitted with rotor design “C”. In this case, flux “Y”

was injected at a rate of 0.3 pounds per minute, for 3 minutes. While this process was similar to process 5 with respect to the

degassing and fluxing technique, the difference in this case was as previously described.

Process 8: Degassing, and mechanized flux delivering unit fitted with rotor design “B” delivering the recommended type of

flux for such application. In this case, flux “F” was delivered at a rate of 0.3 pounds per minute, for 3 minutes.

The corresponding parameters for each of the three processes are summarized in Table 5.

Table 5. Summary of conditions and parameters used per corresponding processes 6, 7, and 8.

Al4C3

MgO

MgAl2O4

Process Equipment Rotor Flux

Flux Method of Addition

Flux Added

Fluxing Time (Min)

Degassing Time (Min)

Total Cycle (Min)

6 Rotary degassing A none N/A 0 N/A 12 12

7 Rotary flux injection C Y Injected 0.3 lbs/min 3 9 12

8

Rotary degassing with mechanized flux

delivery B F Auto

delivered 0.3 lbs/min 3 9 12

Figure 11. Optical microphotograph of sample # 16 showing the presence of carbides (Al4C3) at the filter.

Figure 12. Optical microphotograph of sample # 12 showing the presence of magnesium oxides (MgO)

and spinel (MgAl2O4) at the filter.

As in the previous five first processes, charging and melting were conducted according to the foundry operating procedures.

The only emphasis was in keeping a uniform metal temperature of 1360ºF+/- 20ºF (738ºC +/- 11ºC) at the starting of the

degassing and/or fluxing cycle. To evaluate the hydrogen gas content in a quantitative and semi quantitative form, three

Alscan, three ALSPEK H readings, and three specific gravity measurements were conducted. Moreover, inclusion cleanliness

measurements were conducted using Metalvision MV 20/20 and Prefil. For each of these three processes, at least three

similar trials were performed.

Table 6 summarizes the metallographic analysis results from the Prefil results for processes 6, 7, and 8. Noted that the metal

treatment process and the metal charging conditions had an effect on the molten metal quality. To facilitate the presentation

of the results, and taking into account that the inclusions were only spinel like and TiB2 particles, Table 6 provides the

percentage amount in which the spinel like inclusions were present. Therefore, the remaining percentage not given is related

to the TiB2 particles.

Table 6. Summary of metallographic analysis results for processes 6, 7, and 8.

Process #

Prefil Metallographic

Analysis Factors Process Ingot 100% Ingot/Scrap

60%/40% Scrap 100%

6

Inclusion content (mm

2/Kg)

Before treatment 0.083 0.199 0.250

After treatment 0.092 0.737 0.866

After Holding 0.165 0.284 0.422

Oxide films (number/Kg)

Before treatment 36 47 77

After treatment 41 47 29

After Holding 37 55 23

Spinel like

inclusion type (mm

2/Kg - %)

Before treatment .012 - 15% .032 - 16% .102 - 41%

After treatment .009 - 10% .030 - 4% .036 - 7%

After Holding .015 - 9% .031 - 11% .030 - 7%

7


2/Kg)



After Holding 0.033 0.101 0.016





Spinel like

inclusion type (mm

2/Kg - %)

Before treatment .014 - 100% .016 - 18% .089 -17%

After treatment .004 - 14% .013 - 13% .037 - 12%

After Holding .008 - 24% .018 - 18% .004 - 25%

8


2/Kg)



After Holding 0.245 0.056 0.063





Spinel like

inclusion type (mm

2/Kg - %)

Before treatment .045 - 45% .051 - 8% .244 - 29%

After treatment .012 - 9% .009 - 3% .016 - 25%

After Holding .031 - 13% .015 - 27% .030 - 7%

Process 6 Results Figure 13 shows the Prefil curves obtained from the 100% molten ingot metal in crucible (C4) during day 3 before degassing

(P1), after degassing (P2), and after the ladling out operation (P3). It was noticeable that the molten metal became dirtier as it

went from before degassing to after degassing and after the holding period and the ladling metal out operation. As shown in

Table 6, the inclusion content increased slightly after degassing from 0.083 to 0.092 mm2/Kg and then increased to 0.165

mm2/Kg after the subsequent ladling out operation. However, further consideration showed that the inclusion types before

degassing were 85% TiB2 and 15% spinel-like while after degassing 90% of the inclusions were TiB2 and only 10% spinel-

like as shown in Table 6. From the qualitative and quantitative understanding of the molten process, it is important to take

into account that when grain refinement is included in the process, these types of TiB2 inclusions are invariably present and

noticeable in the analysis and that they are not necessarily detrimental for the process. The presence of more TiB2 inclusions

suspended in the melt after a metal treatment involving rotary technology could be attributed to the rotor action that provides

better mixing and uniformity of grain refinement particles in the molten bath. Evidently, any increase of TiB2 particles

suspended in the molten bath will negatively influence the Prefil cleanliness curve. Sedimentation of higher number of TiB2

inclusions subsequent to rotary treatment during holding periods of time could be expected to influence melt cleanliness

evaluations up to 24 hours. 29

Figure 13. Prefil curves representing process 6, before degassing, after degassing, and after ladling metal out

(following a holding period) from a crucible charged with 100% ingot and rotary degassed without any flux.

