Soldering Behavior of JIS ADC12 Alloy Die Castings and Its Mechanism+1

Yasushi Iwata1,+2, Hiroaki Iwahori1 and Yuichi Furukawa2

1Toyota Central R&D Labs., Inc., Nagakute 480-1192, Japan2Toyota Motor Corporation, Toyota 471-8571, Japan

Die casting, a highly efficient production method, is widely applied to the manufacturing of automotive components. On the other hand,extending the die life is required to meet the needs of reducing the production cost because die costs are included in the manufacturing costs.

As one of the primary factors leading to die failures, soldering was investigated in this study by observing the adhesion of ADC12 alloy tothe die and the reaction between ADC12 alloy and die in die casting process, analyzing the diffusion of concerned elements in ADC12 alloy, andnumerically simulating the flow and the solidification of the molten metal. It was revealed that ADC12 alloy adhered to the surface of the die andcaused the surface layer of the die castings to remain on the surface of the die by breaking the die castings with shearing stress at the time ofejection. In addition, Al­Si­Fe compounds were found at the interface between the die and the remaining ADC12 alloy layer with the repetitionof die casting. These compounds grew gradually with the concentration of Si in the remaining ADC12 alloy layer towards the interface bydiffusion with the further increase of shot numbers. [doi:10.2320/matertrans.F-M2018827]

(Received April 16, 2018; Accepted June 4, 2018; Published July 13, 2018)

Keywords: aluminum alloy, die casting, soldering, SEM, diffusion

1. Introduction

Die casting, a process in which molten metal fills a diecavity at high speed and solidifies under high pressure, iswidely applied in the manufacture of various automobilecomponents because of its high productivity. In order toreduce the cost of die castings, much research has beenconducted1­3) on the behavior of molten metal during cavity-filling and solidification and the associated effects on theinternal and external defects of die castings. Anotherimportant consideration is the lifetime of dies, whose costaccounts for a large percentage of the overall cost. The mainfailure modes of dies, including soldering and heat cracking,have thus received increasing research attention.4)

Soldering is caused by a reaction between a die and analuminum alloy adhered to the die during die casting.5) Thisreaction forms a compound on the surface of the die, theamount of which increases with increasing shot number,eventually leading to die failure. Although mold releaseagents are applied to prevent soldering, uniform coatingof the die is difficult because of high temperature andcomplicated die shape.6,7) In addition, the oils in mold releaseagents become carbonized and affect soldering.8) Therefore,it is important to understand the reaction behavior betweena die and aluminum alloys.

Various investigations have been conducted on the reactionbetween a die and an aluminum alloy when the former isimmersed in the latter.9) During die casting, aluminum alloyssolidify within several seconds after mold filling. The die isthus exposed to the molten aluminum alloy for a very shortduration; in the remaining time, the die is in contact with thesolidified aluminum alloy. Therefore, studies have also beenconducted on the relation between the start time of thereaction and the temperature at the point when the die makescontact with the solidified aluminum alloy.10) However, thereaction behavior between the die and the aluminum alloy

in the practical die casting process, in which the die issimultaneously exposed to both liquid and solid states of analuminum alloy, requires further clarification. The releaseforce required for die castings, as an index of soldering, hasbeen examined for aluminum alloys (JIS ADC10 alloy)11)

with various addition levels of Fe, Mn, etc. and types ofdie material.12) Nevertheless, various aspects of solderingin practical die casting remain unclear. The present reportexamines the adhesion of a practical aluminum alloy (JISADC12) to the die and the reaction behavior between themin the repetitive die casting process without the use of amold release agent to clarify soldering behavior.

2. Experimental Methods

2.1 Casting methodThe die castings were made by injecting molten aluminum

alloy (JIS ADC12) at 924K into a rectangular cavity[100mm (l) © 120 (w) © 10mm (h)] with a mold pin(º10 © 10mm) (hereafter referred to as “pin”) made of JISSKD61 tool steel at the center of the cavity, as shown inFig. 1, with a 135-ton horizontal die casting machine. Thedraft angle of the pin was set to 0°. The die casting conditionsare shown in Table 1. The die castings were made without theuse of a mold release agent using two-step injection (at lowand high speeds, respectively) after die casting twice at a low

Fixed die side Movable die side

20mm

Core pin

Biscuit

Gate

Fig. 1 Die castings for soldering test.

