Effect of metallurgical parameters on the machining behavior of 356 and 319 alloys (drilling and tapping study)

The present study was undertaken to investigate the effect of metallurgical parameters on the hardness and microstructural characterisation of as-cast and heat-treated 356 and 319 alloys, with the aim of adjusting these parameters to produce castings of suitable hardness and Fe-intermetallic volume fractions for subsequent use in studies relating to the machinability of these alloys. Hardness measurements were carried out on specimens prepared from 356 and 319 alloys in as-cast and heat-treated conditions, using different combinations of grain refining, Sr-modification, and alloying additions. Aging treatments were carried out at 155°C, 180°C, 200°C, and 220°C for 4 h, followed by air cooling, as well as at 180°C and 220°C for 2, 4, 6, and 8 h to determine conditions under which specific hardness levels viz., 85 and 115 HBN could be obtained. Addition of Mg to 319 alloys containing beta- and/or alpha-Fe intermetallics was found to produce a remarkable increase in hardness at all aging temperatures in both the unmodified and Sr-modified conditions.; Additions of Mg to 319 alloys coupled with different heat treatment conditions for both 356 and 319 alloys were carried out to get similar levels of hardness for both alloys. Conditions of Sr-modified (200-250ppm) 356 and 319 alloys-containing mainly alpha-Fe-intermetallics related to different levels of hardness (90, 100 and 110 HB) were selected for the drilling and tapping study. Effect of Mg and alpha-Fe-intermetallic volume fraction on the machinability of heat treated 319 alloys was studied for two levels of Mg (0.1 and 0.28%) and 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 conforms to the most common levels of the commercial applications of these alloys.; The most important factors undertaken in the present study that determine the condition of the work material that can influence the outcome of the machinability of 356 and 319 alloys are: (a) Chemistry and additions (Cu, Mg and alpha-Fe-intermetallic volume fractions); (1) Role of Cu-intermetallics when machining 356 (without Cu- aged at 180°C/2h) and 319 (with Cu- aged at 220°C/2h) alloys, both have the same level of hardness (100 HB). (2) Role of Mg addition to 319 alloys at two levels of Mg content (0.1 and 0.28%) given the same aging treatment (220°C/2h) that yields different levels of hardness (90 and 100 HB) and given different aging treatment (180°C/2h and 220°C/2h) that yield the same level of hardness (100 HB). (3) Effect of increasing alpha-Fe intermetallic volume fractions to 319 alloys (2 and 5%) when aging carried out at 220°C/2h and at 180°C/2h that yields hardness of (90 HB) and (100 HB), respectively. (b) Solidification time (25-45)s and quenching rate (100-145)°C/s; (c) Hardness (100 +/- 10 HB). (Abstract shortened by UMI.)...