Figure 14 shows the Prefil curves obtained from the 60% ingot and 40% scrap ratio molten metal in crucible (C8) during day

3 before degassing (P1), after degassing (P2), and after the ladling out operation (P3). In this case, the molten metal became

dirtier as it went from before degassing to after degassing. After the subsequent ladling out operation, the quality of the

molten bath apparently improved from the after degassing quality but was not better than the molten quality before

degassing. As shown in Table 6, the inclusion content increased after degassing from 0.199 to 0.737mm2/Kg and then

dropped to 0.284mm2/Kg after the subsequent ladling metal out operation. In this case, the inclusion types before degassing

were 84% TiB2 and 16% spinel-like while after degassing 96% of the inclusions were TiB2 while the 4% remaining were

spinel-like. The decrease in TiB2 inclusion content from the after degassing cycle to the after holding and ladling out

operation indicated that while the TiB2 particles were settling down, they were still in suspension since the inclusion content

was still greater than the one before the degassing cycle.


(following a holding period) from a crucible charged with 60% ingot and 40% scrap, rotary degassed without flux.

Figure 15 shows the Prefil curves obtained from the 100% scrap returns molten metal in crucible (C3) during day 3 before

degassing (P1), after degassing (P2), and after the ladling out operation (P3). Again, the molten metal became dirtier after

degassing. Also, the quality of the metal improved slightly after the settling and ladling out operation but no better than the

molten quality before degassing. As shown in Table 6, the inclusion content increased after degassing from 0.250 to

0.816mm2/Kg and then dropped to 0.722mm

2/Kg after the subsequent ladling out operation. In this case, the inclusion types

before degassing were 59% TiB2 and 41% spinel-like while after degassing 93% of the inclusions were TiB2 while the 7%

remaining were spinel-like. The oxide film content was reduced after degassing with TiB2 forming the greater % of the

inclusions.

Figure 15. Prefil curves representing process 6, before degassing, after degassing, and after ladling metal out (following a holding period) from a crucible charged with 100% scrap returns, rotary degassed without any flux.


(P1), after rotary degassing and flux injection (P2), and after the ladling out operation (P3). In this particular case there was

not much difference in the molten quality of the metal from before degassing to after degassing/fluxing and after the

subsequent ladling out operation. It can be seen that the metal was very clean during all the three stages of the process. As

shown in Table 6, while the inclusion content increased slightly after the degassing and flux injection process from 0.014 to

0.031 mm2/Kg and remained about the same (0.033 mm

2/Kg) after the settling and ladling out operation, the oxide film

content (No/Kg) changed from 27 to 6 to 14. The inclusion content before degassing and fluxing was mainly oxides. After

degassing and fluxing the oxides were removed and the remaining inclusions were mainly TiB2. The Prefil results indicate

that the process provides very consistent molten quality. Qualitative interpretation of the number of oxide films indicates that

the rotary flux injection process reduced the number of oxide films by 75%. The increase in the number of oxide films after

the holding period and subsequent ladling out operation could be attributed to the normal process of film thickening and or

films introduced by metal disturbances.


(following a holding period) from a crucible charged with 100% ingot and rotary flux injection using rotor “C” and flux “Y”.

A typical graphical representation of the molten metal cleanliness analysis by the Metalvision MV 20/20 system is shown in

Figure 17. This figure depicts the three displays that the instrument shows, all of them as a function of time. The plot on the

top provides the cleanliness value. The plot on the middle provides the average particle size, and the histogram at the bottom

illustrates the particle size distribution, and the particle count for each particle size. The cleanliness level is based on an

arbitrary scale between 0 and 10. The cleanest metal will be at level 10 while the dirtiest metal will be at level 0. The smallest

particle that can be detected and counted is 20 microns (.0008” or 0.02 mm) while the largest detectable particle is 600

microns (0.024” or 0.6 mm).

Because of understandable limitations to present every plot of data from all the evaluations, only two of the plots that were

obtained for process 7 would be used to illustrate some results. Nevertheless, it is important to convey that such results are

representative of the other findings with respect to the difference in metal quality as a function of metal charging and

handling in combination of the degassing/fluxing technique. Besides, the reader may want to further refer to an early article

covering in more detail some of the first work done in this area. 22

Thus, Figures 17 and 18 depict the metal cleanliness evaluation for the same 100% molten ingot metal in crucible (C4)

during day 2 before degassing (P1) and after rotary flux injection (P2) that was previously presented in Figure 16. The

cleanliness value of 2.5 given by the blue line in Figure 17 indicated that the metal was “dirty” as it would be expected with

any metal in the as melted condition. Also Figure 17 shows that the average particle (inclusions) sizes were around 74

microns (0.003” or 0.074 mm) and that some of the particles sizes varied between 200 microns (0.008” or 0.2 mm) and 500

microns (0.020” or 0.50 mm).

