Menghani, Jyoti1; Bhushan, Bharat1; Singh, Balraj1; Suthat, Deepak1; Shah, Dimple1 SVNIT Surat,India
Aluminum, magnesium alloys and plastics have received growing attentions as light-weight materials for automobiles Al alloys with Si as major alloying addition are one of the most effective way to obtain good Al casting mainly because of high fluidity imparted by presence of relatively large volume of Al-Si eutectic. A356(al-7Si-0.3Mg) is hypoeutectic heat treatable Al-Si alloy. Presence of Mg in Al-Si alloy makes it heat treatable.
Light weight, Excellent weldability, Corrosion resistance, Good mechanical strength, Ductility, Hardness, Fatigue strength, Pressure tightness, High fluidity and Good machinability. Good plasticity, High durability limit, Recyclability
Widely used for automotive industry,( for example for cylinder heads and engine blocks) Aerospace parts Military parts
The microstructure and hence Mechanical properties of A356 casting alloys consists of primary aluminium dendrites, eutectic Si, intermetallic compounds and solidifications defects, such as gas and shrinkage porosity and oxides. The volume fraction of dendrites and eutectic particles is determined by chemistry, whereas the size, morphology and distribution of the dendrites, eutectic particles and intermetallics depend on the local solidification conditions
On-going research has sought to enhance the mechanical and wear resistance properties of Al Si alloys by controlling their microstructures via Suitable casting procedures, Heat treatment Addition of minor alloying elements Grain Refinement Modification
Two main processes to give better structure and mechanical properties are : (i) The addition of alloying elements during melting and melt treatment of the liquid alloy through Grain refining and Modification, and/or (ii) Heat-treatment.
Slow solidification of pure Al-Si-Mg produces a very coarse eutectic microstructure consisting of large plates or needles of silicon in aluminium matrix. The silicon particles appear to be interconnected. The coarse eutectic results in low ductility because of brittle nature of large silicon plate. Most of the mechanical properties of castings are determined by the eutectic microstructure.
Al-Si Eutectic phase diagram Al-Si Eutectic phase diagram after Na addition
Conversion of coarse acicular Si needle to fine fibrous structure is called Modification. Modification of Al-Si-Mg alloys is commonly done as it results in refinement of microstructure, by adding MODIFYING AGENT to the melt prior to pouring. Sodium and Strontium are most widely used as a modifiers. Main drawback is Sodium is rapidly lost in the molten Aluminium through its high vapor pressure so that modifying effects are transient. Periodic additions are required to maintain modification levels. Sodium modified alloys are also prone to gas pick up and micro shrinkage.
Strontium additions as Al-Sr master alloy when added retains the modified effects even on remelting. However Sr forms undesirable Fe-Sr intermetallic. In addition Sr shows fading effect (Reduction in effectiveness of modifier on holding modifier in melt for longer time). Rare earth in form of Misch metal (Ce-55%, Nd15%,La-15%,others)increases under cooling which suppresses growth of Si and fading effect seemed much less.
There are two principles through which there is improvement in Mechanical properties: (1)Elements that satisfies modification criterion as proposed by Lu and Hellawel. (2)Elements having atomic size similar to Al.
When Al-Si eutectic solidifies it has been shown that modifying elements become concentrated in the silicon phase rather than Aluminium i.e. a Constitutional effect will be present at the solidification front during solidification. Silicon is semi metal and solidifies in faceted manner. Twins are easily formed in silicon crystals on <111>plane. Atoms of the modifier are absorbed onto the growth steps of Si solid interface.
A growth twin is created at the interface when the atomic radius of the element relative to silicon size exceeds 1.65. This multiple twinning may result in the growth directions of primary silicon to change from anisotropic growth to almost isotropic growth The coarse plate like growth was suppressed, coarse primary silicon in hypoeutectic Al Si-Mg alloy could be modified.
