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 1 10 9 Oxidation Cause: Heat treating without use of protective or in- ert atmosphere; can lead to local weakening of the metal part, and even to burning of the metal in the furnace Prevention: Heat treat in a controlled atmosphere containing approximately 1.5% SO 2 , or 0.2 to 0.5% SF 6 (*) or HFC-134a in dry air, CO 2 , nitrogen, or argon atmo- sphere. Ensure that furnace is clean and completely dry. Fusion voids Cause: Use of improper rate of heating from 260° to 370°C (500° to 700°F) for Mg-Al-Zn alloys, or exceeding recommended temperature in solution heat treating of these alloys or of the alloys that contain zinc, thorium, and rare earth metals as major alloying elements. Fu- sion voids are not normally observed when the solidus temperature of the alloy is exceeded. In this instance, grain-boundary phase will run along the grain boundary, forming long, narrow regions. This is normally accompa- nied by grain coarsening. Prevention: Charge furnace with Mg-Al-Zn alloys at 260°C (500°F) and then heat gradually to solution-treat- ing temperature over a period of 2 h. Control solution temperature so as not to exceed designated temperature by more than 6°C (10°F). Warpage Cause: Lack of support of castings during heat treatment; uneven distribution of heat Prevention: Support long spans of thin cross sec- tion; use jigs for intricate shapes. Distribute load in fur- nace to obtain good circulation of atmosphere. Grain coarsening Cause: Occurs in HK31A as a result of delay in at- taining solution temperature or of holding at solution temperature for an excessive period Prevention: Prior to solution treating of HK31A, fur- nace should be at temperature; castings should be load- ed quickly, and the loaded furnace should be closed and brought to temperature as rapidly as possible. Time at temperature should be controlled. Germination Cause: Grain growth, which occurs in AM 100A, AZ81A, AZ91C, and AZ92A toward the end of the solu- tion-treating cycle Prevention: Use antigermination heat treating schedules. Inconsistent properties Cause: Insufficient or excessive furnace tempera- ture, inadequate circulation of heat in the furnace, faulty temperature control, very slow cooling from the solu- tion-treating temperature, or inadequate solution-treat- ing time for heavy sections Prevention: Check temperature at various positions in furnace with standardized thermocouple. Distribute castings in furnace to provide adequate circulation of heat. Check temperature controls often and ensure that controls are located so as to provide uniformity of tem- perature. Increase solution-treating time to allow com- plete homogenization. (*) SF 6 is banned in some jurisdictions due to its high global-warming potential. CAUSES AND PREVENTION OF PROBLEMS COMMONLY ENCOUNTERED IN HEAT TREATMENT OF MAGNESIUM ALLOYS Heat treatment can improve the mechanical proper- ties of most magnesium casting alloys. In most wrought alloys, maximum mechanical properties are developed through strain hardening, and these alloys generally are either used without subsequent heat treatment or merely aged to a T5 temper. Occasionally, however, solution treat- ment, or a combination of solution treatment with strain hardening and artificial aging, will substantially improve mechanical properties. Three basic types of thermal treating processes are commonly applied to magnesium alloys: annealing, solu- tion heat treatment, and precipitation or aging. In addi- tion, stabilizing and stress-relieving treatments are used in practice. The former is a type of precipitation, and the latter is related to annealing. Precipitation occurs in many magnesium alloys, but precipitation does not always re- sult in hardening. In many alloys, lattice coherence (and lattice straining) is lost early in the formation of precipi- tates. Some examples of significant hardening that does occur from precipitation include magnesium-aluminum alloys, magnesium-yttrium alloys (Fig. 1), andmagnesium- zinc alloys. CAST MAGNESIUM ALLOYS The cast alloys are used in the as-cast, annealed, or precipitation-hardened conditions. In terms of strength- ening by heat treatment, many of the cast alloys can be age hardened. The most common precipitation-hardening treatments for cast magnesium alloys are solution treating and natural aging (T4), natural aging after casting (T5), and solution treating and artificial aging (T6).