Figure 18 shows that after rotary flux injection the molten quality improved by 64% from an average cleanliness value of 2.5

to a 4.1. In addition, the reduced average inclusion particle size from 74 microns (0.003” or 0.074 mm) to 51 microns (0.002”

or 0.051 mm) may indicate that the rotary technology breaks down particles or oxide films into smaller sizes and/or distribute

them more uniformly. The histogram distribution chart indicated that after the rotary flux injection treatment the particles

sizes were in the 30 to 120 microns range (0.0012” or 0.03 mm) to (0.005” or 0.12 mm). However, the majority of the

particles were less than 60 microns in size (.0024” or 0.06 mm).

When analyzing the Metalvision MV 20/20 cleanliness measurements, it was apparent, as it was with Prefil, that in the as

melted condition, melts with 100% ingot had better cleanliness values than melts made with 100% scrap. The still relative

low cleanliness values for degassed and fluxed metal could also be attributable to the TiB2 particles. In all the cases the

cleanliness value after degassing and fluxing did not exceed a 4.5 level, even after the holding period. The Metalvision MV

20/20 also showed that in the as melted condition, particles sizes between 200 microns (0.008” or 0.2 mm) and 500 microns

(0.020” or 0.50 mm) were common, while in the after rotary degassing or rotary flux injection processes the particles sizes

were smaller. However, rotary degassing as in process 1 and 6 yielded similar particle sizes as rotary flux injection in

processes 5 and 7. It appeared that any rotary treatment with and without flux would keep particle sizes in a range between 30

to 160 microns (0.0012” or 0.03 mm) to (0.006” or 0.16 mm). However, the majority of them would be in a range of 40 to 90

microns (0.0016” or 0.04 mm) to (0.0035” or 0.09 mm).

Figure 17. Cleanliness values, as depicted by Metalvision MV 20/20 system, representing process 7 in the as melted

condition. Molten metal in a crucible charged with 100% ingot.

Although in the case of transfer ladles, a low cleanliness value usually between .5 to 1.5 was common just after the ladle had

been filled with molten metal, the particles sizes distribution were more in line with the particles size distribution seen after

any metal treatment involving rotary degassing. Such findings could be attributed to the filtration system in the melting

furnaces and metal splashing while filling the ladle. Molten metal coming into the ladle would have been cleaner as

compared to the resulting heavily oxidized metal in the ladle and therefore contains more smaller particles/oxides sizes.

Figure 18. Cleanliness values, as depicted by Metalvision MV 20/20 system, representing process 7 after rotary flux injection using rotor “C” and flux “Y”. Molten metal in a crucible charged with 100% ingot.


2 before degassing (P1), after rotary degassing and flux injection (P2), and after the ladling out operation (P3). Once more,

there was not much difference in the quality of the metal as it went from before degassing to after degassing/fluxing and after

the ladling out operation. It can be seen that the metal was consistently clean during the three stages of the process. As shown

in Table 6, the inclusion content increased slightly from before degassing at 0.89 to 0.098 mm2/Kg after degassing/fluxing

and then increased to 0.101 mm2/Kg after the subsequent ladling out operation. In this case, the inclusion types before

degassing were 82% TiB2 and 18% spinel-like while after degassing 87% of the inclusions were TiB2 while the 13%

remaining were spinel-like. The oxide film content (No/Kg) changed from 26 to 30 to 7 as a result of the respective process

treatment. Despite minor changes in the Prefil slopes, the graphical representation indicates again that although the charging

mix makes a difference in molten metal quality, rotary flux injection provides a consistent process within the chosen

conditions.


(following a holding period) from a crucible charged with 60% ingot and 40% scrap ratio, and rotary flux injection using rotor “C” and flux ”Y”.


degassing (P1), after degassing (P2), and after the ladling out operation (P3). In this case, the molten metal became cleaner

as it went from before degassing/fluxing to after the holding period and ladling metal out operations. As shown in Table 6,

the inclusion content was reduced by 57% after rotary flux injection from 0.514 to 0.294 mm2/Kg and dropped even further

to 0.016 mm2/Kg after the subsequent ladling out operation. The inclusion types before degassing were 83% TiB2 and 17%

spinel-like while after rotary flux injection 88% of the inclusions were TiB2 while the remaining 12% were spinel-like. The

oxide film content (No/Kg) changed dramatically from 54 to 17 as a result of the rotary flux injection treatment. The slope of

the Prefil curves representing before and after rotary flux injection did not changed appreciably with the drop in inclusion

content as it can be seen in Figure 20.


(following a holding period) from a crucible charged with 100% scrap returns, and rotary flux injection using rotor “C” and flux “Y”.