+1This Paper was Originally Published in Japanese in J. JFS 89 (2017) 757­763.

+2Corresponding author, E-mail: iwata@mosk.tytlabs.co.jp

Materials Transactions, Vol. 59, No. 9 (2018) pp. 1471 to 1476©2018 Japan Foundry Engineering Society

speed for die heating. Pins used for different numbers of shots(1­9, 100 and 300) were prepared using the above die castingmethod. A K-type thermocouple with a diameter of 0.1mmwas installed in the center of the pin to monitor thetemperature during die casting. After die casting, theappearance of the pins was observed before and after theadhered aluminum alloy was removed via dissolution with10% NaOH aqueous solution. Furthermore, the occurrenceand development of soldering were investigated by observingthe cross-sections of pins using scanning electron microscopy(SEM). Since Fe in aluminum alloys is effective inpreventing soldering,11) in this study, JIS AD12.2 alloycontaining a relatively low Fe content was used to facilitatethe investigation of soldering behavior. Table 2 shows thecomposition of the used alloy (in terms of mass fractions).

2.2 Analytical methodThe filling and solidification behaviors of the molten

aluminum alloy during die casting were simulated using thecasting simulation software TopCast (Toyota CommunicationSystem Co., Ltd.). The parameter values used in thesimulation are shown in Table 3.

The diffusion distance of each element was estimated asfollows. The diffusion of an element in a solid can beexpressed according to Fick’s second law as:

@C

@t¼ Dk

@2C

@x2ð1Þ

where C is the concentration and Dk is the diffusioncoefficient. The temperature dependence of the diffusioncoefficient Dk can be expressed in terms of the Arrheniusequation13) as:

DK ¼ D0 exp�Q

RT

� �ð2Þ

where D0 is the frequency factor, Q is the activation energy,R is the gas constant, and T is the temperature.

The variation of the concentration with time t, C(x, t), canbe derived from eq. (1) as:

Cðx; tÞ ¼ A exp � x2

4Dkt

� �ð3Þ

where A is a constant independent of time and the diffusioncoefficient; it is set to a value such that the concentration ofFe becomes 100% at the interface between the die and thealuminum alloy.

The change in concentration on the left side of eq. (1)reflects the macroscopic change in the element amount andthus the diffusion that results from the jumping motion ofthe atoms of the element. The diffusion distance, x, of anatom can be expressed according to random walk theory14)

[parabolic rule of diffusion, see eq. (4)], which states that thediffusion coefficient and the jumping motion are proportionalto each other. The diffusion distance of each element wasestimated as:

x ¼ffiffiffiffiffiffiffiffiffiffi2Dkt

pð4Þ

where t is time.Table 4 shows the physical property values15) used in the

calculations.

3. Results and Discussion

3.1 Behavior of molten metal and occurrence ofsoldering

Figure 2 shows the flow behavior of the molten metal inthe die cavity for the tested castings. The molten metalflowing out of the gate hits the pin at the center of the cavityand flows into the left and right sides of the cavity. Afterreaching the upper side of the cavity, the molten metal beginsto fill the left and right sides of the cavity near the gate andthen near the back side of the pin. Figure 3 shows thetemperature distribution on the movable die surface at thetime of ejection (6 s from the start of injection) from the 1st tothe 4th shot. The temperature of the pin after the 1st shot is693 to 733K (yellow region) and its distribution is almostuniform. With increasing number of die casting cycles, the

Table 1 Die casting conditions used for this research.

Table 2 Chemical composition of AD12.2 die casting alloy for thisresearch.

Table 3 Thermal properties of AD12.2 alloy for numerical simulation.

