Research project

Target industries

Automotive industry, vehicle technologies, manufacturing technology, tool manufacturing, production technologies, design, building construction, materials

Challenge

Ferritic heat-resistant stainless steels such as 1.4509 (X2CrTiNb18) are used in the automotive industry for manufacturing exhaust manifolds, manifold or catalytic converters, for instance. Using hot metal gas forming (HMGF), which is temperature-assisted hydroforming at temperatures above 800 °C, even complex geometries with high local strains can mostly be formed in just one step. However, the process reliability and economy of the process are not yet fully exploited when using a heating technology that is purely external of the tool or integrated into the tool. In the case of heating outside the tool, the material's formability is reduced due to the material-related temperature limit during heating and due to the large temperature loss until start of forming. If the heating is only realized inside the mold, the achievable cycle time is too long for economic manufacturing.

Solution

A new HMGF process strategy with two-stage heating was developed for the material 1.4509. The preheating of the tube took place in a chamber furnace. The second heating stage was realized by conduction integrated into the tool (direct resistance heating), which enables fast heating of the tube. In a forming study, parameters such as forming pressure, calibration time and conduction temperature were varied and compared with the results of a process with pure furnace heating. As a result, the two-stage process improved process stability by 75% and shape accuracy by up to 23%. Thinning was reduced by up to 10%. Compared to a heating solution purely integrated into the tool, the cycle time was shortened by 70%. In addition, the new process strategy led to a significant improvement in component surface quality.

_{This work has been funded within the MANUNET ERA-NET project "Hydroforming of Stainless Steel Tubes (HySST)" by the German Federal Ministry of Education and Research (BMBF) and the Department for Economic Development and Infrastructure of the Basque Country (Spain).}