(P1), after degassing and flux delivered (P2), and after the ladling out operation (P3). In this particular case while there was

an appreciable difference in the quality of the metal before and after degassing and fluxing, there was not a major difference

between the molten quality as it went from the after degassing and fluxing to after the ladling out operations. It can be seen

that the metal was very clean after melt down. As shown in Table 6, the inclusion content increased after degassing from

0.088 to 0.240 mm2/Kg and then remained at about the same level at 0.245 mm

2/Kg after the subsequent ladling out

operation. In this case, the inclusion types before degassing were 55% TiB2 and 45% spinel-like while after degassing/fluxing

91% of the inclusions were TiB2 while the remaining 9% were spinel-like.


(following a holding period) from a crucible charged with 100% ingot and rotary degassed with flux using rotor “B” and flux ”F”.


1 before degassing (P1), after degassing and flux delivered (P2), and after ladling out three inches of metal from the top

surface after a holding period of 2 hours (P3). As shown in table 6, the molten metal became cleaner as it went from before

degassing/fluxing to after the holding period and ladling metal out operations. As shown in table 6, the inclusion content was

reduced by 51% after rotary degassing and flux usage from 0.641 to 0.314 mm2/kg and even then dropped to 0.056 mm

2/kg

after settling and ladling metal out. In this case, the inclusion types before degassing were 92% TiB2 and 8% spinel-like while

after degassing 97% of the inclusions were TiB2 while the remaining 3% were spinel-like. The inclusion types after the

holding period were 73% TiB2 and 27% spinel-like. The oxide film content (No/Kg) changed dramatically from 152 to 9 as a

result of the rotary degassing with flux usage and then went up slightly to 27 after the subsequent ladling metal out operation.

As seen in Figure 21, the slopes did not change much with the decrease in inclusion content before and after degassing.

Figure 22. Prefil curves representing process 8, before degassing, after degassing/fluxing, and after ladling metal out

(following a holding period) from a crucible charged with 60% ingot and 40% scrap, using rotor “B” and flux “F”.


degassing (P1), after degassing and flux delivered (P2), and after the ladling out operation (P3). In this case, the molten

metal became cleaner as it went from before degassing to rotary degassing with flux usage to after the subsequent ladling out

operation. As shown in Table 6, the inclusion content was reduced by 74% after rotary degassing with flux treatment from

0.843 to 0.219 mm2/Kg and even then dropped to 0.063 mm

2/Kg after the ladling out operation. The inclusion types before

degassing were 71% TiB2 and 29% spinel-like, while after rotary degassing and flux treatment 75% of the inclusions were

TiB2 and the remaining 25% were spinel-like. The oxide film content (No/Kg) changed dramatically from 148 to 13 as a

result of rotary degassing with flux treatment, and then increased to 32, which were still very low and better than the number

present before treatment.

Figure 23. Prefil curves representing process 8, before degassing, after degassing/fluxing and after ladling metal out

(following a holding period) from a crucible charged with 100% scrap returns. Process with rotor “B” and flux “F”.

In summary, the results from the second phase, indicated that regardless of the type of molten metal treatment used, the kind

of metal charged had a direct effect on the initial molten metal cleanliness, after melt down. The cleanest molten metal was

obtained when the charge consisted of 100% ingot, and the dirtiest molten metal when the charged consisted of 100% scrap.

The metal cleanliness, given by inclusion content, ranged between 0.083 and 0.014 mm2/kg when 100% ingot was used, and

from 0.250 to 0.843 mm2/kg when 100% scrap was used. The inclusion content when an ingot/scrap ratio was used ranged

from 0.089 to 0.641mm2/kg.

Likewise, the molten metal cleanliness given by the other two factors (number of oxide films and the inclusion type) behaved

in similar manner. The fewest oxide films for melts consisting of 100% ingot, and the most oxide films for melts made with

100% scrap returns, and between them for melts with the ingot/scrap ratio. The only exception was one case in which 148

oxide films were counted for a melt made out of 100% scrap and 152 for a melt made out of the 60%/40% ingot/scrap ratio.

A higher number of spinel type of inclusions were detected in the as melted bath condition than in the after rotary degassed or

rotary flux injected molten bath. However, the opposite occurred after the molten metal was treated; fewer spinel type

inclusions and more TiB2 type inclusions than in the as melted condition. Thus, the rotor degassing action provided better

mixing and more uniformity dispersion of TiB2 particles in the molten bath and floated some of the oxides to the surface,

where they were removed during dross skimming.

As the number of TiB2 particles became more uniformly suspended in the bath, obviously the Prefil cleanliness measurement

was negatively affected. Nevertheless, grain refinement should not be considered necessarily detrimental for the Prefil

analysis.

When the initial inclusion content in the molten metal before any treatment was less or equal than 0.250 mm2/Kg, the

degassing and or fluxing treatment caused the inclusion level to increase to more than the initial level that was present in the

as melted condition.

