Effect of lanthanum addition on microstructure and hardness of brass alloys produced by rheological squeeze casting

The effect of La addition (0–0.30 wt%) on the microstructure and hardness of rheological squeeze casting brass alloys was experimentally investigated. The rheological squeeze casting process is improved by controlling the wall surface crystals and melt flow rate to realise the preparation of semi-solid melt with flow, and a brass alloy workpiece with La is produced. The microstructure and properties of the brass alloy samples were investigated using metallography, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and hardness testing. The results indicate that the hardness of the rheological squeeze casting brass alloy is increased by 20.4% from 108 to 130 HBW with an increase in the La content from 0 to 0.30 wt%. The microstructural analysis results show that La significantly refines the primary α-phase grains, and the main mechanism is the constitutional undercooling and heterogeneous nucleation caused by the La enrichment in the front of the solid–liquid interface. The squeeze pressure promotes undercooling, which improves the nucleation rate and affects the solute diffusion and nucleus growth. The dual effects of these two aspects aggravate the grain refinement process, consequently increasing the number of grain boundaries and improving the hardness of the brass alloy.