Highly heat-resistant tool surface layers by extending the tailored forming technology to hot solid forming tools

The objective of the transfer project was the cost-effective manufacture of tools with improved service performance in die engraving regions subjected to severe thermomechanical loads. This results in significant potential for application in hot bulk metal forming. In conventional dies made of hot-work tool steel, tempering effects in the surface region lead to a reduction in hardness and the formation of wear, which can ultimately be identified as the cause of failure. In contrast, Inconel, as a representative nickel-based alloy, exhibits high high-temperature strength and excellent tempering resistance. However, due to its high material costs and poor machinability, its use has so far not been economically feasible. By developing forming operations on pre-joined hybrid semi-finished products, the surface enlargement occurring during forging is intended to be exploited in order to reproducibly and material-efficiently introduce Inconel onto the hot-work tool steel substrate as a protective layer.

In the initial phase of the project, worn tools provided by the project partner were examined with regard to their wear characteristics. The analyses were conducted using both experimental measurement techniques and numerical methods. Based on the findings, requirements for hybrid tools to be manufactured by means of tailored forming were defined.

To produce the required hybrid semi-finished products, material combinations of hot-work tool steels and nickel-based alloys were joined by rotary friction welding. Subsequently, these hybrid semi-finished products were subjected to a forming process. For the design of the forming process, material models were developed that served as the basis for numerical simulations.

The objective of the process design was to achieve a controlled, forming-induced distribution of the difficult-to-machine Inconel. By adjusting suitable process parameters, targeted influence could be exerted on both the surface enlargement and the layer thickness distribution of the Inconel.

Using the developed process chain, hybrid high-temperature-resistant tool mandrels were manufactured. These mandrels were subsequently integrated into conventional hot-work tool steel dies and tested in automated, industry-relevant series forging processes over more than 2,000 forging cycles. Comparative investigations with conventional nitrided hot-work tool steel tools demonstrated that a deliberately introduced wear-protection layer made of nickel-based alloys can almost completely prevent the occurring wear.

At present, transfer studies on the application of scaled tools are being carried out at the industrial partner within the transfer project.