Table 4 Physical properties used for diffusion analysis.14)

Y. Iwata, H. Iwahori and Y. Furukawa1472

temperature of the pin increases, starting from the tip of thefixed die side; the temperature throughout the pin exceeds733K (orange region). The temperature of the root portionof the pin (at the movable die side) is lower than that of thetip of the pin. Figure 4 shows the changes in the maximumand minimum values of the measured pin temperature withincreasing number of die casting cycles and a comparison ofthe measured and calculated temperatures T of the pin afterthe 5th shot. As shown, the temperature of the pin rises withincreasing number of die casting cycles. The temperatureobtained in the simulation agrees with the measured value.

Figure 5 shows the results of the soldering of thealuminum alloy on the pin after the 1st to 5th, 8th and 9thshots, respectively. The aluminum alloy adhered to the entiresurface of the tip of the pin after 3 shots, when the maximumtemperature of the pin exceeded 730K, as shown in Fig. 4.The portion of the pin to which the aluminum alloy adheredcorresponds to the fixed die side, where the molten metal hitthe pin at a higher velocity; the pin thus had the highesttemperature, as described above. Then, the adhesion areaspread gradually after the 4th shot, covering about 60% ofthe entire surface from the pin tip, until the 9th shot.

Figure 6 shows the variation in the thickness L of thealuminum alloy layer adhered to the pin with the number ofshots up to the 300th shot. The thickness of the aluminumlayer adhered to the pin was 10 µm from the 3rd shot tothe 9th shot. After that, with increasing shot number, thealuminum layer became thicker, peeled away, and a newaluminum layer adhered; this process then repeated. Thethickness of the aluminum layer after the 300th shot wasabout 18 µm. The surfaces of the pins to which aluminumlayers adhered were examined.

Figure 7 shows the surfaces of the pins after the aluminumadhesion layer was removed. Only tool marks (machiningstriations) were observed on the surface of the pin before diecasting. After the 4th shot (two shots after the aluminumalloy began to adhere), still only tool marks were observed(i.e., there were no erosion traces such as pits). Then, after the9th shot, a number of pits of submicron size were observedon the surface of the pin. After the 100th shot, largecontinuous pits were clearly observed, and after the 300thshot, many large erosion marks were also observed besidethe pits.

Figure 8 shows the simulated temperature distributions ofthe 1/4 cross-sections of the die castings near the gate sideat the time of the completion of filling and at the time ofdie opening for the 5th shot in Fig. 4. Immediately after thecompletion of filling, the temperature of the die castingsaround the pin was 840K or lower (as indicated by the bluecolor), which was lower than that of the region away from thepin (860K, as indicated by the light blue color). Similarly,at the time of die opening, the temperature (750K or lower,as indicated by the light blue color) of the die castings aroundthe pin was lower than that (790K, as indicated by the greencolor) of the region away from the pin.

These results suggest that the soldering was due to thebreakage of the die castings, as reported by Aoyama et al.16)

The strength of the region away from the pin was lower

Slow Shot Zone Fast Shot Zone

Molten metal

Core Pin

Fig. 2 Molten metal flow in cavity by numerical simulation.

Core pin 773K

373K

573K

Temp.

1st cycle(6s) 2nd cycle(45s) 3rd cycle(84s) 4th cycle(123s)

20mm

Number of cycles (Elapsed Time)

Fig. 3 Temperature distributions of movable die at ejection time bynumerical simulation.

Fig. 4 Comparison of calculated and measured temperatures of core pinduring die-casting.

Al Adhesion areas

1st 2nd 3rd 4th 5th 8th 9th

5mm

Observation siteFixed die side

Shot number

Fig. 5 Change of soldering behavior with the increase of shot number.

Fig. 6 Change of Al layer thickness with the increase of shot number.

Soldering Behavior of JIS ADC12 Alloy Die Castings and Its Mechanism 1473

because of its higher temperature, compared to that of theregion just around the pin; the region away from the pin wasthus broken by the shear force generated when the die castingwas pulled out from the pin.