When the initial inclusion content in the molten metal before any treatment was more or equal than 0.514 mm2/Kg, the

degassing and or fluxing treatment caused the inclusion level to decrease more than 40% of the initial level.

With regards to the cleanliness evaluations after the holding periods and subsequent ladling out operations, it was difficult to

find a trend correlating how the sedimentation (of the higher number of TiB2 particles subsequent to any of the rotary

treatments) behaved as a function of the process utilized. Perhaps, the exception was that as long as 100% ingot was used

rotary flux injection provided the more uniform sedimentation process.

The optical microphotograph shown in Figure 24 illustrates the typical types of oxides and inclusions observed in the samples

in processes 6 through 8. As previously discussed, each specific sample would have a different level of inclusions. The

presence of TiB2 is identified with the number 1, the presence of oxide films as number 2, the presence of spinel (MgAl2O4)

as number 3, and the spinel like as number 4.

Figure 24. Optical microphotographs showing typical inclusion type showing TiB2 as 1, oxide film as 2, spinel as 3,

and spinel like as 4.

THIRD PHASE In the third phase of the study, the effects of different metal charging combinations on the quality of the finished castings

were evaluated with the objective to optimize raw material cost and minimize casting defects. Three different charging

1

3

4

4

2

combinations of T-bar, RSI, and in-house scrap castings at four different percentages were evaluated in combination with the

effects of two cleaning molten metal treatment processes. Table 7 provides the fundamental framework of the blending mix

used in each of the four different trials evaluated. The sum of the individual percentage of the three different raw materials,

which were used in each trial, makes up the 100% of the required hourly charging rate. The precise blending percentages are

proprietary and can not be disclosed.

In each of these four trials, different reverberatory dry hearth furnaces were charged accordingly. Once the molten metal was

available, it was tapped out into 1,700 pounds transfer ladles. Consecutive ladles coming from the same furnace were treated

accordingly. PoDFA samples were obtained from the transfer ladle for the first, fourth and last tap of each trial. Sampling

was done before and after each respective molten metal treatment. The molten metal temperature at the degassing operation

was at 1380ºF +/- 30ºF (748ºC +/- 16ºC). Tables 8 and 9 summarize the typical PoDFA analysis results (total inclusion

content in mm2/Kg and the oxide films in number/Kg) for the four trials and their correlation with rotary degassing and rotary

flux injection.

After the molten aluminum in the transfer ladles was properly treated, the molten metal was delivered to different holding

furnaces. Each holding furnace fed individual casting machines. Since the casting quality of four different casting programs

was evaluated, similar and dissimilar castings were produced using the two types of molten metal treatments being assessed.

The total number of castings that were produced and rejected was closely monitored to properly correlate the combined effect

of the charging combination mix and the molten metal treatment used upon the final casting quality. Total final scrap

percentages varied from 0% to 5.1% at the casting cell and from 0% to 3.0% at x-ray. However, the scrap was easily related

to specific casting programs rather than to the melting and molten metal process.

Based on the fact that one casting program yielded 100% good castings in x-ray, regardless of the casting machine, mold

number, charging mixed or the metal treatment being used, the process assessment was based on the PoDFA results related to

the treatment used when the x-ray scrap was 0%. Thus, Tables 8 and 9 also identify, in gray, the combinations of metal

treatment and trial number in which the casting programs with 0% rejections at x-ray resulted.

Table 7. Charging combinations for each trial.

Table 8. Typical inclusion content per trial as a function of metal treatment process.

Trial

Percentage T-

bar

Percentage

RSI

Percantage

In-House

Scrap

1 T1 2R1 S1

2 T1 R1 1.75S1

3 .90T1 2R1 1.25S1

4 .70T1 3R1 S1

Charging combination

Before After Before After Before After Before After

Rotary

degassing 0.064 na 0.028 0.100 0.052 0.148 0.037 0.220

Rotary flux

injection 2.023 0.169 na 0.043 0.022 0.087 0.043 na

Total inclusion content (mm2/Kg)

Trial 1 Trial 2 Trial 4Trial 3

Table 9. Typical number/Kg of oxide films per trial as a function of metal treatment process.

Table 8 also shows that apparently the molten metal becomes dirtier as it went through metal treatment. Similar grain

refinement effects as the ones previously reported for the Prefil analysis apply in this case, since the inclusion content goes

from lower to higher levels after rotary metal treatment. However, it appears that there is a threshold value over which

inclusion content levels on the high side before metal treatment were reduced after the rotary metal treatment. For example,

Table 8 shows that in trial 1 the inclusion content before metal treatment was 2.023 mm2/Kg and after rotary flux injection

went to 0.169 mm2/Kg. From the perspective of the metal treatment process, it was obvious that while rotary flux injection

provided three different conditions for 0% x-ray scrap, rotary degassing provided only two. Still, it would not be

straightforward to substantiate rotary flux injection over rotary degassing since the final inclusion content although with

small differences between them is very similar, especially after taking into consideration the different starting levels.

