Tribological Evaluation of Different Lubricant Formulations for Hot Forming of Aluminum

Recent efforts in reducing the carbon dioxide (CO₂) emissions in transportation lead to increasing interest in lightweight materials (such as e.g., aluminum or magnesium alloys). High strength aluminum alloys seem promising, owing to their good strength-to-weight ratio which would allow their use in structural car parts (currently mostly cosmetics use). These alloys (e.g., 6XXXAQ1, 7XXXAQ2) are hot formed, for thermomechanical strengthening and mitigating their poor room temperature formability. However, hot forming of aluminum leads to galling, which currently limits mass production. For hot forming structural components, limited working solutions with 6XXX aluminums exist, and few are implementable to high production rates. During forming of these alloys, the role of surface engineering or lubrication are not fully understood. In this article, two tool steels were tested in hot strip tribometer against AA6010S alloy. One tool steel was coated with two diamond-like carbon (DLC) (a-C:H, ta-C). The tribological performance of two high temperature lubricants (polymer vs. salt) was evaluated and compared to a reference graphite lubricant. The main findings are:<br/>- Both polymer and salt lubricants led to low friction and wear by effectively separating the contacting surfaces;<br/>- DLC coatings show a lower mechanical anchorage with lubricants, leading to earlier lubricant removal and initiation of material transfer. Especially, the rougher ta-C coating led to the earliest lubrication failure. Its performance in the contact is the poorest when lubricated with polymer lubricant;<br/>- The salt lubricant is the best solution in this contact, through forming a thick lubricant layer, actively separating the Al and tool steel surfaces.