Identification of advanced die coatings, their failure mechanisms, and progress towards lube-free aluminum high pressure die casting

Abstract

In this work, the effects of Al/(Al+Cr) ratio, nanolayering with TiCN, surface roughness of the coated H13 steel, and coating defect population were evaluated for five different AlCrN-based PVD coatings (Chapter 2). In addition, the failure mechanisms of core pins coated with Si-DLC by plasma-assisted chemical vapor deposition, or with AlCrN/TiCN or Al2O3 by PVD and positioned directly in front of the gate of a die used to produce large automotive die castings were investigated (Chapter 3). Regardless of coating type, five failure mechanisms were observed including mechanical erosion, chemical soldering, gross chemical soldering, build-up on coating, and thermal spalling, although the extent of each mechanism did vary with coating type. Selection maps were created by combining the wear resistance and the fraction of the core pin surfaces experiencing soldering and used to determine the best coatings for avoiding chemical and mechanical degradation leading to soldering. Successful lube-free aluminum HPDC can be achieved (Chapter 4) using two simple dies that were coated with an AlCrN PVD coating but contained no core pins or other features that were non-perpendicular to the ejection direction. It was possible to run both of these dies in the lube-free condition and produce over 200 lube-free castings. It was observed that aluminum build-up on the die surface was an intermittent phenomenon as it did not lead to chemical soldering. There was no chemical reaction or intermetallic formation in the interface regions between the aluminum and the H13 steel, or between the aluminum and the AlCrN coating. A mechanistic lube-free model was developed to understand the correlation between surface roughness, wetting angle and pressure and explain why lube-free aluminum HPDC was possible for these simple geometries and why it becomes challenging for more complex geometries.