The effect of powder addition manner and volume fraction of reinforcement on tribological behavior of Al7075/B4C surface composite produced by friction stir processing

Al7075/B4C surface composites were fabricated by friction stir processing using four passes. The B4C powders were added into the prepared grooves with 1 and 2 mm width at the surface of Al7075 alloy in two different manners. In the first one, the B4C powders were added prior to the four consecutive passes in one stage. In the second manner, the powders were added in two stages, prior to the first pass and following the second one. The microstructural evaluations showed that the increase in the volume fraction of reinforcement significantly reduced the matrix grain size. Meanwhile, the reinforcement had a more homogeneous and uniform distribution in the samples processed through four consecutive passes. The maximum hardness and wear resistance was achieved in the one-stage powder-added samples, containing higher volume fraction of the reinforcement. A direct relationship was observed between the wear resistance and the composite layer hardness. The wear mechanism in the Al7075 substrate and the non-reinforced friction stir processed sample was the delamination of unstable aluminum oxide tribolayer. However, in the composite samples, a mechanically mixed layer, containing aluminum, chromium, and iron mixed oxides was formed along with B4C on the worn surface. In the two-stage powder-added samples, containing lower amounts of reinforcement, the detachment of mechanically mixed layer resulted in three-body abrasive wear condition and high friction coefficient. However, the most stable mechanically mixed layer was formed on the surface of the one-stage powder-added sample containing higher amounts of B4C.

Effects of MoS2 on Tribological Properties and Mechanically Mixed Layer of Al Matrix Composites

Al matrix composites containing MoS2 at weight fractions in the range of 0%, 3%, 10%, were fabricated by hot press method, respectively. The effect of MoS2 on tribological properties and mechanically mixed layer of Al matrix composites were investigated on a pin-on-disc tester at load of 20~50N and sliding speed of 0.23~0.69m/s. Worn surfaces and mechanically mixed layer of composites were characterized by scanning electron microscopy(SEM). Results indicate that the hardness and compression strength of composite with 3% content of MoS2 has a little higher than that of composite with 0% content of MoS2, but the hardness and compression strength of composite with 10% content of MoS2 were dropped sharply. With the increase of MoS2 content, the composite with 10%MoS2 has a low and stable coefficient at load of 30~50N. The wear rate of composite with 10%MoS2 at load of 50N and sliding speed of 0.69m/s has a sharp increase and there wasn’t mechanically mixed layer(MML) present; the composites at load of 30N and sliding speed of 0.69m/s have low and stable wear rates and there were MML present with wavy shape across entire wear track.

Laser Developed Al-Cr Surface Alloys: Microstructure, Mechanical and Wear Behaviour

Surface alloys with composition ranging from 10 to 20% Cr were produced by laser surface alloying. Their microstructure consists of faceted plate-like Al4Cr intermetallic compound particles dispersed in a matrix of α-Al solid solution. During remelting, heterogeneous nucleation of eutectic Al7Cr/α-Al occurred in the undercooled liquid ahead of the columnar solid-liquid interface, followed by equiaxial solidification, resulting in a microstructure formed of equiaxed cells. Al-Cr alloys present Young’s modulus and hardness values that increase with increasing volume fraction of intermetallic compounds. Wear resistance, measured in dry sliding conditions, increases with increasing load due to the protective effect of a stable mechanically mixed layer that forms at the surface of the samples and the steel counterbody. Alloys formed of equiaxed eutectic cells provide better wear resistance than those formed of large plate-like particles since a thinner, more stable and harder mechanically mixed layer is formed, which offers best protection against wear.