South Asian Journal of Engineering and Technology Vol.2, No.22 (2016) 245– 255

Role of Copper on the Microstructure and Mechanical properties of Forged A356 Aluminum alloy

T.Prabhu,E.R.Sivakumar,M.Chandrasekar

Department of Mechanical Engineering , Sasurie College of Engineering,Tiruppur-638056, Tamil nadu, India

Abstract

Aluminum alloys are widely used in many manufacturing areas due to good castability, lightness and mechanical properties. In general, the mechanical properties of the castings are controlled by their microstructure. The mechanical properties of A356 are governed by casting soundness, the amount, size and morphology of the constitutive phases such as alpha-aluminum (Al) phase, eutectic silicon (Si) particles and the amount of strengthening precipitates Mg2Si formed in the alpha-aluminum (Al) phase. These factors are influenced by how the Al-Si binary eutectic nucleates and forms structure during solidification and by the heat treatment process involving Solutionising and Artificial ageing. The cast-forged alloy has given the best hardness, strength, ductility because of the matrix strengthening and homogeneous distribution of Eutectic Silicon particles due to forging process. Study reveals that at micro level forged structure is more refined than in the as cast conditions. This is due to the work-hardening effect, where the original structure is destroyed during the forging and recrystallization helped in creating large number of nucleating sites leading to fine grain structure. The purpose of this research is to investigate the effect of Copper (Cu) in Cast and Forged A356 alloy (Al-7Si-0.3Mg). The microstructure of the samples has been studied by optical and scanning electron microscopy. Materials in both as-cast, as forged and heat treated status have been investigated through tensile test bars to get the mechanical properties of the different conditions.

Keywords: A356 Aluminium Alloy, Forging, Casting,

1. Main text

1.1. Aluminum Introduction:

Properties of Al are usually enhanced by the addition of major alloying elements such as Cu, Si, Mg, Mn, Zn, Li, Ni and then subjecting the alloys to various thermal, mechanical and thermo mechanical treatments. Some of the minor alloying elements added to aluminum are Na, Sr, Sb, Ba and Ca to induce specific changes in the microstructure. Al alloys are available in both cast and wrought forms and about 20% of aluminum produced is used in the cast form mainly in the transport sector [2].

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9% Electrical 25% 26% Construc tion 20% Packagin 20% g

Figure.1.1. Aluminum’s major application

1.2. Aluminum is one of the most abundant metals available in the earth's crust as bauxite with wide range of applications in the modem world. There are many reasons for aluminum's continued expansion into newer and wider fields of application. Light weight, excellent specific strength, high thermal and electrical conductivities, high reflectivity, good corrosion resistance, excellent workability, and attractive appearance are some of aluminum's most appealing properties. However, its relatively low strength and poor castability limit its use largely to the production of rotor castings for electrical motors and other applications in which high electrical conductivity is required.

1.1 Role of Alloying Elements:

The Aluminum Association’s Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot lists for each alloy 10 specific alloying elements and also have a column for “others”. Not all of the listed elements are major alloying ingredients in terms of an alloys intended uses; and some major elements in one alloy are not major elements in another. Also, some elements, like Sr for example, can be very important to microstructure control and mechanical properties but are not specifically identified in the Aluminum Association document and are instead are merely included in the category “others”. For the purposes of understanding their effects and importance, alloying elements for the majority of alloys are probably best classified as major, minor, microstructure modifiers or impurities; understanding, however, that impurity elements in some alloys might be major elements in others [2]:  Major elements typically include silicon (Si), copper (Cu) and magnesium (Mg).  Minor elements include nickel (Ni) and tin (Sn) -- found largely in alloys that likely would not be used in high integrity die castings.  Microstructure modifying elements include titanium (Ti), boron (B), strontium (Sr), phosphorus (P), beryllium (Be), manganese (Mn) and chromium (Cr).  Impurity elements would typically include iron (Fe), chromium (Cr) and zinc (Zn).

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1.3. Tables

Young’s Alloy % Alloy Composition 0.2% PS UTS %E Modulus VHN No Reduction (E)

1 A356 131 185 3.25 3.32 87 245

2 A356 + 0.65% Al-3Ti 132 188 4.05 5.85 88 252

3 A356 + 0.60% Al-3B 135 192 4.52 6.74 89 260

4 A356 + 0.20%Al-10Sr 139 195 4.78 7.15 92 281 A356 + 0.65% Al-3Ti + 5 0.60% Al-3B + 0.20% Al- 142 202 4.92 7.55 100 298 10Sr Microstructural Studies:

a b c

d

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e Fig. 3.5 a-e SEM photomicrographs of A356 alloy (a) as cast alloy; (b) with 0.65% of Al-3Ti grain refiner; (c) with 0.60% of Al-3B grain refiner; (d) with 0.20% of Al-10Sr modifier and (e) combined addition of 0.65% Al-3Ti, 0.60 % of Al-3B grain refiner and 0.20% of Al-10Sr modifier.