3.2 Reaction between adhered aluminum alloy layerand pin

In order to investigate the state of the adhered aluminumalloy layers after the 9th shot when fine pits (erosion traces)began to appear on the surface of the pin, the 100th shot,and the 300th shot, the cross-section (circle in Fig. 5) wasobserved using SEM. The SEM images of the adheredaluminum alloy layers for each shot are shown in Fig. 9.A number of fine crystals with a size of 1 µm or less wereobserved at the interface between the adhered aluminumalloy layer and the pin after the 9th shot (i.e., 6 shots aftersoldering). After the 100th shot, besides the fine crystals,large crystals with a size of several micrometers were alsoobserved at the interface. After the 300th shot, many largecrystals with sizes of several micrometers were observed.

In order to clarify the causes of the precipitation of thesecrystals, the distributions of elements on the cross-sectionswere measured using energy-dispersive X-ray spectroscopy(EDS). Figure 10 shows the distributions of Al, Si, Fe,and Mn along the cross-section of the adhered aluminumalloy layer (point A in Fig. 9) near the interface after the 9thshot. Although the adhered aluminum alloy layer wasexposed to the high-temperature molten metal alloy for 6shots after soldering took place, all of the elements werealmost uniformly distributed throughout the layer withoutremarkable segregation.

Figure 11 shows the EDS analysis results of the finecrystals at the interface between the adhered aluminum alloylayer and the pin after the 9th shot. The concentrations ofFe and Si in the fine crystals (Point 1) were slightly higherthan those in an area without crystals (Point 2) located atthe same distance from the interface. These results suggestthat the fine crystals are Al­Si­Fe-based compounds. TheFe in these compounds likely comes from impurities in thealuminum alloy and the pin (via elution). In order to confirmthis supposition, the diffusion behavior of Fe in a die castingcycle was examined.

As shown in Fig. 4, the temperature of the pin increasedsharply after the injection of molten metal and varied in therange of 470 to 760K during die casting. Figure 12 showsthe calculated diffusion concentrations of Fe in the aluminumalloy layer when the pin was kept in contact with the

0shot

4shot

9shot

100shot

300shot

Pit

Pit

Machining striations

Pit

Erosion trace

Pit

Fig. 7 Change of pin appearance with the increase of shot number.

Low temp. area

PinAt 5th shot

10mm

At ejection time

Temp.(K)

840

790

740

Temp.(K)

870

820

920

At filling up time

Pin

Low temp.

10mm

Fig. 8 Temperature distribution of castings at 5 shots by numericalsimulation.

Core pin(SKD61)

9

100

300

Shot No. SEM Image / Side view of interface at an Al layer

A

A B

Fine crystal

Core pin

Core pin

Enlarged view of A

Enlarged view of A

Enlarged view

Adhesionlayer

Adhesion layerAdhesion layer

Fine crystalCrystal

Crystal

Fig. 9 SEM micrographs of cross sections near the interface of Al layer.

X-r

ay in

tens

ity

Distance from interface , d/μm

Accelerating voltage 10kV

AlSi

FeMn

Pin

Al Layer

X-r

ay in

tens

ity

Distance from interface , d/μm Backscattered electron image

Analysis position

Cu

Fig. 10 Energy dispersive X-ray spectroscopy of Al layer (Shotnumber: 9).

Y. Iwata, H. Iwahori and Y. Furukawa1474

aluminum alloy layer in the temperature range shown inFig. 4 for a die casting cycle time of 39 s. The diffusion rateof Fe depends on the temperature of the adhered aluminumalloy layer; the diffusion concentration increases withincreasing temperature. For example, if the temperature ofthe adhered aluminum alloy layer is 542K, the Feconcentration at a distance of 0.1 µm from the surface ofthe pin is 0% and thus Fe hardly diffuses. When thetemperature of the adhered aluminum alloy layer increasesto 752K, the concentration of Fe rises to about 30% at adistance of 1 µm from the surface of the pin.

