Fracture property correlation of Rheocast (EMS) and Thixocast 6061 Aluminium alloy

Shailesh Kumar Singh1a, Prosenjit Das2b, 3, K. Chattopadhyay1b

and Pradip Dutta2c

1Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

2Near-Net Shape Manufacturing Technologies Group, CSIR-Central Mechanical Engg. Research Institute, Durgapur-713209, India.

3Department of Mech. Engineering, Indian Institute of Science, Bangalore 560012, India

E-mail1a: shailesh.iisc@gmail.com, E-mail2b: prosenjit@cmeri.res.in

E-mail1b: kamanio@materials.iisc.ernet.in E-mail2c: pradip@mecheng.iisc.ernet.in

Keywords: Rheocast, Thixocast, Al 6061 alloy, Fracture, Tensile properties, Microstructure.

Abstract

The present study has been taken up to establish fracture property correlation of the semi solid

processed 6061 aluminium alloy. Electro Magnetic Stirring (EMS) has been performed to get non-

dendritic billets and then the alloy has been thixocast using a High pressure die casting machine.

Tensile tests have been performed to estimate mechanical properties of the semi solid processed

alloy. Fracture behaviour of the alloy has been investigated at the intermediate EMS cast state and

subsequent final thixocast state, employing uniaxial tensile tests at 0.001s-1

strain rate and at room

temperature. Fracture surface morphology of the EMS cast state shows that the presence of micro

porosity is responsible for crack initiation and final fracture. Our results confirm that the dimpled

rupture of primary Al phase is responsible for improved mechanical properties in the thixocast state

of the alloy.

Introduction

Discovery of thixotropic behaviour of the metallic alloys between solidus and liquidus temperature

and their shear rate dependent viscosity in the early 1970s [1] paved the way for the development of

semisolid processing (SSP) routes. Basically there are two variants of SSP namely, rheoforming and

thixoforming [2, 3]. To manufacture components of industrial relevance with their intricate details

in near net shape, the SSP routes that are adopted are Rheocasting [4-7] and Thixocasting [8-12]. In

case of rheocasting superheated melt is processed to form semi-solid slurry and shaped in the form

of components whereas thixocasting involves an intermediate solidification step and requires

specially prepared non-dendritic billets having near spherical microstructural morphology.

Although developed in early 1930s, 6061 aluminium alloys have remained important general-

purpose alloys for aerospace and automotive sectors because of their good strength, formability,

weldability and corrosion resistance. In order to make complex shapes using these alloys, forging

and machining operations generally need to be performed on them, which not only increases the

cost but also reduces the productivity. To overcome these problems semisolid forming technique is

perceived as a suitable alterative. Although cast alloy series is most popular for semisolid

processing, it is also possible to make net shape component using conventional wrought alloys such

as 6000 series [13, 14] through the semisolid processing route. Notable effect of semisolid

processing has been observed towards spherodisation of primary Al-phase present within the alloy,

which explains the improved deformation behaviour of the thixocast alloy compared to its wrought

form.

In order to qualify the thixocast components of 6061 alloy for in-service conditions,

investigation of its fracture behaviour is of utmost importance. Fracture is the result of continued

deformation processes and hence there is possibility that fracture surfaces may preserve the

Solid State Phenomena Vols. 217-218 (2015) pp 405-411© (2015) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.217-218.405

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information of the deformation behaviour of the material. The mechanism of ductile fracture

consists of initiation, growth, and coalescence of microvoids [15, 16]. Several researchers have

attempted to correlate the fracture behaviour and mechanical properties. Groselle et al. [17] found

that in case of cast Al – Si alloy, fracture mechanism depends on the size and shape of Si or Fe-rich

brittle phases. They observed that presence of large and acicular particles decreases mechanical

properties, reducing elongation to fracture and the ultimate tensile strength. Nagumi et al. [18]

investigated the fracture surface morphology of Al-Mg-Si alloy at different solid fraction. It was

observed that the grain size near the fracture surface increases with increase in solid fraction. The

fracture surface morphology tends to flatten with increase in solid fraction. Sunwoo and Morris [19]

observed the fracture morphology in different zone of welded 2090 aluminium alloy. For base metal

in the peak-aged condition, the fracture surface appears planar-transgranular, while for the over-

aged conditions the surface appears ductile with subgrain delamination. Jiang et al. [20] studied the

fracture behaviour of A356-T6 aluminium alloy produced by EPSC-VL (expendable pattern shell

casting process with vacuum and low- pressure), gravity casting (GC) and lost foam casting (LFC).

They observed that for samples produced by EPSC-VL process the fracture path passes through the

eutectic phase along the grain boundaries of the α-Al primary phase, and its fracture mode is

intergranular. The fracture path for the samples produced by gravity casting method passes through

the eutectic silicon particles, and exhibits a transgranular fracture mode. Again, for the case of

sample produced from lost foam casting, the fracture path goes through shrinkage pores due to the

high shrinkage porosity level in these samples and it exhibits a brittle fracture nature. Similarly,

fracture mechanisms were also investigated for other metals. Miura et al. [21] observed that by

increasing temperature and strain rate, voids in fracture surface becomes more spherical in shape in

carbon steel. Strain rate and deformation temperature have significant impact on the dimple size and

shape on fracture surface.