Figure 3.5 a shows the SEM photomicrograph of A356 alloy in the absence of grain refiner. From figure it is clear that in the absence of Al-3Ti master alloy, A356 alloy shows coarse columnar α-Al dendritic structure and unmodified needle/plate like eutectic silicon. With the addition of 0.65% of Al-3Ti the master alloy, A356 alloy shows response towards grain refinement with structural transition from coarse columnar dendritic structure to fine equiaxed structure as shown in figure 3b. With the addition of 0.60% of Al-3B master alloy, the structure ofA356 alloy changes from columnar to finer equiaxed α-Al dendrites compared to the addition of Al-3Ti grain refiner as clearly observed in figure 3c, while eutectic silicon remains unmodified as expected. This could be due to the presence of AlB2 particles present in the Al-3B master alloy and these particles are act as heterogeneous nucleating sites during solidification of α -Al. While addition of A356 alloy to 0.20% of Al-10Srmaster alloy, the plate like eutectic Si is converted in to fine particles and α -Al dendrites remain as columnar-dendritic structure only as clearly seen in figure3d. However, figure 3e shows the simultaneous refinement (α-Al dendrites) and modification (eutectic

Si) of A356 alloy due to the combined action of AlB2 and Al4Sr particles present in Al-3B grain refiner and Al-10Sr modifier respectively. 3.4 Influence of copper

The addition of copper to Al–Si alloys enables the formation of Al-Al2Cu eutectic and/or Al2Cu phases and other intermetallic compounds, which increase strength and machinability of casting parts. Copper also increases heat treatability of the alloy. On the other side, copper reduces resistance to general corrosion and in specific compositions and material conditions increases stress-corrosion susceptibility. Addition of copper decreases significantly the melting point of the alloy. Therefore, the copper increases the solidification range of the alloy reducing the hot tear resistance and increasing the potential for inter-dendritic shrinkage.

3.4.1 Effect of copper on mechanical and microstructural properties of aluminum alloys In this study, effect of copper and silicon content on the mechanical properties of Al–Cu–Si–Mg alloys has been investigated. Al–Cu–Si–Mg alloys with 1, 3, 4.5, 6% Cu were utilized for this purpose. After melting and Na modification, alloys were cast in metal moulds at 780°C and solidified. They were solution treated at 490°C for 4 h and then quenched. Samples were aged at 180◦C for 5, 10, 15, 20 h to observe the effect of aging on mechanical

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properties. Strength of aluminum alloys after aging can be considerably increased by addition of copper, silicon and magnesium. The mechanism of the strength increase cannot be easily explained by addition of various alloying ׳due to the presence of copper, and S ׳and θ ״elements and phase transformations alone. However, precipitation of θ

Mg2Si) due to the presence of magnesium and silicon are expected. A good combination of mechanical) ״and β are present. Solidification of ״and β ׳S ,׳properties can be achieved when all these hardener precipitates, namely θ Al–Cu–Si–Mg alloys is observed to be dendritic in nature. Rapid increase in secondary dendrite arm spacing is detected as moving from mould walls towards the mould center. This is due to the decrease in the cooling rate during solidification. Mechanical properties of Al–Cu–Si–Mg alloys largely depend on the heat treatment. Thus, characteristics of heat treatment play a vital role for a good combination of microstructure and mechanical properties. Copper content in Al–Cu–Si–Mg alloys affect the mechanical properties. By increasing copper content, tensile strength and hardness increase due to the precipitation hardening and elongation decreases. It is found that increasing copper content from 1to 6%, tensile strength increases from 152 to 402 MPa and hardness increases from 45 to 118 HB. Early hardening is observed in Al–Cu–Si–Mg alloys during first few hours of aging and hardness increase is completed within 15 hrs. After 15 hrs, over aging occurs [6]. Copper has been used in the ternary system of Al–Si–Mg alloy in different solidification conditions to increase the mechanical properties of the existing alloys. Copper in the range of 0.2–2.5 wt. % has been used in A356 aluminum alloy and cast at different solidification conditions. Sand, graphite, copper and cast iron moulds have been designed and manufactured to produce tensile test specimens at different cooling rates during solidification of the alloys. The microstructures of the alloys have been studied through optical and scanning electron microscopy (SEM). Mechanical properties of different alloys at various solidification conditions have been investigated in both non-heat treated and heat treated conditions. The fracture surfaces of the tensile test samples have been studied. All intermetallic compounds formed during solidification have been investigated through SEM– EDS [7].

Figure.3.6 Effect of copper on porosity formation in Al-Si-Mg alloy. As seen in Figure 3.6 porosity of the samples increases with increasing copper. Two mechanisms may explain this observation. First, copper causes ternary eutectic reaction at about 525°C. The copper content in the eutectic melt is high which increases volumetric shrinkage during solidification and porosities of the sample. Second, hydrogen activity coefficient decreases with increasing copper content and hydrogen solubility decreases. Therefore, porosity forms rapidly during solidification in the alloys containing copper. As seen in, the effect of

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copper on porosity formation decreases with increasing cooling rate. It seems that porosity formation is related to the diffusion phenomena. Therefore, porosity formation will be more difficult with increasing cooling rate.

