The safety, performance, and minimal impact on costs [1].

The effect of Al-5Ti-1B master alloy addition on the microstructure, hardness and
mechanical properties of hypoeutectic Al-7.6Si alloy
Abstract:
In this investigation, hypoeutectic Al-7.6Si alloy with (2.0 wt.%, 4.0 wt.% and 6.0 wt.% ) and without Al-5Ti-
1B master alloy addition has been developed through gravity casting method. The effects of the Al-5Ti-1B grain
refiner on the microstructural morphology, hardness, mechanical properties and fracture behaviour of the Al-
7.6Si alloy have been studied. The cast unmodified hypoeutectic Al-7.6 Si alloy consisting of needle and rodlike
eutectic Si particles with very sharp corners inside the ?-Al phase. The addition of Al-5Ti-1B master alloy
in the Al-7.6 Si alloy, TiB2 is formed and these TiB2 are acting as a potential site for nucleation of ?-Al grains.
Due to this, the grain refined alloys have globular ?-Al grains and a fibrous eutectic Si phase. The addition of 4
wt.% Al-5Ti-1B in the alloy, the average diameter of ?-Al grains decrease to 42.5 ?m from 14.19 ?m (as cast
Al-7.6Si alloy) and roundness of ?-Al grains increases to 0.696 from 0.33 (as cast Al-7.6Si alloy). The bulk
hardness, ultimate tensile strength (UTS) and elongation (%) of the modified alloy are increased. Further,
factographs showed that the cleavage fracture is reduced in the modified alloy and fine dimple formation is
increased.
Keywords: Al-7.6Si alloy; Al-5Ti-1B grain refiner; microstructure evolution; hardness; mechanical properties;
Fracture Behaviour
1. INTRODUCTION
Nowadays, it is gradually becoming very important for automotive industries to increase energy efficiency to
reduce fuel consumption and pollution. Environmental concerns, government-mandated fuel efficiency
standards, raw-material constraints and global competition are driving the automotive industry to reduce fuel
consumption while maintaining safety, performance, and minimal impact on costs 1. Switching to light-weight
materials, without sacrificing safety and performance, is being looked at as the most cost-effective way to
address these challenges. As a result, more emphasis is being laid on the increase in production and use of
aluminium silicon cast alloys. The Al-Si alloys are widely used because of their excellent properties such as
high fluidity, high specific strength, good wear resistance, good corrosion resistance, low thermal expansion,
high recyclability and low cost of manufacturing 1,2. Some of the typical applications include cylinder heads,
aircraft stabilizer, and crankcase for small engines, cellular phone castings and domestic food components 3.
The microstructure of the hypoeutectic Al-Si alloys has coarse-columnar primary ?-Al phase and needle-like or
plate-like eutectic silicon phase 4. The Hypoeutectic Al-Si alloy contains a large volume fraction of primary
?-Al phase 5. Therefore, the shape, size and distribution of ?-Al in the microstructure directly influence the
mechanical properties 6 of the alloy. Thus, the grain refinement of the Al-Si alloy is needed to achieve the
fine-grained structures with superior mechanical properties. Previously, few researchers have investigated the
effects of varying ratios Ti and B on the Al-Si alloy 4,5,7-10. Recently, the effects of Al-Ti-B-RE master alloy
(RE: rare earth) addition on grain refinement was investigated in the Al-Si alloy. The plate-like or needle-like
morphology of eutectic silicon acts as sites of internal stresses development, which provide an easy path for
cracks to propagate resulting in fracture 4. Because of it, the strength and ductility of Al-Si alloy are not up to
mark 9. Therefore, the modification of eutectic silicon is carried out to convert the plate-like or needle-like
morphology to the fine fibrous structure with superior mechanical properties. Sodium (Na)12, strontium (Sr)
13 and antimony (Sb) 14 are used for this purpose. However, antimony is generally avoided as it leads to the
formation of toxic stibine gas (SbH3) in the foundry 15. Modification of eutectic silicon has also been
investigated using high-intensity ultrasonic vibration 16.