The present study is taken up to establish fracture property correlation of the EMS cast 6061

and Thixocast 6061 aluminium alloy. Fracture behaviour of the alloy is investigated at the

intermediate EMS cast state and subsequent final thixocast state, employing uniaxial tensile tests at

room temperature considering strain rate of 0.001s-1

.

Experimental Procedure

As-received 6061 wrought aluminium alloy (composition is given in Table 1) is used as a raw

material for semisolid processing. The liquidus temperature of the 6061 alloy is 652℃ [22] .Around

25 kg of the alloy is melted in a silicon carbide crucible in a resistance heating furnace at 850 ±

3°C. Coverall powder (M/S Foseco) has been used as a fluxing agent to the melt to facilitate

removal of oxides and to protect the melt from further oxidation. The slag formed after flux

addition within the melt is skimmed off and the melt is degassed subsequently using two

hexachloroethylene tablets for about 2 minutes. Magnesium (0.5 wt% of melt weight) is then added

to the melt in order to compensate the loss occurring due to slag formation. TiB2 (0.2 wt% of melt

weight) is used as grain refiner for melt treatment. The melt is then allowed to soak for 10 minutes

and subsequently transferred into a crucible (around 1.5 kg). When the temperature reaches 670°C,

the metal is poured into the preheated metal mould kept inside the electromagnetic stirrer.

Electromagnetic force field is applied on the melt to generate melt convection for shearing action at

the solid/liquid interface of the solidifying melt, which results in the formation of semi-solid slurry

of the said alloy having fragmented dendritic primary particles. The electromagnetic stirrer used for

the present investigation is of linear type which consist of six primary coils displaced equally,

stacked on top of one another. Electromagnetic stirrer is excited by a three-phase power supply to

generate an axially travelling magnetic field. The mould is cylindrical in shape, made of stainless

steel (SS 304 grade, non-magnetic and corrosion resistant material), and aligned co-axially within

the stirrer. The inner mould walls are coated with a layer of boron nitride to prevent the molten

metal from sticking. There is a gradual taper along the inner mould wall to ease the billet removal

after casting. The electromagnetic stirrer is operated at 300A current, 50Hz frequency for a stirring

time of 30 seconds. After 30 seconds of stirring time, water supply is switched on for the cooling

406 Semi-Solid Processing of Alloys and Composites XIII

water to flow through the cooling channels placed at the circumference of the metallic mould, in

order to chill cast the solidifying melt within the metallic mould. The solidified cylindrical billets

(figure 1) are machined to the required length of 130 mm and diameter 60 mm for induction heating

(figure 1). The billets are then brought into the semisolid state (640°C) and transferred into the shot

sleeve of the die casting machine. The solid fraction of this alloy at 640 °C is 0.34 [22]. The dies are

preheated up to about 200°C. Figure 1 shows a die for a connecting rod used with Buhler VISION

B 42N Die Casting machine (with maximum locking force capacity of 420 tonnes), where

thixocasting has been performed. During thixocasting, the billet is pushed by the plunger whose

movement is governed by the velocity specified as a function of stroke. In the present work,

injection speed of 1.9 m/s with an intensification pressure of 1000 bar is considered to perform

thixocasting. The complete experimental set for the preparation of EMS cast and Thixocast

component is shown in figure 1.

Microstructure of the rheocast billet and thixocast cast component (figure 1) is observed with

the help of optical microscope. Optical micrographs are shown in Fig. 2, depicting macro porosity

free castings in both the casting techniques under investigation. In case of EMS casting, the

geometry of the mould is very simple (cylindrical), and molten metal is poured into it by gravity.

Subsequently the electromagnetic field is switched on, and under the action of electromagnetic

force the dendrites undergo shearing to produce non-dendrite structure. Whereas, geometry of

thixocast component is complex, so pressure is applied to feed the semisolid material into complex

geometries.

To investigate mechanical properties of the said alloy at rheocast and thixocast state, tensile

samples are prepared from rheocast billets and thixocast cast components according to ASTM E8

M-01 with gauge length of 20mm and gauge diameter of 4mm. The tensile strength measurements

of the specimens are performed at room temperature at a strain rate of 0.001s-1

. The fractured

samples are then examined under scanning electron microscope.

Table 1: The chemical composition of experimental alloy (in wt %)

Al Si Mg Fe Cu Mn Ti Cr Zn

Bal. 0.62 0.91 0.22 0.19 0.07 0.03 0.08 0.10

Figure 1: Experimental set up for the preparation of EMS cast and Thixocast components.

Solid State Phenomena Vols. 217-218 407

Results and Discussions

Microstructural morphology

Figure 2 (a, b) shows the optical micrographs of the rheocast (EMS) and thixocast Al 6061 alloy.

