The Effect of HIP Processing on the Properties of A356 T6 Cast Aluminum Steering Knuckles

Hot Isostatic Pressing (HIP) has been routinely used to densify castings for aerospace and medical applications for over 30 years. While HIP is widely known to improve the toughness and fatigue life of castings through the healing of internal porosity, it has been perceived as too expensive for most cast aluminum alloys for automotive applications. Recent developments suggest that the cost effectiveness of certain special HIP processes should be revisited due to reductions in process cost and improvements in throughput. This paper will evaluate the Densal® II process applied to a front aluminum steering knuckle. Two casting processes representing differing levels of relative cost and quality were evaluated. The first was Alcoa's VRC/PRC process, a metal mold process with bottom fill, evacuation before fill and pressurization after fill. This is considered to be a premium quality, but higher cost casting process that is already qualified for this application. The second was a high-rate, bottom fill, Disamatic sand casting process operated by Robinson Foundry. This process is considered a lower cost process. All parts were evaluated in the as cast + T6 condition and after Densal® II + T6 processing to evaluate microstructure and density. The tensile properties and fatigue curves were established and compared. Fracture analysis using Scanning Electron Microscopy was performed to help understand the fatigue results. The total costs are estimated for both casting techniques and the suitability of the resultant products for critical automotive structures is discussed.