Microstructure and Mechanical Properties of Aluminum Cast Alloy A356 reinforced with Dual-Size B4C Particles

The paper details the studies carried out on a dual-size particulate composite prepared by stir casting using A 356 aluminum alloy and B4C powders. Three composite compositions, viz., A356 plus 2% B4C (44µm size and 105µm size in 1:1 ratio), 4% B4C (3:1 ratio), and 6% B4C (1:3 ratio) were cast in finger molds, from which test specimens were prepared for hardness and tensile tests as well as for metallography. Vickers hardness tests, tensile tests and microstructure analysis using an optical microscope were conducted. The results obtained indicated that the B4C particles were evenly distributed in the alloy matrix. EDS also revealed the presence of B4C in all the three composites.  In general, the hardness and tensile strengths increased with increase in concentration B4C powders. While the increase in hardness was increases less than 15%, there was significant increase (more than 35 %) in tensile strength. However, the ductility represented by % elongation, which was already very low in A 356 cast alloy (24.2%,), further decreased in composites. Tensile fractography results shows inter crystalline fracture where the breakage in the B4C particle instead of deboning were observed.


INTRODUCTION
luminum alloys find application in diverse fields owing to their high strength-to-weight ratio, low cost, and ease of fabrication [1][2][3]. Substituting the base aluminum alloys with particulate composites has the advantage of improving the mechanical strength parameters without seriously disturbing the thermal and electrical conductivities and other properties A [4][5][6][7]. The common reinforcement particles used in AMMCs are graphite, SiO 2 , SiC, Al 2 O 3 and B 4 C used in small percentages of about 2 to 8% [8][9][10][11][12]. Some researchers have used hybrid reinforcement, i.e., using two types of particulates [13][14], while some others have employed dual or treble sizes of same particulate [15][16]. The automaker Honda uses a hybrid AMMC for some of its engine blocks [17]. Of the reinforcement particulates available, B 4 C is perhaps the hardest. Hence an AMMC with B 4 C particles may be expected to have higher strengths than other AMMCs. The work presented here, which is part of a larger comprehensive study, is based on aluminum alloy particulate composites with B 4 C particles. In this study, two sizes of B 4 C particles (44 µm, and 105 µm) in different ratios were employed to obtain three AMMCs in the as-cast condition. These were evaluated for their microstructural features and mechanical properties using standard testing procedures.

MATERIALS AND METHODS
he cast aluminum alloys A 356 was selected for the study. The chemical composition of the alloy as tested and as required by standard is given in Tab. 1. Two sizes of B4C powder, one fine and the other coarse, viz., 44 µm and 105µm were procured from reputed suppliers. The chemical assay of B 4 C powders is given in Tab   T

Preparation of materials
The AMMCs were prepared by the well-established stir casting method. Pieces of A 356 alloy cut from ingots were placed in a graphite crucible and melted in an electric resistance furnace (schematic diagram of stir casting is shown in Fig.1). The melt was degassed using hexachloromethane tablet, and held at above 800 0 C, and stirring started. When a nice vortex was formed, pre-weighed quantities of B4C powders were introduced, and stirring continued. After about three minutes, the melt was skimmed and the liquid metal poured into preheated finger molds. The three compositions of the AMMC, viz., 2% B 4 C (44µ size and 105µ size in 1:1 ratio), 4% B 4 C (3:1 ratio), and 6% B 4 C (1:3 ratio), as shown in Tab. 4, were prepared as detailed above. From these cast fingers, test specimens for hardness, microstructure and tensile tests were machined as per ASTM standards   Table 4: Material composition with different ratios of fine and coarse size B4C particle.

Microstructural examination
The test specimens were polished to 6-0 smoothness and etched with Keller's Reagent. An Optical Microscope (Model NIKON LV-150, Fig.2) was used for the purpose.

Hardness tests
Hardness tests were performed on specimens polished to 5-0 smoothness with a Vickers Micro Hardness Tester (Model VHM-102, Fig. 3(a)). The applied load was 100 g for 10 s. Fig. 3(b) shows the schematic of hardness specimen.

Tensile tests
The tensile test specimens were of 6.35 mm diameter and gauge length of 26 mm, conforming to ASTM E8 Standard. The tensile tests were accomplished using a Hounsfield-type testing machine (Model TUE-C-400, Fig. 4(a)) at a strain rate of 0.2 mm/s, at room temperature. Fig 4(b) shows Schematic of Tensile specimen.

