Investigation on machinability in turning of as-cast and T6 heat-treated Al-(3,7,12%)Si-0.6%Mg alloys

The machinability in turning of Al-Si-Mg alloys under as-cast and heat-treated conditions has been investigated. Three alloys with 3%, 7% and 12%Si and 0.6%Mg contents (wt%) were prepared. Unsteady solidified ingots were obtained in a vertical upward directional solidification apparatus using an instrumented cylindrical water-cooled mold. Samples extracted from the ingots were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) examinations and by Brinell hardness tests, before and after T6 heat treatments (540 degrees C solutioning temperature for 10 h, water quenching at 25 degrees C and artificial aging at 155 degrees C for 5 h). Specimens in both the as-cast and heat-treated conditions were subjected to machinability tests as a function of cutting force in face turning operation using a CNC lathe machine, fixing parameters as cutting depth, feed rate, cutting speed, and cutting tool. The as-cast microstructure consists of a dendritic alpha-Al rich phase and an interdendritic eutectic microconstituent (alpha-Al + Si particles). As the alloy Si content increased, the solidification cooling rate increased, the amount and size of alpha-Al dendrites decreased while the amount of eutectic mixture increased. After heat treatment, Si particles were found to be more rounded and an increase in fraction of Mg2Si was observed. The hardness profile declines along the length of the as-cast ingots, however the values improved with both alloy Si content and heat treatment. The as-cast alloys showed maximum cutting force at the beginning of the process, decreasing during machining progress, whereas in the heat-treated condition the cutting forces showed values almost constant. The results suggest a more stable machinability behavior during turning of the heat-treated alloys as a consequence of a more homogenous microstructure and hardness.

Automated summary

The machinability of Al-Si-Mg alloys under as-cast and heat-treated conditions was investigated. The results showed that the heat-treated alloys exhibited more stable machinability behavior due to improved microstructure and hardness.