Graduate Thesis Or Dissertation
 

Design of advanced aluminum silicon alloy compositions and processing

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  • Part I discusses the development of an aluminum-magnesium-silicon alloy that may combine strength, extrudability, favorable corrosion resistance with low cost and scrap compatibility. The first part of the study determined the effects of small composition, heat treatment and mechanical processing changes on the ambient temperature tensile properties of the alloy. A combination of magnesium and silicon of about 2%, 1% copper, 0.2% chromium and 0.1% vanadium can produce a T6 alloy with significant higher strength, fatigue and corrosion fatigue properties for both ingot and extrusion than those of 6061 but with only a modest increase in cost. The new alloy has been designated as AA6069. The second part of the study determined the T6 properties of 6069 alloy. The tensile test results of cold and hot extrusions of hollow, solid bars, and high pressure cylinders indicate that the T6 properties ranged from 55-70 ksi (380-490 MPa) UTS, 50-65 ksi (345-450 MPa) yield strength, and 10-18% elongation. It also appears that the fracture toughness and general corrosion resistance in saline environment are comparable or better than those of 6061 T6. Part II attempted to evaluate the formation, formability, thermal and mechanical properties of semi-solid A356, A357 and modified aluminum silicon semi-solid alloys. The semi-solid alloy microstructure was produced in this study by purely thermal treatment rather than conventional and expensive electromagnetic or mechanical stirring. Three heat-up stages in semi-solid treatment were evaluated. Stage I is related to the heating of the alloy in the solid state. Stage II is related to the eutectic reaction. Stage III is related to the heating of the semi-solid slurry. Stage II requires the longest time of the three heat-up stages due to the endothermic reaction on heating. An increase of furnace temperature can greatly reduce the time of stage II. The atmosphere (vacuum, air, argon) of the semi-solid treatment does not appear to greatly affect the T6 properties of semi-solid alloys. The microstructure and T6 properties of semi-solid A356 do not appear sensitive to the homogenization treatments before semi-solid treatment. The porosity of semi-solid ingots and pressed parts increases as the cooling rate decreases in unformed and subsequent-to-moderate pressure forming. The T6 properties basically appear sensitive to voids, with a degradation of properties as the void concentration increases. The formability of A357 may be improved as the spheroidal particle size decreases. Hence, formability may improve with decreasing ingot grain size. The mechanism of coarsening of the solid phase at isothermal temperatures is related to Ostwald ripening and/or "merging" of particles. The mechanical properties of die-casting parts show that the method of thermal treatment to produce a spheroidal microstructure is an effective method for industrial production of semi-solid aluminum-silicon alloys.
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