Experimental investigations on hot forming of AA6082 using advanced plasma nitrocarburised and CAPVD WC: C coated tools

The use of lubricant for hot stamping process of sheet material can reduce the tendency of adhesion between work-piece and tool significantly and the friction coefficient. However, the post-process of cleaning the formed part and lubricating the tools before each stamping operation can compromise the manufacturing efficiency. It is especially challenging for hot stamping because reducing lubricant could lead to severe adhesion between tool and blank during stamping. Hence, it is desirable to develop an advanced tooling technique suitable for hot stamping processes of aluminium alloys. In this paper, an innovative tooling technology enabling hot and cold forming of aluminium with little lubricant has been developed using plasma thermochemical treatment and Cathodic Arc Physical Vapour Deposition (CAPVD) technologies. The forming performance was validated on a top-hat part stamping test to benchmark the performance of the developed tools at different forming temperature, blank-holding force (BHF) and lubrication state. The results show that WC: C coating prepared by CAPVD adequately reduced the aluminium sticking on the tool surfaces, to the extent that it achieved an 80% lubricant reduction in the hot forming stamping of a top-hat part with a drawing depth of 70 mm. The morphology of die showed that aluminium adhesion at the corner area of the die where the contact pressure doubled was still noticeable, while no severe adhesion was observed on the top flat surfaces. A further investigation of tribology on hot and cold stage tribometers was deployed to quantify the friction coefficient and wear resistance of tooling materials which were found correlated to the material's universal hardness and time-dependent adhesion rate of aluminium.