Mixtures of rosette and equiaxed particles of primary Al phase are observed within the micrograph

of rheocast sample. Dendrite fragmentation mechanism is identified as the governing mechanism

towards non-dendritic microstructure formation during EMS. Optical micrograph of the thixocast

specimen of the said alloy shows mostly spheroidal primary grains. Liquid entrapment observed

within the primary Al grains is attributed to the rapid coalescence of the dendrite arms under

applied pressure. Liquid segregation is also observed from the micrograph due to the separation of

solid and liquid phases during pressure driven flow of the melt towards the die cavity.

Figure 2: Microstructure of a) EMS cast 6061 Al alloy and b) Thixocast 6061 Al alloy

Mechanical properties

Tensile results

The load stroke curve obtained from tensile test is converted to engineering stress – strain curve and

subsequently to the true stress – strain curve using the standard equations. Figure 3 shows the

photograph of the tensile specimen used for conducting the uniaxial tensile test and its fracture part

after the test. Figure 4 shows the true stress – strain curve for EMS cast and thixocast 6061

Aluminium alloy at a strain rate of 0.001s-1

. It is obvious from the figure that thixocast 6061 alloy

exhibits maximum tensile strength (UTS) of 210MPa while EMS cast alloy shows UTS of 167

MPa. Similarly, Thixocast 6061 alloys show 26% elongation to failure, which is significantly above

that of EMS cast alloy (7% elongation to failure) at room temperature. Thus, the thixocast alloy

exhibits 10% higher tensile strength and 72% higher ductility compared to those of the EMS cast

state. The phenomenon behind this is explained subsequently.

Thixocasting deals with the solidification under pressure, which significantly reduces the

amount of the microporosity present within the cast parts compared to that of EMS cast parts of the

said alloy. This is responsible for improved ductility and higher tensile strength of thixocast alloy.

Another reason which is greatly responsible for improved tensile behaviour of the thixocast alloy is

the smaller grain size of the primary Al particles and higher degree of sphericity compared to that of

EMS cast state.

Moreover, on comparing the two curves it is observed that slope of stress – strain curve is

high for EMS cast part which indicates that the work hardening for EMS cast part of 6061 alloy is

more prominent at room temperature in comparison to thixocast 6061 alloy. This is because EMS

cast 6061 alloy offer higher resistance to dislocation motion due to the presence of rosette

(a) (b)

408 Semi-Solid Processing of Alloys and Composites XIII

(fragmented dendritic) like microstructure as compared to globular microstructure of thixocast 6061

alloy.

Fracture surface morphology

Figure 5 (a, b) and 5 (c, d) shows the fracture surface morphology of the rheocast and thixocast

samples failed under tensile loading respectively. In case of rheocast samples shrinkage porosity

triggers crack initiation and it progresses through the region of localised and clustered pores. In

addition to that, fracture surface also shows the evidence of improper feeding of liquid metal into

the inter spheroidal space of the rheocast sample (figure 5 a), as there are regions where failure has

occurred around the spheroids. This is because of the presence of shrinkage porosity due to inability

of feeding the solidifying liquid between the spheroids during volume contraction. Hence, presence

of high porosity level brings the mechanical properties down in case of rheocast samples.

In contrast to EMS cast (rheocast) samples, thixocast samples shows improved tensile

properties, and the reason is evident from their fracture surface morphology. Figure 5 (c) and 5 (d)

show mostly dimpled fracture with local presence quasi-cleavage regions in case of thixocast

samples. The ductile fracture behaviour of the thixoformed samples can be explained as the

formation of voids at eutectic region and then the microvoids are joined together by dimpled rupture

of primary Al grains within them. This failure mechanism is responsible for higher percentage

elongation to failure in this case. The mean dimple diameter for thixocast 6061 alloy at a strain rate

of 0.001 is 7 µm. Hence, thixoforming of the EMS cast 6061 Al alloy facilitates uniform

distribution and small dimple size and promotes higher ductility.

Figure 4: True Stress-Strain curve of

investigated alloys at room temperature and

strain rate 0.001s-1

Figure 3: Digital photograph of the tensile

specimen prior and after testing.

Solid State Phenomena Vols. 217-218 409

Figure 5. Representative SEM Fractograph of a) EMS cast 6061 alloy b) EMS cast 6061 alloy at

high magnification, c) Thixocast 6061 alloy, d) Thixocast 6061 alloy at high magnification

Conclusions

This paper presents the fracture behaviour of the 6061 alloy at the intermediate EMS cast state and

subsequent final thixocast state, using uniaxial tensile tests at 0.001s-1

strain rate and at room

temperature. The following main conclusions are drawn:

• Thixocast 6061 alloy shows better ductility and strength as compared to EMS cast 6061

alloy which led to the inference that the high degree of spheroidization in the microstructure

enhances the ductility of the material. Moreover, response to the flow stress is better for

thixocast part in comparison to the EMS cast part as the latter exhibits higher work

hardening.

• Thixocast 6061 show higher frequency of dimples with uniformity in shape, while rheocast

fracture exhibits less frequency of dimple with non – uniformity in shape and size. Hence,

thixoforming of the EMS cast 6061 Al alloy facilitates uniform distribution with small

dimple size, and promotes higher ductility.

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