March_2022_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 2 4 2 Solution heat treating is used with cast alloys to re- duce the brittle eutectiferous networks that form during casting. Solution heat treating is conducted at 390° to 530°C (730° to 980°F). Solution-heat-treated castings have a more uniform matrix, which improves both strength and ductility. The aging treatment normally does not improve the tensile strength very much, but the yield strength is in- creased along with some reduction in ductility. However, the ductility decrease still leaves enough ductility for most casting applications. Some wrought alloys also can be hardened by heat treatment. Magnesium alloys AZ80 and ZK60 are com- monly hardened by heat treatment. In general, wrought magnesium alloys that can be strengthened by heat treat- ment are grouped into five general classes according to composition: • Magnesium-aluminum-zinc (example: AZ80A) • Magnesium-thorium-zirconium (example: HK31A) • Magnesium-thorium-manganese (examples: HM21A, HM31A) • Magnesium-zinc-zirconium (example: ZK60A) • Magnesium-zinc-copper (example: ZC71A) Six common problems that may be encountered in heat treating magnesium alloys are oxidation, fusion voids, warpage, grain coarsening, germination, and incon- sistent properties. A summary of the causes and preven- tion of problems encountered in heat treatment of magne- sium alloys is described in the sidebar. Magnesium alloys are usually solution treated in a protective atmosphere with special temperature monitor and control devices to prevent magnesium fires, which are very difficult to extin- guish. The parts are then air quenched and aged. For more specific information on heat treating, see Ref. 2. ~HTPro Note: This article is an excerpt from the book Practical Heat Treating, Basic Principles by Jon L. Dossett. It is adapted from Chapter 12, Heat Treatment of Aluminum and Other Nonferrous Alloys. The book is available in print and in the ASM Digital Library at doi.org/10.31399/asm. tb.phtbp.9781627083263. References 1. C.R. Brooks, Heat Treatment, Structure and Properties of Nonferrous Alloys, American Society for Metals, 1982. 2. T. Abbott, Heat Treating of Magnesium Alloys, Heat Treating of Nonferrous Alloys, Vol 4E, ASM Handbook, ASM International, 2016, p 640–649. 10