Figure 13 shows the calculated diffusion distances d of theconsidered elements when the pin was kept in contact withthe aluminum alloy layer at 650K (the average temperaturein Fig. 4) for 39 s. Although aluminum hardly moves in iron,iron moves about 0.2 µm in aluminum. The moving distanceof silicon in aluminum is as long as 0.82 µm. Therefore, theerosion (pits) of the pin can be considered to be caused by thediffusion of iron from the pin into the adhered aluminumalloy layer. In addition, for the 1st shot after the aluminumalloy adhered to the pin, the diffusion distance of iron isas small as about 0.2 µm, and thus no Al­Si­Fe-basedcompound is likely to have formed. However, 6 cycles afterthe aluminum alloy adhered to the pin, the diffusion distanceof iron increased to 1.2 µm, which is six times as large as thatafter the 1st shot.

According to a report on the reaction initiation temperatureof Fe/Al by Aoyama et al.,10) the reaction proceeds in a veryshort time at a temperature high enough. For example, thereaction of Fe/Al occurs in 6 s (corresponding to the curingtime) at 822K and in 36 s (total curing time for 6 cycles) at799K. From Fig. 8, it can be seen that the lower limit of the

temperature range of the die castings around the pin changesfrom 740 to 820K from the completion of filling to dieopening. The reaction initiation temperature during the 6thshot is in this temperature range. During the 6th cycle, ironcan diffuse into the adhered aluminum alloy layer to adistance of 1.2 µm from the surface of the pin and thetemperature of the adhered aluminum alloy layer exceedsthe reaction initiation temperature of Fe/Al due to the heatsupplied from the side of the die casting. As a result,compounds with a size equal to or less than the diffusiondistance of iron (1.2 µm) form on the surface of the pin. Thisis confirmed by Fig. 9, where the thickness of the compoundobserved after the 9th shot (6 cycles after the adhesion ofthe aluminum alloy) is 1 µm or less.

Figure 14 shows the EDS line spectra of aluminum andsilicon in the adhered aluminum alloy layer (point A inFig. 9) after the 100th shot. A backscattered electron image isalso shown in the figure. Figure 15 shows the EDS analysisresults of the compound. The silicon concentration in theadhered aluminum alloy layer near the surface of the pin afterthe 100th shot is higher than that after the 9th shot. Althoughthe detailed mechanism requires further examination,considering that the diffusion distance of silicon in aluminumis relatively long (see Fig. 13), silicon in the adheredaluminum alloy layer can diffuse to and concentrate at theinterface under high temperature during 100 cycles of diecasting. Therefore, it can be considered that the Al­Si­Fe-based compound forms and grows on the surface of the pinthrough a reaction between the concentrated silicon and theiron diffused into the adhered aluminum alloy layer from

Accelerating voltage 10kV

Point1

Al layer

Point2

Point1 Point2

X-ra

yin

tens

ityX-ray energy X-ray energy

EDS of each point

Fig. 11 Energy dispersive X-ray spectroscopy of each point after 9 shots.

Fig. 12 Relationship between iron concentration in aluminum layer anddistance from pin surface.

Fig. 13 Calculated values of diffusion distance of each element (Al,Fe, Si).

0 40

100

X-r

ay in

tens

ity , I

(a.u

.)

Distance from interface , d/μm

6μm

Al

SiPin

Al layer

Backscattered electron image

AlSi

Analysis position

SiAl

Si

Al

Fig. 14 Energy dispersive X-ray spectroscopy of Al layer (Shot number:100).

Soldering Behavior of JIS ADC12 Alloy Die Castings and Its Mechanism 1475

the pin. Therefore, the concentration of aluminum near thesurface of the adhered aluminum alloy layer (Point 3 inFig. 15) is high and the concentration of silicon is higherthan that of aluminum in the area without crystals near theinterface with the pin (Point 2). Furthermore, the compoundobserved on the pin after the 100th shot is an Al­Si­Fe-basedcompound, the same as the fine compounds precipitated afterthe 9th shot.

