Self-lubricating laser claddings for friction control during press hardening of Al-Si-coated boron steel

Abstract

In recent years, the use of Al-Si-coated ultra-high strength steels has grown rapidly in hot stamping applications in order to protect the workpiece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperatures can lead to unstable friction, increased wear and damage both the tool and the workpiece surfaces.In the present study, self-lubricating coatings incorporating silver and molybdenum disulfide have been prepared by means of laser cladding, aiming at controlling friction and wear in hot stamping. The friction and wear characteristics of these coatings have been investigated at high temperatures against Al-Si-treated boron steel using two different test configurations featuring open and closed tribosystems respectively. A significant reduction in friction as well as decreased material transfer from the Al-Si-coated workpiece to the tool surface have been observed for the self-lubricating claddings. The closed configuration tribological tests have been found to underestimate friction and wear of the tool/workpiece system, and may be less suited for the lab-scale simulation of hot metal forming compared to an open test configuration. These findings are considered relevant as most of the published results in the available literature have been obtained by using closed configurations tests.