Moreover, the controlled ratio of in-house scrap castings in the mix also contributes for a good starting quality of the molten

aluminum.

Table 9 shows some of the oxide films numbers that were obtained during the assessment. As shown, several of the values

were not obtained due to typical operational anomalies when running PoDFA. Therefore, a careful consideration can only be

valid with respect to the oxide films that varied from 5 to 53 before any metal treatment, and the ones that were present after

the rotary degassed melts for trials 2 and 4, which were the trials that yielded acceptable castings after x-ray. With respect to

the three trials with rotary flux injection melts (that yielded acceptable castings after x-ray) it is obvious that the data after

rotary flux injection is not available. Nevertheless, taking into consideration other available data from the other casting

programs in which the oxide films content changed from 8 to 19, 14 to 16, and 37 to 6, when going from before to after

rotary flux injection, it could be expected that the values missing in Table 9 would have been within similar ranges. In

addition, all the compiled data in such trials was also comparable with the values presented in Tables 4 and 5.

Since the corresponding specific gravities were pretty much the same after rotary degassing or after rotary flux injection, both

metal treatment processes were considered to be operationally optimized for the application and therefore were regarded as

providing the same degassing efficiency. Rotary flux injection did not reduce the degassing cycle from the rotary degassing

unit.

FOURTH PHASE In the fourth phase of the study, the effects of the different techniques of using fluxes in the metal treatment of transfer ladles

and crucibles upon the amount of dross and its constituents were conducted. Table 10 shows the typical chemical

composition of the resulting dross after the corresponding metal treatment. Table 10 depicts typical chemical composition of

the dross after molten metal treatment of crucibles and ladles in the range of 900 to 2,000 pounds capacity. The process

parameters for processes 6, 7 and 8 have been previously stated. The “only flux by manual mixing” condition refers to the

manual application, mixing, and skimming of flux before the degassing operation. Rotary degassing with flux on top

indicates that as the rotary degassing started, flux “Y” was manually added on top of the molten metal and then let stay

during the 12 minutes degassing process, without any manual stirring or activation. Thus, once the degassing was completed

the dross was skimmed off.

Table 10 shows that rotary flux injection is the most effective process to reduce aluminum content in the dross and that

manual fluxing is the worst process, leaving 84% metallic aluminum in the dross as compared to 6%. The second best

process to reduce metallic aluminum content in the dross is rotary degassing with a mechanized flux delivery system yielding

17% aluminum in the dross. Table 10 also shows that rotary degassing yields 85% metallic aluminum in the dross. In

addition, it also shows that if flux were used during the degassing cycle, the aluminum content could be reduced to 77%.

Before After Before After Before After Before After

Rotary

degassing 12 na na 17 21 na 7 17

Rotary flux

injection 5 na 27 23 53 na 11 na


Trial 1 Trial 2 Trial 3 Trial 4

Figure 25 depicts the amount of dross that can be generated in a 1,200 pounds transfer ladle or crucible furnace during a

manual flux operation without the involvement of any rotary metal treatment. Figure 25 shows that while 1.2 pounds of flux

generated 2.25 pounds of dross; 5.5 pounds of flux generated 6.5 pounds of dross. Therefore, it appears that an increase in the

amount of flux usage does not proportionally increase the amount of dross produced. It could be argued that if 53% and 84%

from the 2.25 and 5.25 pounds of dross produced respectively was mainly flux weight. (eg 2.25 pounds of dross minus 1.2

pounds of flux equals 1.05 net pounds of dross. If 1.05/2.25 = 0.47, consequently 1 - 0.47 = 53%). Accordingly, the results

indicated that if the flux addition surpass an application rate of 0.1% by weight of total metal melted, the manual fluxing

operation becomes inefficient.

Table 10. Typical constituents present in dross as a function of the molten metal treatment used.

Figure 25. Generation of dross as a function of a manual fluxing operation in transfer ladles and crucible furnaces.

Figure 26 depicts the amount of dross that can be generated in a 1,200 pounds transfer ladle or crucible furnace during a

rotary degassing process in which no flux is used at all. It is very typical in foundries, which operate a sound degassing

process to be able to obtain the corresponding levels of low hydrogen gas in the melt with 12 minutes of rotary degassing.

