? Web viewKeywords: Aluminium alloys; High pressure die casting; Mechanical property; Microstructure
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A High Strength Aluminium Alloy for High Pressure Die Casting
Shouxun Ji, Feng Yan, Zhongyun Fan
BCAST, Brunel University London, Uxbridge, Middlesex, UB8 3PH, UK
Keywords: Aluminium alloys; High pressure die casting; Mechanical property; Microstructure
In order to achieve the mechanical properties of thin-wall components made by high pressure die-casting at a level of 300MPa of yield strength, 420MPa of UTS and 3% of elongation, the composition of the new aluminium alloy and casting parameters have been optimized to reduce casting defects in final components. In the present paper, we describe the alloy development in an industrial scale HPDC system.
Aluminium alloys are being increasingly used as lightweight materials in transport since weight reduction has been proved as one of the most effective approaches to improve fuel economy and reduce CO2 emissions [[endnoteRef:1],[endnoteRef:2]]. High pressure die casting (HPDC) is a widely used process to make thin-wall aluminium components for structural applications. The solution and ageing heat treatment have been used to enhance the mechanical properties of castings [[endnoteRef:3]]. However, the existing aluminium alloys for HPDC process are mainly based on Al-Si, Al-Si-Cu, and Al-Mg-Si systems. Most of these alloys are not particularly developed for property enhancement by heat treatment, especially not suitable for solution heat treatment because of the unsatisfied mechanical properties for industrial application [[endnoteRef:4],[endnoteRef:5]]. Therefore, the development of high strength aluminium alloys becomes significant to promote the application of aluminium alloys where an improved strength and lightweight are essential. [1:  T. Zeuner, L. Wrker. Process Optimisation of Cast Aluminium Chassis Components WFO Technical Forum 2003, p3.1-3.14.] [2:  S. Ji, D. Watson, Z. Fan, M. White. Development of a super ductile diecast AlMgSi alloy Mat. Sci. Eng. A 556 (2012) 824833.] [3: S. Ji, F. Yan, Z. Fan, Casting Development with a High Strength Aluminium Alloy, Materials Science Forum 828-829 (2015) 9-14.] [4:  V. S. Zolotorevskii, N. A. Belov, M. Glazoff. Casting aluminium alloys, first ed., Elsevier, London, 2007, pp. 386-396.] [5:  S. Ji, F. Yan, Z. Fan, Development of a high strength AlMg2SiMgZn based alloy for high pressure die casting, Materials Science and Engineering: A 626, 165-174.]
In the present paper, we describe the alloy development including the effect of Mg, Mn and Zn in the Al-Mg2Si system on the mechanical properties and microstructure of the cast alloys. The effect of solution and ageing heat treatment on the mechanical properties and microstructure was also studied
Commercial purity aluminium, magnesium, zinc, Al-20wt.%Mn, and Al-50wt.%Si master alloys were used as base materials to make the aluminium alloys. 10kg melt were made each time using a clay-graphite crucible in an electrical resistance furnace. During manufacturing, Al, Si and Mn were melted first and 15 ppm Be was added to the melt 10 minutes prior to the addition of Mg ingots. The melt was degassed at 750C using a rotatory degassing equipment with a rotating speed of 500 rpm for 5 minutes. After degassing, the melt was maintained at a temperature of 700C. A mushroom casting with a bottom flat of 6010mm was made by a steel mould for composition analysis. The casting was cut 5mm off from the bottom and ground down to 800 grid sand SiC paper. The composition was obtained from a well-calibrated optical mass spectroscope, in which at least five spark analyses were tested and the average value was taken as the chemical composition of the alloy.
The castings were produced using a 4500 kN cold chamber high pressure die casting machine, in which most of the operation parameters including injection speed and intensification pressure are monitored. One set of die was used to cast standard tensile samples, during which three 6.35mm round samples and three flat tensile samples at 2mm, 3mm, and 5mm were made in one shot. The mechanical properties were obtained from 6.35mm round tensile samples. All parameters had been optimized for the HPDC machine during casting.
The tensile tests were conducted following ASTM standard B557, using an Instron 5500 Universal Electromechanical Testing System equipped with Bluehill software. All tensile tests were performed at ambient temperature. The gauge length of the extensometer was 25 mm and the ramp rate for extension was 2 mm/min. The microstructure of each alloy was examined using a Zeiss optical microscope with quantitative metallography, and a Zeiss SUPRA 35VP SEM.