RESULTS AND DISCUSSIONS
Microstructure he photomicrographs of the as-cast structures of base A 356 alloy, and AMMCs with A 356 and 2% B 4 C, 4% B 4 C, and 6% B 4 C respectively is shown in the Fig.5.(a to d), From Fig. 5(a), it is seen that the cast structure of the base A 356 alloy is dendritic or semi-dendritic. The Al-Si eutectic phase forms the dendritic structure with in a matrix of primary α-Al phase. Figs. 5 (b), (c)and (d), being the microstructures of cast 2%B 4 C, 4% B 4 C, and 6% B 4 C respectively, indicate that the structures of all three cat composites are similar. There is nearly uniform distribution of B 4 C particles in all three composite castings. This is true for the fine B4C particles, as well as for the coarse B4C particles. The microstructural examination reveals simple primary α-Al phase as matrix in which there are dendrites of the intermetallic Al-Si phase. In the composites, there is distribution of the B 4 C particles, which are mostly spheroidal in shape. [18]. investigation. The graphical depiction of EDS analysis of A356 aluminum alloy, A356+2% B4C, A356+4% B4C and A356+6% B 4 C composites is shown in Figs. 6(a to d) respectively. From Fig. 6 (a) is the EDS spectrograph of A356 aluminum alloy. The spectrum confirms the existence of aluminum as the uppermost element followed by silicon and magnesium alloys. From Fig. 6(b to d) is the EDS spectrograph of A356 and B 4 C composites spectrum which evidence the existence of boron (B), carbide (C), in the carbon form. Figure 6: (a) shows the EDS spectrum of A356 alloy    Fig. 7 is a bar graph of the hardness values of the A 356 alloy and the three composites, hardness being measured in VHN. It is seen from Fig.7 that while the composites seem to have higher hardness than the base A 356 alloy, the increase in hardness is being 6%, 3.4% and 14.14% respectively for the composites with 2%, 4% and 6% B 4 C. Among the Dual size particle composites, the composite with 6%B 4 C of the ratio 1fine and 3coarse size particles shows marginally higher hardness. This may be due to the higher ratio of coarse size B4C particle. The higher amount of coarse particle in AMMC shields the finer particles, additionally coarse particles helps to carry a extra portion of the applied load in comparison with the finer size particles [19]. The composite with 4% B4C of the ratio 3 fine: 1coarse particle particles shows slightly lower hardness. This may be due to the higher amount of fine size B4C particle. The higher amount of the fine size is more prone to the particle clustering hence the dispersion of fine size particle is very difficult [20][21].  Fig. 8 plots the UTS of the base alloy and three composites in their as-cast condition. While there is no significant increase in UTS by adding 2% or 4% B 4 C (increase in UTS by 3.8% and 7.5% respectively), there is a significant increase achieved by adding 6% B 4 C with the ratio of 1fine and 3coarse size. (an increase of 38.8%). The B 4 C ceramic particle acts as a barrier against the plastic deformation in the A356 matrix material to counter the tensile load. The composite with 6% B 4 C of the ratio 1fine:3 coarse size particle exhibits higher UTS this may be due to the increase in the wt% of B 4 C reinforced particle [22][23]. and also a significant increase due to the higher ratio of coarse size B 4 C particle in the composites. Higher ratio of coarse size B4C particle acts as an effective stress carrier between reinforcement and matrix material. [24]  Ductility (% of Elongation) Fig. 9 is a plot showing the ductility (represented by % Elongation) of the Base A356 alloy and the three composites. It is evident from Fig. 7 that there is a significant loss in ductility in composites as compared with the A356 alloy. The loss in ductility, as calculated by the reduction in % Elongation is found to be 24.2%, 27.1%, and 28.6% respectively for the 2%, 4%, and 6% B 4 C composites. It is further evident that the % Elongation decreases as the B 4 C particulate quantity increases [25] irrespective of the sizes of the B4C particle. (though initially there is a drastic drop in % Elongation from the Base A 356 alloy and 2% B4C.this is due to the presence of needles like of silicon in the α-Al Matrix (silicon is the second highest constituent of A356 alloy) is responsible for the reduction in ductility [26][27]. The reason is that the crack nucleation at the inter-dendritic region and propagation along these dendrites is the primary mechanism for failure of A356 alloys. all the A356 composites exhibited brittle failure along with quasi-cleavage feature, as shown in Fig.  10 (b to d). the micro cracks and quasi-cleavage feature is common feature of materials with lower elongation. This may be due to the detachment of coarse size B4C Particle. A small dimple was also absorbed on fracture area of the composite, fracture of coarse size B 4 C causes pullout due to the decrease in ductility as compared to the A356 alloy. From all this observation it is considered that as the load increase B 4 C particle of both size, breaks instead of debonding hence the nature of fracture in the composites is brittle & quasi cleavage type. [28][29].