THERMOMECHANICAL MECHANISMS THAT CAUSE ADHESION OF ALUMINUM HIGH PRESSURE DIE CASTINGS TO THE DIE
Date of Award
Open Access Dissertation
Doctor of Philosophy in Materials Science and Engineering (PhD)
Administrative Home Department
Department of Materials Science and Engineering
Committee Member 1
Committee Member 2
Committee Member 3
In high pressure die casting (HPDC) of aluminum, cast material adhering to die is a significant defect. Adhesion occurs in two primary ways. The casting may stick preventing its removal from the die. Aluminum can also adhere to the die and buildup in local areas on the die surface with additional casting cycles. This second form of adhesion is called soldering. Lubricant is the best technology to control all forms of adhesion, but it comes at the cost of casting porosity, blisters, reduced die life, and increased die casting machine wear. New strategies to prevent adhesion are desired to eliminate the downsides of spray lubricants.
Thermodynamically, there is a drive to form intermetallic phases between the aluminum casting and steel die. The kinetically controlled formation of these phases has been understood as the primary mechanism which causes sticking and soldering. Assuming this mechanism, adhesion should be eliminated by coating the die surface with a non-reactive material, but laboratory and industrial experiments show that this approach is partially effective. Also, kinetics based predictions cannot identify the areas where adhesion is most severe. Additional mechanisms are needed to further predict and reduce these defects.
Friction is such a mechanism, and it has been overlooked as a significant cause of sticking and soldering in HPDC. Sticking results from the thermal contraction of the casting onto the die. This creates a friction force that resists ejection and may be larger than the ejection force capability of the die casting machine. Soldering is a special case where the local shear stress due to friction exceeds the local shear strength of the casting. This typically happens at high temperatures and can be predicted by the ratio of the local ejection shear stress and temperature dependent shear strength.
Four aspects of HPDC adhesion were investigated to support this friction mechanism. First, the accepted theory of a kinetically controlled approach to the thermodynamic equilibrium does not adequately predict sticking and soldering. Next, the decrease in sticking force with increasing draft angle is predicted by a friction model. Third, soldering is shown to occur when the shear stress at ejection exceeds the strength of the casting via hot ejection test and computer models of industrial castings. Finally, the friction approach is applied to a range of casting conditions and alloys with discussion of optimization opportunities.
Monroe, Alex, "THERMOMECHANICAL MECHANISMS THAT CAUSE ADHESION OF ALUMINUM HIGH PRESSURE DIE CASTINGS TO THE DIE", Open Access Dissertation, Michigan Technological University, 2021.
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