1. Effect of extra Mg in Al-Mg2Si alloys
Fig. 1 shows the equilibrium phase diagrams at Al-rich side for Al-Mg2Si system. The pseudo-binary Al-Mg2Si system behaves like a normal binary eutectic system with a eutectic point at 13.9 wt.% Mg2Si. It shows that a narrow three-phase region for L+-Al+Mg2Si co-existence between 591 C and 578 C. With extra Mg in the Al-Mg2Si system, the shape of equilibrium phase diagram exhibits some changes. The eutectic point is shifted towards a lower Mg2Si concentration and the area of co-existence for the three phases is enlarged. This means that, for a given Mg2Si content, an extra Mg addition results in a decrease of the volume fraction of the primary -Al phase and an increase of the volume fraction of Al-Mg2Si eutectic phase.
To confirm the calculated results, a series of experiments were performed with different amounts of extra Mg in the Al-Mg2Si alloys. The representative microstructures are shown in Fig. 2. The primary phase presented in the as-cast microstructure was different. With 1.9wt.% extra Mg, the alloy showed a typical hypoeutectic microstructure, consisting of dendrites or fragmented dendrites of primary -Al phase surrounded by Al-Mg2Si eutectic phase. However, the microstructure was much different when the extra Mg was 5.8 wt.%. The coarse primary -Al phase almost disappeared and the microstructure was dominated by fine primary -Al phase and Al-Mg2Si eutectics. In general, the experimental results were consistent with the calculation results, although the Al-Mg2Si alloys without extra Mg cannot be obtained in high pressure die casting because of the severe die soldering problem.
Temperature ( oC)
Fig. 1 Equilibrium phase diagrams of Al-Mg2Si pseudo-binary system and Al-Mg2Si-Mg ternary system with 6wt.%Mg.
The yield strength, UTS and elongation of the as-cast Al-Mg2Si-Mg alloys can be varied by the content of extra Mg. The yield strength was increased with increasing Mg content, but showed no significant difference with increasing the Mg2Si phase. Conversely, the elongation was decreased with the increase of Mg and Mg2Si in the alloys. The UTS were different for the alloys with different extra Mg contents. The decrease of the UTS was significant for the Al-9.9wt.%Mg2Si and Al-12.7wt.% Mg2Si alloys with the increase of extra Mg in the alloys. However, it is noted that the yield strength is much lower than the target of 300MPa.
2. Effect of Mn in Al-Mg2Si-Mg alloys
Fig. 3 shows the typical microstructure of the intermetallics in the Al-7.8wt.%Mg2Si- 5.8wt.%Mg-0.59wt.%Mn alloy with 0.12 wt.% Fe. Intermetallics with two different sizes were observed and the volume fraction of the intermetallics increased with the increase of Mn addition. The quantitative analysis showed that the average size of the fine intermetallics were about 0.8m. An obvious increase of the volume fraction of the Mn-rich intermetallics could be seen for the increasing Mn content. The composition of the Mn-rich intermetallic phase was measured with different amounts of Mn additions and the results confirmed that the Mn-rich intermetallics were -AlFeMnSi phase. The measured constituent of the intermetallics could be described by Al42-46(Fe,Mn)3.5-Si7. The Mn-rich intermetallics are essentially associated with Fe in the die cast alloys.
The increase of Mn content in the alloys resulted in a slight increase of the yield strength and UTS, but a slight decrease of the elongation for the Al-7.8wt.%Mg2Si-5.8wt.%Mg alloy under as-cast condition. The elongation was decreased from 6.9% to 6.5% when the Mn content was increased from 0.19wt.% to 0.59wt.%, which is the level of the commonly used in the die-cast Al-Mg-Si alloy.
Fig. 2 Optical micrographs showing the effect of excess Mg on the as-cast microstructure of Al-7.8 wt.%Mg2Si alloy with (a) 1.9wt.%Mg, (b) 5.8wt.%Mg.
Fig. 3 The morphology of Mn-rich intermetallic phase and AlMg intermetallic phase in the Al-7.8 wt.%Mg2Si-5.8 wt.%Mg-0.59wt.%Mn alloy with 0.12wt.%Fe.
3. Effect of Zn in Al-Mg2Si-Mg-Mn alloys
The typical microstructure of Al-Mg2Si-Mg-Zn-Mn alloy is shown in Fig. 4. An AlMgZn phase was observed, which were characterised by irregular shapes. The Zn-containing intermetallic compounds were located at the boundaries of the primary -Al phase or between the eutectic cells and the primary -Al phase. The measured volume fractions of the AlMgZn phase were increased significantly with increasing the Zn addition. The quantitative measurement confirmed that the intermetallic compounds contained 251 at.% Mg, 100.7 at.% Zn, which corresponded to a formula of Al13Mg5Zn2. Therefore, the Zn-rich intermetallic phase was essenti