Once the aluminum alloy adheres to the pin, finecompounds comprising aluminum, silicon, and iron form atthe interface between the pin and the aluminum alloy layerover several shots of die casting. With an increasing numberof die casting cycles, the compound becomes coarse due tothe accumulation of silicon on the surface of the pin in theadhered aluminum alloy layer and the elution of iron from thepin.

4. Conclusion

To prevent soldering in the die casting process, weinvestigated the adhesion and reaction behavior between analuminum alloy and a mold pin without the use of a moldrelease agent. The following conclusions were obtained.(1) The thickness of the adhered aluminum alloy layer on

the pin is about 10 µm after several shots and increasesto 20 µm with a further increase in the number of shots.The adhesion of the aluminum alloy to the pin is likelydue to the fracture of the die casting a short distanceaway from the surface of the pin caused by the shearforce of ejection.

(2) During die casting, a compound of Al­Si­Fe forms onthe surface of the pin; the aluminum and silicon arefrom the adhered aluminum alloy layer and the ironis from the pin (via elution). With the precipitation ofthis compound, silicon concentrates in the region ofthe adhered aluminum alloy layer in the vicinity of thepin.

(3) The size of the Al­Si­Fe-based compound thatprecipitates at the interface with the pin in the adheredaluminum alloy layer depends on the diffusion rate ofiron in aluminum. Since the movement distance of ironin one cycle of die casting is as small as 0.2 µm, it islikely that only physical adhesion occurs at the initialstage, with soldering occurring later via a reactionbetween the adhered aluminum alloy layer and the pinwith increasing number of die casting cycles.

REFERENCES

1) Y. Iwata, S. Dong, Y. Sugiyama and H. Iwahori: Mater. Trans. 54(2013) 1944­1950.

2) Y. Iwata, S. Dong, Y. Sugiyama and H. Iwahori: Mater. Trans. 55(2014) 311­317.

3) Y. Iwata, S. Dong, Y. Sugiyama and H. Iwahori: Mater. Trans. 53(2012) 483­488.

4) H. Sasaki: Katagijyutsu 16 (2001) 9.5) H. Sasaki: Lecture of Aluminum Die Casting, Tokai Branch, Japan

Foundry Engineering Society, (2015) pp. 19­26.6) H. Yamada, H. Iwahori, H. Ito, Y. Esaki, K. Yamada, K. Suzuki, H.

Takeuchi, M. Kazino, Y. Iwata, H. Tachikawa and S. Kin: Rep. JFSMeeting, 164 (2014) p. 12.

7) S. Kin, H. Tachikawa, H. Suzuki, Y. Nomura, T. Hayafuji, Y. Hisanagaand Z. Hachisuka: Rep. JFS Meeting, 164 (2014) p. 13.

8) H. Sasaki: Sokeizai 51 (2010) 10.9) N. Komatsu, M. Nakamura and H. Huzita: Light Metals 18 (1968) 9.10) S. Aoyama: Katagijyutsu 4 (2001) 28.11) N. Nishi and S. Kami: J. JFS 70 (1998) 9.12) K. Wakatsuki, H. Uno and N. Yokoi: Denkiseikou 85 (2014) 1.13) S. Arrhenius: Doctoral dissertation, Stockholm, Royal publishing

house, P.A. Norstedt & söner, (1884) p. 89.14) M. Kotani and T. Sunada: Contemp. Math. 338 (2003) 271­305.15) The Japan Institute of Metals and Materials: Metals databook

(3rdEdition), (Maruzen Co., Ltd., Tokyo, 1993).16) S. Aoyama, M. Sunada, K. Sakamoto and T. Umemura: Light Metals

41 (1991) 6.

Point1Point2

Point3

Point1 Point2 Point3Accelerating voltage 10kV

X-r

ayin

tens

ityX-ray energy → X-ray energy→ X-ray energy→

EDS of each point

Fig. 15 Energy dispersive X-ray spectroscopy of each point after 100shots.

Y. Iwata, H. Iwahori and Y. Furukawa1476