Inclusions in Permanent Mold Cast Magnesium
This research is part of an ongoing effort to increase the use of magnesium alloys to a significant level in the aerospace and automotive industries and to reduce vehicle weight, fuel consumption, and emission of harmful gases. ABDALLAH ELSAYED, RYERSON UNIVERSITY (TORONTO) CURRENTLY AT NEMAK CANADA CORPORATION (WINDSOR, CANADA); COMONDORE RAVINDRAN AND ELI VANDERSLUIS, RYERSON UNIVERSITY (TORONTO); SOPHIE LUN SIN, RYERSON UNIVERSITY (TORONTO) CURRENTLY AT INRS (QUEBEC CITY, CANADA).
minum alloys to reduce vehicle weight in aerospace and automotive applica- tions. Magnesium alloys are about 35% lighter than aluminum alloys. However, only 0.3% of the total automotive vehicular weight in North America is composed of magnesium alloys, while 8.3% is composed of aluminum alloys. In terms of total weight, each pas- senger car contains only 11.02 lbs. (5 kg) of magnesium, yet 264.5-308.6 lbs. (120–140 kg) of aluminum. Te widespread use of magnesium
M
alloys for aerospace and automotive applications is hindered by their high reactivity, which increases the probabil- ity of inclusion formation during cast- ing processes. Inclusions in magnesium alloys compromise corrosion resistance, increase porosity, produce unfavorable surface finishes, and reduce mechanical properties, in particular ultimate tensile strength and elongation. Two major types of inclusions in magnesium alloys occur: intermetallic and non-inter- metallic. Intermetallic inclusions are almost always iron-rich phases, while non–intermetallic inclusions include sulfides, fluorides, sulfates, chlorides, nitrides, and oxides, with oxides being the most dominant. Avoiding inclusions in magnesium alloys is difficult due to the many
46 | MODERN CASTING March 2017
agnesium alloys have been gaining consid- eration as possible alternatives to alu-
oxygen to form MgO. Even with the use of protective atmospheres such as sulfur hexafluoride (SF6
, CaCl2 ), reac-
become entrapped in the melt. In addi- tion, melt turbulence during melting, handling, and pouring can be a source of inclusions in magnesium castings. A wide variety of inclusion assess-
tion products of MgO and MgF2 can
ment techniques are available for magnesium and its alloys. Tese tech- niques vary from simple observational methods, such as metallographic and fracture bar examinations, to highly sophisticated online methods, such as liquid metal cleanliness analyzers. Since no industry standard for examining inclusions in magnesium alloys exists, the techniques employed are foundry- dependent, which complicates com- parisons between facilities. Tis article aimed to characterize and examine the effects in ZE41A and AZ91D magnesium alloys and their influence on microstructure and mechanical properties. By better understanding and documenting metal handling, the resulting scrap reduc- tion, casting quality enhancement, and associated cost reductions will improve foundry competitiveness. Tis
sources from which they arise. Inclu- sions can arise from reactions with air where magnesium reacts with oxygen to form MgO, reactions with fluxes entrapping flux components (e.g., MgCl2
) and flux reactions with
research is part of an ongoing effort to increase the use of magnesium alloys to a significant level in the aerospace and automotive industries and to reduce vehicle weight, fuel consumption, and emission of harmful gases. Te general procedure was the same
for both alloys and involved the produc- tion of permanent mold tensile castings and fracture bar castings from multiple foundries and characterizing them according to their mechanical proper- ties, grain sizes, microstructures, and inclusion contents. Te microstructures, inclusions, and grain sizes were charac- terized using scanning electron micros- copy (SEM) and optical microscopy. Te mechanical properties were assessed using uniaxial tensile testing. For both ZE41A and AZ91D alloy
castings, the average yield strength, ultimate tensile strength, and elongation decreased between the start and end of the production run. Te results from examination of grain size, microstructure and inclusion analysis indicate that the loss in properties was predominately caused by the accumulation of oxides. For example, the AZ91D castings demon- strated a ~5-10% decrease in UTS and a ~20-30% decrease in elongation, with a smaller change in yield from start to end of production. For both alloys, an increase in grain
size was observed between the start and end of the production run, but the reduc- tion in mechanical properties was mainly
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