Effect of pouring temperature on A356-TiB₂ MMCs cast in sand and permanent moulds by in-situ method

2.1 Fabrication of composite

Commercial A-356 was used for the production of Al based MMCs by in-situ technique. The stoichiometric mixture of prebaked potassium hexa flouro titanate (K₂TiF₆) and potassium tetra flouro borate (KBF₄) was added to molten metal for 6 wt% TiB₂. Fluxes melted immediately to form a molten layer on the molten metal surface. The stirring of molten metal was continuously up to 30 min. K₂TiF₆ and KBF₄ salts were added into molten Al resulting in exothermic reaction to form in-situ TiB₂ particulates in Al. The in-situ formed A356-TiB₂ MMCs were cast in both sand and permanent moulds at different pouring temperatures of 780°C, 790°C, 800°C, 810°C and 820°C [10].

2.2 Evaluation of cast composite

Specimens from cast components of A356-6 wt% TiB₂ composite were examined using optical microscope, SEM and XRD techniques. The fracture toughness specimens were pre cracked in accordance with ASTM E399 to provide a sharpened crack of adequate size and straightness. Fracture toughness test was carried out in the Instron 8801 testing machine. Tensile properties of the composite are evaluated using ASTM standards E8-03 on standard 12 mm diameter specimen. Tensile tests were also carried out at UTM servo hydraulic testing machine. The hardness tests were carried out by Brinell hardness testing machine.

3.1 Microstructure analysis

The optical micrograph of sand and permanent mould Al/TiB₂ MMCs is prepared using optical microscopy. Figure 1 shows a typical microstructure of the TiB₂ particles were found to be distributed along the grain boundary during solidification still existed, despite the formation of TiB₂ in the melt and very minimal agglomeration of the particles was observed [11]. In the case of Al/TiB₂ MMCs cast in permanent moulds due to sudden cooling fine grained structure are seen as found in optical micrograph (Figure 1A and B). In the case of Al/TiB₂ MMCs cast in sand moulds due to slow cooling coarse grains are obtained as seen optical micrograph (Figure 1C and D).

Figure 1:

(A and B) Optical micrographs of A356/TiB₂ permanent mould, A356/TiB₂ sand mould composite and closed grain structure refinement (C) and (D).

Figure 2A shows the SEM micrograph of permanent mould which shows the presence of TiB₂ particles and its distribution in the aluminium metal matrix. The morphology of the TiB₂ particles is typically hexagonal or nearly spherical and there are clear interfaces between particles and matrix. It can be seen in the micrograph of TiB₂ content some TiB₂ particles distributed evenly also some small particle agglomerate to form some particle clusters [12]. The TiB₂ particles are visible and the average size of particles is 1.5–2.0 μm, sudden cooling fine grained structures are seen as found. The permanent mould is high temperature and fluidity of molten metal are also high then the TiB₂ particles will disintegrate into smaller sizes (1.0–1.5 μm) observed. The more number of TiB₂ particles will get trapped in the freezing metal because more amount of cooling MMC is circulated in that condition.

Figure 2:

SEM micrograph shows Al/TiB₂ prepared 820°C at permanent mould (A) TiB₂ evenly distributed in matrix, and (B) interface of the reinforcing TiB₂ particle hexagonal shape l in matrix.

Figure 3A and B shows the micrographs of sand mould which shows the presence of TiB₂ particles and its distribution in the aluminium metal matrix. It can be seen in the micrograph of TiB₂ content the particles are evenly distributed. The TiB₂ particles are visible and the average size of particles is 2.0–2.5 μm. These strong interfaces between the particle and matrix but slow cooling coarse grains are obtained. Hence in sand mould which is lowest corner of the ingot more molten metal will get circulated molten metal will get trapped [13]. Dispersion of reinforcing particles and assume a needle shape and flaky form. It is seen that the presence of Al₃Ti hinders the uniform distribution of the reinforcement. The size of TiB₂ particles will be more for higher pouring temperatures at the same time the cooling rate is also low the TiB₂ will disintegrate into larger sizes. Hence the size of the TiB₂ particles, 2–2.5 μm as observed.

Figure 3:

SEM micrograph shows Al/TiB₂ prepared 800°C at sand mould (A) TiB₂ evenly distributed in matrix, and (B) interface of the reinforcing TiB₂ particle with the soft metal matrix.

The XRD patterns of the in-situ composites TiB₂ particles have formed as evidenced by XRD both permanent and sand mould with different conditions as shown in (Figure 4A and B). The peak intensity is increasing with increasing amount of TiB₂ particles. Apart from the Al and TiB₂ peaks, the XRD patterns also show peaks corresponding to Al₃Ti (θ), as expected from the phase diagram. After pouring as the cooling rate is low the TiB₂ particles grow in size affecting the mechanical properties. The effect of pouring temperature exclusively can be understood by analyzing the percentage of particles. At lower pouring temperature of 750°C at 50% TiB₂ content is found the fluidity will be less [14]. The pouring temperature of 810°C the TiB₂ content is maximum at 70–80% where the fluidity are maximum and first to be filled in the mould.

Figure 4:

(A) XRD of extracted second phase particles boundaries of A356/TiB₂ permanent mould, (B) A356/TiB₂ sand mould.

3.2 Mechanical properties of Al/TiB₂ MMCs

Figure 5 shows the comparison of UTS bar graph values with permanent mould and sand mould conditioned samples. In permanent mould casting have more UTS with higher temperature as the cooling rate is high and TiB₂ particles formed is are finer [15]. The Brinell hardness test results are shown in Figure 6A. In permanent mould condition as the temperature increases hardness also increases in range tested. In sand mould condition fissure cracks were formed due to Al₃Ti formation and it affect the hardness of Al/TiB₂ composites at temperature above 800°C. The fracture toughness of sand mould and permanent mould were improved considerably by in-situ TiB₂ reinforcement. The fracture toughness improvement was more in permanent mould as compared to sand mould composite as shown in Figure 6B.

Figure 5:

Effect of processing temperature on UTS of Al/TiB₂ MMC on sand and permanent mould.

Figure 6:

(A) Comparison of hardness in different mould condition with different temperature of Al/TiB₂ MMCs, (B) Comparison of fracture toughness in different mould condition with different temperature of Al/TiB₂ MMCs.