Figure 3.7 Effect of cooling rate and copper on tensile properties of Al-Si-Mg alloy. DAS decreases with increasing cooling rate as seen in Figure 3.7 Also, dendrites have partially refined with increasing copper. Casting microstructure will become finer by decreasing DAS. This will improve the tensile properties of cast parts. On the other hand, the size of silicon particles and secondary phases become smaller with increasing cooling rate. Iron bearing inter metallic platelets also become smaller by decreasing DAS. Therefore, both ultimate tensile strength and percent of elongation increase with increasing cooling rate. This is clearly shown in Fig 3.8. The results show that ultimate tensile strength increases with copper content up to 1.5 wt. %. UTS increases because of precipitation of copper-bearing phases in the interdendritic spaces caused by increasing copper. On the other hand, porosities, which are the sites of stress concentration, increase with copper content and cause decrease of strength. So, copper has two counter-sides effect on UTS. It seems that the reduction trend of UTS in the alloys containing more than 1.5 wt.% copper is related to the negative effect of porosities, which over come to the positive effect of copper-bearing precipitates.

Figure 3.8 Density variations with increasing copper in Al-Si-Mg alloy

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(a) As cast (b) Heat treated Figure 3.8 Increase in tensile strength with increasing copper in Al-Si-Mg alloy. Fig.3.8 shows the tensile test result. Ultimate tensile strength of the alloys increased with heat treatment, mold cooling rate and copper content. Tensile strength decreased in alloys having more than 1.5 wt. % copper in all solidification conditions. Also, yield strength of the alloys in the non-heat treated conditions was not dependent on solidification conditions.

Figure 3.9 SEM showing effect of copper addition in Al-Si-Mg alloy. As seen in figure 3.9, copper phases form as blocky or eutectic colonies in the interdendritic spaces based on cooling rate. Needle-like iron-bearing intermetallic and Al2Cu phases form simultaneously during a paratactic reaction. Silicon phases become fine and close to spherical shape by increasing cooling rate (graphite mould) and/or heat treatment process of silicon phases during heat treatment is done as follows. First, fragmentation of silicon branches and second, spheroidization of these fragmented silicon branches. The rate of first and second stages of this

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process increases with increasing cooling rate. Therefore, spheroidization takes place more easily in the alloys solidified at high cooling rates. Porosity nucleates in the interface of dendrites and eutectic cells. Dendrites and eutectic cells become smaller at high cooling rates. Therefore, porosities become smaller. In general fracture of these alloys is controlled by silicon phases and interdendritic phases. In high cooling rate, silicon shape changes to spherical, and the type of fracture transforms from brittle to ductile. So, percent of elongation improves. A Fine

Al2Cu precipitate caused by heat treatment slows the mobility of dislocations and ductility of the matrix reduces. Consecutively, percent of elongation decreases. When a sample solidifies at low cooling rate, fragmentation of silicon phases and spheroidization of them are postponed therefore, samples produced in sand mold exhibits lower percent of elongation compared to the samples solidified in graphite mold. Also, change of platelet iron-bearing intermetallic to the Chinese script phases at high cooling rates is an effective parameter in reducing brittleness of the alloy. Secondary dendrite arm spacing: Depending of the rate of cooling a different result of the dendrite structure is achieved. SDAS is the distance between these dendrites. With a high solidifications rate the result will be a short SDAS which provides o fine eutectic structure.

RESULT : In the present studies we aimed at enhancement of mechanical properties of A356 alloy by addition of copper. From the properties it is found that the alloy with this addition is suitable for industrial applications such as cylinder head, Brake calipers and Wing structures due to their excellent combination of properties.  In nil copper content alloy, the size of eutectic silicon particles alone have been reduced. The morphology of the microstructure changed after T6 heat treatment.  Silicon needles vanish and highly refined grains are observed. Also, the elongation of the grains is clearly observed in the direction perpendicular to the force of forging.  In 0.5% copper content, increase the number of Si particles size especially in spherical form leads to the improvement in the mechanical properties, First, copper causes ternary eutectic reaction at about 525°C.  The copper content in the eutectic melt is high which increases volumetric shrinkage during solidification and porosities of the sample. Second, hydrogen activity coefficient decreases with increasing copper content and hydrogen solubility decreases. Therefore, porosity forms rapidly during solidification in the alloys containing copper.  In 1.5% copper content, it is clearly observed at micro level the forged structures are more refined and clear precipitation at the grain boundaries with alloy as ternary intermetallic particles.  Forged microstructure becomes finer by decreasing dendritic arm spacing. This would improve the tensile properties of the parts. On the other hand, the size of silicon particles and secondary phases become

smaller. A fine Al2Cu precipitate caused by heat treatment throughout the grain boundaries, both ultimate tensile strength and percent of elongation increased.

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CHAPTER 7

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