||Microstructures and elevated temperature hardness and wear behaviors in five hyper-eutectic Al-Si alloys, namely A390 (modified), BH390 (un-modified), BH390 (modified), BH390 + 5% Si (modified) and BH390 (die cast cylinder), are investigated in this study by using OM, SEM, GDOS, EPMA, HV, HRB and AFM. The wear tests were performed in a linearly reciprocating ball-on-plate sliding wear test apparatus in accordance to ASTM G-133 specifications.
Micro-hardness in the eutectic micro-constituents of these Al-Si alloys showed an increase by a T5 heat treatment. It was also shown that the eutectic micro-constituents in BH390 alloys (having slightly higher Mg and Zr contents than that of A390 alloy) are harder than that of A390 alloy. The room temperature HRB hardness test results also showed the BH390 alloys to be harder than the A390 alloy. It was noticed that the room temperature HRB hardness of the BH390 + 5%Si alloy (modified) is very close to that of the BH390 alloys. On the other hand, the high temperature hardness test (at 225oC) showed that the HRB hardness of the BH390 + 5%Si alloy (modified) is higher than the rest of Al-Si alloys. This indicates that the greater amount of primary Si particles in the BH390 + 5%Si alloy (modified) can effectively enhance its high temperature hardness.
The room temperature wear test results showed that the BH390 (die cast cylinder) to be the highest wear resistant alloy among the Al-Si alloys. The reason of that lies in the fact that the BH390 (die cast cylinder) specimen was chemically modified and chill casted (pressure die cast) to results in a microstructure with very fine primary Si particles (d≦18 μm) to resist wear from happening. As for high temperature wear test (at 225oC), the BH390 + 5%Si alloy (modified), having a greater amount of primary Si particles, exhibits the highest wear resistance among the Al-Si alloys.
When subjected to an etching treatment (using 10% NaOH for 45 s) the primary Si particles are exposed as islands above the eutectic micro-constituents matrix for about 0.3~0.5 μm in height. Since these exposed Si particles can effectively reduce the wear loss of the relatively softer eutectic micro-constituents matrix, the etched Al-Si alloys therefore exhibited improved wear resistance than those un-etched under test conditions at both room and elevated temperatures and with or without lubricant.