However, it was not unusual to find foundries using longer degassing cycles without full investigation of why the need of

longer degassing cycles to achieve the desired gas levels. In addition, it was not uncommon to observe rotary degassing

cycles around the 30 minutes range. As Figure 26 shows, there is a tremendous increase in dross generation from a 12-minute

cycle to a 32-minute cycle. The rotary degassing process can generate 6.25 pounds of dross in 12 minutes and 14.25 pounds

in 32 minutes. When no flux is used at all, these amounts of dross would have 85% metallic aluminum as given in Table 10

Process Equipment Al KCl Si02 AL2O3 CaCl2

Complex

oxides

6 Rotary degasser 85% 9% 6%

7 Rotary flux injection 6% 12% 16% 52% 4% 10%

8

Rotary degasser with

mechanized flux

delivery 16% 18% 20% 34% 4% 8%

N/A

Only flux by manual

mixing 87% 10% 3%

N/A

Rotaring degassing

with flux on top 77% 13% 8% 2%

Manual Fluxing Operation Using Flux "X"

0

1

2

3

4

5

6

7

1.2 2 3 4 5.5

Pounds of flux added

Po

un

ds o

f sk

imm

ed d

ross

for process 6. The difference may be 5.31 pounds of metallic aluminum versus 12.11 pounds of metallic aluminum in the

dross if rotary degassing is properly used.

In addition, other findings showed that in all the processes that were evaluated, the amount of dross generated during a 12

minutes metal treatment cycle, independently of equipment used and type of material used, was between 6 and 6.5 pounds.

The main difference when analyzing the dross was in the amount of metallic aluminum content on it as depicted in Table 10.

Thus, rotary flux injection would generate the same amount of dross as the other metal treatment processes but the advantage

would be in the lower metallic aluminum that would contain.

Figure 26. Generation of dross as a function of rotary degassing in transfer ladles and crucible furnaces.

CONCLUSIONS The kind of metal charged into a crucible furnace had a direct effect on the initial molten metal cleanliness after melt down.

Cleaner molten metal was obtained when the charge consisted of 100% ingot, and dirtier molten metal when the charge

consisted of 100% scrap returns. Molten metal cleanliness, measured by inclusion content, ranged between 0.083 and 0.014

mm2/kg when 100% ingot was used, and from 0.250 to 0.843 mm

2/kg when 100% scrap returns was used. The inclusion

content when a 60%/40% ingot/scrap ratio was used ranged from 0.089 to 0.641mm2/kg.

Prefil, Metalvision MV 20/20 and PoDFA analysis showed that the inclusion levels in the molten bath, in the as melted

condition, were lower than in the treated condition (after rotary degassing and/or rotary flux injection). Nevertheless, proper

understanding of the grain refinement and molten metal process was critical for proper interpretation of the apparent increase

of inclusion content since the TiB2 inclusions detected by such analytical techniques were not necessarily detrimental for the

metal treatment process.

While a higher number of spinel-like of inclusions were detected in the as melted condition than in the after treated condition,

the opposite occurred after the molten metal was treated; fewer spinel like-inclusions and more TiB2 type inclusions. The

presence of more TiB2 particles suspended in the melt after a metal treatment involving rotary technology could be attributed

to the rotor action that provided better mixing and more uniform dispersion of TiB2 particles in the molten bath. Less spinel

type of inclusions would be expected, as many of these particles floated to the surface, and were then removed during the

dross skimming operation.

When the initial inclusion content in the molten metal, before any treatment, was less than or equal than 0.250 mm2/Kg, the

degassing and/or fluxing treatment caused the inclusion level to increase to more than the initial level.

Rotary degassing (no flux)

0

2

4

6

8

10

12

14

16

12 32

Degassing time in minutes

Pounds o

f skim

med d

ross

It appeared that there was a threshold value over which inclusion content levels on the high side before metal treatment were

reduced after metal treatment. When the initial inclusion content in the molten metal before any treatment was greater than or

equal to 0.514 mm2/Kg, the degassing and or fluxing treatment caused the inclusion levels to decrease more than 40% of the

initial level.

It was difficult to find a trend correlating how the sedimentation of the higher number of TiB2 particles that were present after

the holding periods behaved as a function of the metal treatment process utilized. The only result that was noticeable was in

the case of the crucible charged with 100% ingot and processed with rotary flux injection. Such a process provided similar

inclusion contents after degassing and after the holding period with respect to the as melted condition.

While the rotary flux injection process provided the cleanest molten metal regardless of which type of rotor was used, rotary

degassing without any usage of flux provided the lowest molten metal quality, especially in melts that consisted of raw

materials other than 100% ingot. Flux “Y” provided the cleanest molten metal.

Although, rotors “B” and “C” provided similar and clean molten metal conditions after rotary flux injection with flux “Y”,

from the operational perspective rotor “C” was better because the flux tended to stick between the two flanges of rotor “B”

causing plugging, requiring careful and constant cleaning after every cycle. Any flux sticking between openings caused

splashing during the metal treatment.

Rotor designs “A” and “B” provided similar cleanliness conditions. However, rotor “B” provided a slightly process

consistency. Neither of rotors designs “A” nor “B”, when used with manual flux additions, delivered the same molten

cleanliness as the one obtained when the rotary flux injection unit was fitted with rotor design “C” while using flux “Y”.

Rotary flux injection could reduce the amount of flux that is used through the manual application process during rotary

degassing, by as much as 78%, demonstrating the effectiveness of rotary flux injection.