Previously, some works have been carried out on grain refinement of hypoeutectic (Si<12wt%) Al-Si alloys. In an earlier work, Kori et al. 8, investigated the effect of Al-5Ti-1B (up to 0.1wt%Ti) master alloy on Al-7Si alloy. It was reported that the effect of the Al-5Ti-1B master alloy was poor with respect to boron based master alloy due to the poisoning effect of Si. The poisoning effect of Si is attributed to its affinity with Ti where it forms titanium silicide, which coats the nucleating agent in the master alloy rendering them ineffective 17. However, study the effect of Al-5Ti-1B addition on microstructure, hardness, mechanical properties and fracture behaviour is not presently available in existing standard literature. Kori et al. 5 studied the poisoning effect of Al-5Ti-1B master alloy addition on Al-7Si alloy. It was concluded that the poisoning effect is less at an optimum addition of 0.024wt.% Ti and it was substantiated by measuring the dendritic arm spacing values. Still, there is no study on mechanical properties. Lee et al. 7 investigated the effect of Al-5Ti-1B master alloy addition on Al-8Si alloy with titanium additions up to 0.15wt%. It was observed that there is no significant effect of Ti on grain refinement beyond 0.05wt% Ti in the alloy. In this research, hypoeutectic Al-7.6 Si alloy has been successfully developed through gravity casting route with (2 wt.%, 4 wt.% and 6wt.%) and without Al-5Ti-1B master alloy. The main objective of the study is to investigate the effect of higher concentration Al-5Ti-1B master alloy addition on mechanical properties of the hypoeutectic Al-7.6Si alloy. Finally, a structure-property correlation is established to understand the various aspects of properties enhancement. 2. EXPERIMENT The Hypoeutectic Al-7.6Si alloy was developed through melting and gravity casting routes. Initially, small pieces of commercially pure Al (99.7%) were placed inside a clay graphite SiC crucible and melted using an electrical resistance furnace at 7600C. After melting of commercially pure Al, small pieces of pure silicon (99.16%) was inserted into the melt with the help of graphite rod and wait for 45 minutes for complete Si dissolution. After homogenization, the Al-5Ti-1B master alloy was added to the molten mixture. Then wait for 20 minutes and then the molten mixture was degassed with hexachloroethane (0.1 wt.%). After that, slag was removed from the top portion of the melt and it was immediately poured into a cast iron mould, designed as per BS1490 standard 18, 19. After solidification, samples of 10 mm × 10 mm were machined out from the as-cast billets for optical microscopy and hardness test. The metallography samples were polished through the standard procedure of aluminium alloy and etched with Keller's reagent (1% HF, 1.5% HCl, 2.5% HNO3 and distilled water). The optical microscopy was carried out by Leica optical microscope. Hardness test was carried out on the Vicker's hardness testing machine. PANalytical X'PERT PRO machine using Cu-K? radiation was used for XRD analysis. Further, tensile test specimens were machined out from the cast billet as per ASTM standard. Tensile tests were carried out on Tinius Olsen Universal Testing Machine under ambient conditions. Fractured surfaces of the modified and unmodified alloy were investigated under FESEM (Carl Zeiss, Sigma, UK) to understand the mode of fracture. The volume percentage (serologically equal area percentage) of different phases and size of the primary ?-Al were measured using the ImageJ image analysis software. 3. RESULTS AND DISCUSSION 3.1. Microstructural evaluation The optical microstructure of the Al-7.6Si alloy with and without Al-5Ti-1B master alloy addition is shown in Figure 1(a)-(d). The unmodified hypoeutectic Al-7.6Si alloy (Figure 1(a)) exhibits the presence of gray colour combined dendritic plate-like and needle-like particles with very sharp corners inside the ?-Al phase. This gray colour phase was identified as eutectic Si phase by the EDS spot analysis (Fig. 2). The eutectic Si is evenly distributed within the primary ?-Al phase and the ?-Al phase is exhibit like a matrix.