A schematic illustrating primary Si growth in Al-Si alloy (a) Anisotropic growth in unmodified alloy (b) Isotropic growth in modified hypoeutectic Al-Si due to impurity induced twinning.
Element Atomic Radius Ratio r/rsi Modifying effect Sr 2.16 1.84 + Na 1.97 1.68 ++ Ca 1.87 1.59 + La 1.87 1.59 + Ce 1.76 1.56 + Si 1.755 1.00 +
Stirring treatment of molten metal has significant effects on the solidification microstructure of A356 alloy, which includes grain structure, distribution of inclusions, refinement of secondary phases, etc.
The most important aim of tribology science is the precise determination of the nature of the dependence between the surface interaction behavior and the fundamental properties, which themselves are related to microstructural features. Wear of a material is controlled by 1.Material characteristics including metallurgical and mechanical properties of alloy 2.Operating parameters such as applied pressure, sliding speed, environment and the type of sliding interaction. Coarse and needle-shaped eutectic and large primary silicon particles increase adhesive wear
The grain morphology of various phases in Al Si alloys influences the wear and mechanical behavior. In general, fine, spherical and uniformly distributed micro-constituents can improve the wear and mechanical properties. Thus wear behavior of polyphase alloys is complex and not simple function of composition
Considering A356 (Al 7Si 0.3 Mg),an important alloy for automobile industry, particularly for wheel production effect of Rare earth (R.E) and Stirring on A356 in terms of refinement of microstructure, Wear and hardness was undertaken. In present study, it was considered that R.E addition contributes not only as modifier but acts as alloying element
In this work systematic studies on effect of individual or combined addition of Rare earth and stirring on eutectic Si and primary Al solidification of A356 was performed. The relationship between mechanical properties and microstructure are also discussed in order to improve mechanical properties and microstructure of alloy and help enterprises enhance the quality of product.
Fresh ingots of A356 of about 600 gms weight were melted in crucible type electric resistance furnace. Melting was carried out under cover flux. Lime coated clean iron tools were used. Rare earth (0.5wt%, 1.5wt%) is added in form of pure metal wrapped in Al foil was added to the melt. Stirrer was attached to vertical drilling machine and stirring was carried out for about five minutes at varying stirring speed of 400 and 600 rpm. Increasing stirring time beyond 5 minutes resulted in solidification of molten alloy.
Metallography Metallographic samples were prepared as per normal procedure and etched in 0.5% HF solution. Optical Microscopy was used to see Microstructure. Wear surface was observed at various magnification in SEM (Hitachi 3400). XRD Observation: The alloyed and unalloyed LM25 have been characterized by X ray diffraction(xrd) using Cu kα radiation in as cast and heat treated conditions.
Wear tests were carried out,using a Pin-On-Disctypewear-testingmachine(TR-20,DUCOM)as per ASTMG99 90 The normal load/pressure was applied on pin by dead weight through a pulley string arrangement. The system had maximum loading capacity of 200N. Disc was rotated by DC motor, having a speed range of 0 2000 rpm to yield sliding speed of 0 10m/s.
Alloy were machined to produce wear pin of size Wear test specimens are rounded bars with flat surface having dimensions of 20mm length 10mm diameter. The flat portion of 10mm diameter of the test specimen was in contact with a rotating disc. One end of wear pin was polished to 0.5μ. Wear pin was held against counter surface of EN24 hardness Rc 63.
Tribological characterization was done by using Pin on disc wear testing machine with the process parameters selected were 45N load 240rpm and for distance of 6m The wear test was carried out at temp of 40C SEM of wear surfaces was carried out to study the mode of wear. Wear behavior of samples(a356,a356 with0.5 wt%r.e and 1.5wt%R.E)with and without stirring was studied using pin-on flat type wear testing unit (DUCOM pin on disc wear tester).
The microstructure of unalloyed A356 with stirring and without stirring indicates that even though rare earth was not added the morphology of eutectic Si is altered due to the stirring effect. The stirring has direct effect on grain refinement and indirect effect on modification, morphology of eutectic structure.