While rotary flux injection is the most effective process to reduce aluminum content in the dross; manual fluxing is the worst

process, leaving 84% metallic aluminum in the dross as compared to 6%. The second best process to reduce metallic

aluminum content in the dross is rotary degassing with a mechanized flux delivery system yielding 17% aluminum in the

dross. Rotary degassing without any flux usage yields 85% metallic aluminum in the dross. However, if flux were used

during the degassing cycle, the aluminum content could be reduced to 77%.

If the flux addition surpass an application rate of 0.1% by weight of the total metal melted in a 1,200 pounds transfer ladle, or

crucible furnace during a manual flux operation, without the involvement of any rotary metal treatment, the manual fluxing

operation becomes inefficient. While 1.2 pounds of flux generated 2.25 pounds of dross, 5.5 pounds of flux generated 6.5

pounds of dross.

The rotary degassing process can generate 6.25 pounds of dross during 12 minutes degassing cycle, and 14.25 pounds of

dross in a 32 minutes cycle. When no flux is used at all, these amounts of dross would have 85% metallic aluminum. The

difference may be 5.31 pounds of metallic aluminum versus 12.11 pounds of metallic aluminum in the dross if the rotary

degassing unit parameters are properly set.

The amount of dross generated during 12 minutes metal treatment cycles, independently of equipment used and type of

material used, was between 6 and 6.5 pounds. The main difference in the dross was not the weight but the metallic content

that varied according to the process and flux used. The rotary flux injection generated the same amount of dross as the other

metal treatment processes but with the lowest metallic aluminum content.

Once the optimized working parameters are established to be able to degas, a given crucible furnace and/or a given transfer

ladle, in the shortest amount of time, the rotary degassing and the rotary flux injection processes degassed the molten bath in

the same amount of time. Rotary flux injection is not faster than rotary degassing. Both processes provided the same

hydrogen gas levels in the melts.

The molten metal in the as melted condition typically had average particle (inclusions) sizes between 200 microns (0.008” or

0.2 mm) and 500 microns (0.020” or 0.50 mm). After any rotary treatment, with and without flux, the particles sizes ranged

from 30 to 160 microns (0.0012” or 0.03 mm) to (0.006” or 0.16 mm). The great majority was in a range of 40 to 90 microns

(0.0016” or 0.04 mm) to (0.0035” or 0.09 mm).

Alscan and ALSPEK H provided on-line measurement of hydrogen gas content in the molten bath. However, ALSPEK H

also was the only analyzer that provided practical hydrogen monitoring during the degassing cycle as well as under the

stringent timing constraints.

Hydrogen gas levels of 0.08 ml/100 g Al were typically related to specific gravities of 2.63, and levels of 0.19 ml/100 g Al to

specific gravities of 2.50 from samples solidified under 27.5 +/- .5 inches of mercury.

Several assessments showed that although the inclusion content among melts was the same, the hydrogen gas content was

different among them. Thus, findings revealed inefficient degassing practices.

The different combinations and proportions of T-bar, RSI, and scrap return of metal charged in the reverberatory furnaces did

not have a significant impact in metal cleanliness variations in the molten metal in the transfer ladle after it was filled. With

the exception of one trial, similar cleanliness values at the ones obtained after the melt down of 100% ingots in crucible

furnaces were detected.

Apparently the rotary degassing and the flux injection processes provided similar molten metal quality after the treatment of

the transfer ladles having the T-bar, RSI, and scrap return molten metal combinations. Although rotary flux injection

provided three trials for 0% scrap castings in x-ray, as compared to only two trials from the rotary degassing process, it was

not sufficient evidence to consider the rotary flux injection process over the rotary degassing process. First of all, the final

molten metal inclusions content between the two processes were very similar especially after taking into consideration the

different inclusion starting levels. Secondly, the controlled ratio of in-house scrap castings in the mix also contributed to the

high-starting quality of the molten metal. Thirdly, the fact that 0% scrap was obtained in only a certain casting design implied

to consider a solution outside the metal treatment process.

Despite operational anomalies, analysis costs, and/or production constrains to use present day metal cleanliness equipment,

aluminum foundries could benefit from their use to set and monitor their melting practices, and eventually as a process

control in a none too far future.

Most of the results pointed at rotary flux injection as the most efficient and consistent metal treatment process to obtain

cleaner molten metal. Nevertheless, a reality that was observed in all these years of trials was that the majority of the

foundries that had such technology lacked the discipline to constantly clean the shaft after treatment to avoid the always-

present threat of plugging the shaft. Hence, in those foundries very ineffective manual flux additions or not flux usage at all

was observed.

It is up to each aluminum foundry to define the type of the molten metal cleanliness level they can live with. Thus, each

foundry has a choice between implementing discipline to use rotary flux injection or settle for avoiding the headaches of

reinstituting discipline and use a different technology for metal cleaning while still is better than manual or no flux additions.

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Cleaner Aluminum Melts in Foundries: A Critical Review and Update

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