Effect of metallurgical parameters on the machinability of heat-treated 356 and 319 aluminum alloys

The present study was undertaken to investigate the effect of metallurgical parameters on the drilling performance of heat-treated 356 and 319 alloys. The most important metallurgical factors considered in the present study which determine the condition of the work material that can influence the outcome of the machinability are: chemistry and additions (Cu, Mg and alpha-Fe-intermetallic volume fractions), cooling rate and quenching rate, hardness (HB). Additions of Mg to 319 alloys and different heat treatments for 356 and 319 alloys were employed to obtain similar levels of hardness in both alloys. Conditions of Sr-modified (200-250 ppm) 356 and 319 alloys containing mainly alpha-Fe-intermetallics and related to different levels of hardness (90, 100 and 110 HB) were selected for the drilling study. The effects of Mg and alpha-Fe-intermetallic volume fractions on the machinability of heat-treated 319 alloys were studied at two levels of Mg (0.1 and 0.28%) and at two levels of alpha-Fe-intermetallic volume fractions (2 and 5%), respectively. The range of the hardness and Fe-intermetallic volume fractions used in this study conform to the most common levels observed in the commercial applications of these alloys. It was found that a higher Mg content results in a higher cutting force at the same level of hardness. This can be explained by the fact that a high volume fraction of Mg-intermetallics or precipitates can be formed within the alloy matrix in the high Mg-content 319 alloys compared to the low Mg-content ones. The low Cu content in 356 alloy resulted in a higher cutting force compared to 319 alloys exhibiting the same level of hardness. This may be explained by the improvement in the homogeneity of the alloy matrix hardness in 319 alloys on the basis of the combined effect of Cu-and Mg-intermetallics, where hardening occurs by cooperative precipitation of Al2Cu and Mg2Si phase particles, compared to only Mg2Si precipitation in the case of 356 alloys. The morphology of iron intermetallics was found to affect the cutting force results when the aging was carried out for 2 h at 180 degrees C and not at 220 degrees C. It seems that cutting force and moment are only slightly influenced by cooling and quenching rates. Heat treatments that increase the hardness will reduce the built-up-edge (BLYE) on the cutting tool. Hardness affects the machinability of 319 alloys in that machinability improves as the hardness increases. It is observed that both cutting force and moment increase with the hardness while the build-up on the cutting edge decreases. The low Mg-content 319 alloys (0.1% Mg) yielded the longest tool life, more than twice that of 356 alloys (0.3% Mg) and one-and-a-half times that of the high Mg-content 319 alloys (0.28% Mg). It is customary to rate the machinability of the 319 alloy as higher than that of 356 alloy, and the machinability of the low Mg-content 319 alloy as higher than that of the high Mg-content one. Deceptive chip formation (welding) was observed on 356 and 319 alloys (M1 and M3). Full, half turn and helical chips are generated for both 356 and 319 alloys at the start of a cutting operation when the drill is new (shearing process). As the drill begins to wear, the chips gradually become deformed. as both shearing and deformation occur. (c) 2006 Elsevier B.V. All rights reserved.