I(A)0wt%R.E without Stirring I(B)0 wt %R.E Stirring at 400 rpm I 0 wt %R.E Stirring at 600 rpm II(A)0.5wt%R.E without Stirring II(B)0.5 wt %R.E Stirring at 400 II 0.5wt %R.E Stirring at 600 rpm
III(A)1.5wt%R.E without Stirring III(B)1.5 wt %R.E Stirring at 400 rpm III 1.5wt %R.E Stirring at 600 rpm
The as cast specimen clearly shows the large and steep silicon needles present in the microstructure of A356 base alloy. It decreases the tensile strength of the material. After stirring the base alloy the specimen produced by casting is seen with some blunt and small needles which consequently increase its properties up to a certain level. The modifiers produce the blunt and somewhat globular needles which can be seen in the image above. This type of grain structure is considered to be great in terms of strength analysis
The stirring has direct effect on grain refinement and indirect effect on modification, morphology of eutectic structure. It can be seen from microstructure that the case where metal is stirred the volume of interdendritic liquid between primary islands is reduced. This physical constriction put constraints on the size of Silicon needles upon eutectic transformation. The effect of varying amount of rare earth (0.5 wt% and 1.5 wt %) on as cast microstructure and microstructure obtained after stirring is shown. Rare earth addition seems to modify structure appreciably in both with and without stirring. However addition of 1.5 wt% rare earth results in recoarsening of Si needles and new phases may have formed.
A typical XRD diffractogram for 0.5wt% rare earth inoculated A356 with stirring of 400 rpm is shown in Figure The search done by XRD software doesn t give any lanthanum-aluminium based compounds. Following compounds are found on the basis of test carried out: Al4Ce, CeMg2Si2, CeMg, MgAl2O4, AlCe3, MgO. It is the limitation of software used to not being able to identify the less quantity lanthanum based compound. If this is compared to unalloyed A356, then new rare earth based compound phases are observed which gives higher strength to alloy and cause discontinuous dendrites.
Fig (a) 0%R.E 400 rpm Fig (b) 0%R.E 600 rpm
Considering the case of unalloyed A356 and 400 rpm stirring on examination of the worn surfaces at higher magnification, (fig a) evidence of extensive plastic flow and cracking was observed. Cracks may initiate in the highly work-hardened layer, starting from the subsurface region. When cracks grow and get interconnected, a layer of metal is removed leading to delamination wear. Extent of deformation is more however cracks are less in case of unalloyed A356 and 600 rpm (fig (b))
Fig (c)0.5%r.e Without stirring Fig (d)1.5%r.e Without stirring
Considering case of Rare earth added A356, (fig c & d) observing the worn surface at high magnification evidence of extensive plastic flow was observed. However in fig (c) the hollow portion is seen. It may be blow hole or any other casting defect. In Fig (d) small porosities are observed. Also extent of work hardening of surface is observed. Wear debris were not observed because delaminated wear debris particles were severely work-hardened and fractured and they did not stick to the sliding surface and were removed.
The combined effect of stirring at varying speed and varying amount of rare eath (0.5wt% and 1.5wt%) on the wear behavior of A356 gravity castings has been studied. Based on the wear testing and metallographic examination conducted for the specimens, the following conclusions can be drawn: 1. Microstructural analysis shows that rare earth addition is able to refine grain size by reducing the dendrite size and produces more fibrous eutectic silicon phase. 2. Wear behaviour of A356 alloys mainly depend on the shape, type, size and size distribution of α-al grains and silicon particles in the matrix. 3. The A356 alloys investigated exhibit plastic deformation and work hardening during wear testing. A356+1.5wt.R.E cast alloys work-hardened to a greater extent than the unalloyed A356. 4. Wear occurs by plastic deformation and cracking of the matrix followed by delamination of flakes in case of unalloyed A356 at 600 rpm stirring
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