High pressure gas expands forming

Roger Bishop reports on an emerging process that converts metal tubes into automotive components

A new process to convert metal tubes into car components – such as engine cradles and subframe components, could work out cheaper than existing techniques such as hydroforming. Heat stands for Hot Expansion Air Technology Forming. It uses inner gas pressure to form pre-heated hollow shapes and is said to offer high degrees of metal deformation at lower cost than conventional hydroforming. Potential applications are those that are either feasible with hydroforming or which require a couple of dies and annealing/forming steps and are therefore expensive. The process is being further developed and brought to market by Heatform of Wiesbaden in Germany – whose co-founder, Dr Peter Amborn, invented and extensively patented the process – and Schuler Hydroforming of Wilnsdorf. Schuler will offer Heatforming lines worldwide and help customers with die construction, prototyping and contract manufacture. Similar to a glass-blower shaping hot glass, Heatforming uses air, nitrogen or argon as the pressure medium and expands the circumferences of metallic hollow bodies, tubes and profiles to more than 270% in one process step. The process is more akin to high pressure tube hydroforming in that an axial force is imposed on one end of the tube to ‘feed’ material into the die during the forming process. Air is sufficient as a pressure gas for aluminium. Nitrogen and argon are used with scaling-sensitive metals such as steel and titanium, respectively. A significant advantage of using gases as the active medium is rapid feed and ventilation and the fact that the die remains clean. These factors lead directly to shorter cycle times compared with tube hydroforming using liquid media. Generally, the pressures used are between 300 and 500 bar. First applications and prototyping will be in aluminium, brass and titanium. All commercially available aluminium alloys have been used successfully with changes in circumference in excess of 270% and process temperatures of 530-560ºC. Brass alloys, like aluminium, can be formed at high strain values and emerge from the die ready for polishing. Titanium alloys can be formed in a single forming process at around 850ºC achieving high values of strain. Parts for the automotive industry are “in preparation” for serial production within the next year. Development of the process for steels and high strength steels is a focus of the Schuler cooperation. First case studies and prototypes are being produced now but serial production of parts is still a few years away. “The requirements and challenges are definitely higher than for aluminium or brass due to the higher temperatures,” said Karl Kipry, Heatform’s CEO. A paper on the process suggests that Heat forming should be suitable for all steel alloys that do not permanently lose their properties above 450ºC. Current developments focus on the range of 750-950ºC and include stainless steels. Specific advantages of the process beyond the material and cost savings thought to be possible include: large variations in cross-sections with high degrees of detail; small corner radii; forming of thick tubes and high-strength steels; and the ability to control wall thickness and thinning – even to go from thin to thicker wall thicknesses. For the automotive industry, the ability to form a higher spectrum of alloys than with hydroforming, the opportunities to reduce component weight and the ability to integrate complex part features will all appeal. POINTERS Heatform uses hot gas to form metal tubes into shaped car components The technique can expand a tube to 270% of its original size in a single step It could prove to be more cost effective and flexible than hydroforming, which is currently used to make these parts