The importance of the development of a light alloy for use in parts that are subjected to elevated temperatures has already been emphasized in many papers, among which that by S. D. Heron on Air-Cooled Cylinder Design and Development4 should be particularly mentioned. It was with this purpose in view that the foundry of the Engineering Division of the Air Service at McCook Field undertook a brief survey of the alloying, the casting, the heat-treatment, the physical properties and the metallography of an aluminum-copper-nickel-magnesium alloy of the Magnalite type as sand-cast under ordinary foundry conditions.
It was found that the alloying involved no particular difficulty. The casting, however, showed the necessity for proper pouring temperatures, gating and placing of the chills and the risers. Several photographs are shown illustrating satisfactory and unsatisfactory methods of molding pistons and air-cooled cylinder-heads.
The heat-treatment of these alloys must be carefully conducted, since not only is the alloy tender at the quenching temperature, but it is also very susceptible to small variations in quenching temperature. With proper attention to manipulation, consistent results can always be obtained; and rejections from warpage and distortion are negligible. The treatment best combining the factors of physical properties and economy is to heat the alloy at 975 deg. fahr. for 5 hr. and then to quench in boiling water or cool in air according to the design of the casting. The alloy is subsequently aged at 212 deg. fahr. for 16 hr., an overnight treatment.
The physical properties of this alloy at ordinary temperatures as cast are very much improved by suitable heat-treatment; and at 600 deg. fahr. the alloy shows the remarkable property of being slightly stronger
than it is at room temperature, either as cast or heat-treated. The extended heat-treatments partaking of the nature of the malleableizing process in cast iron produce even better results than the short treatment described above, but the former is probably not economically justifiable, although it produces ultimate tensile-strengths as high as 45,000 lb. per sq. in. and Brinell hardness values as high as 105.
The average properties of the sand-cast and heat-treated alloy compared with some of the better known light casting-alloys are tabulated.
The metallography of the alloy is quite complex. A number of photomicrographs are included which not only show the constituents, but the changes in structure resulting from heat-treatment.