TPT September 2007

Technology Update

Cold welding of hot exhaust system parts

Presswerk Struthütten GmbH, part of the PWS Group, Germany, specialises in the production of metal-based components. Presswerk also offers solutions in the field of wire and pipe brackets, hydro-forming and the provision of operating resources. The company specialises in the hot side of exhaust systems, where the parts for manifolds must withstand exhaust gases of up to 900° over many years. Such production requires high-quality materials, optimum design and appropriate production technology, and PWS only uses stainless steel plate for these ‘hot products’.

PWS decided on a new cold process (CP) from German welding specialists Cloos. While current pulsed arc welding processes are unipolar/positively poled, the Cloos cold process uses changing polarities, so the relative heat input in the wire electrode or workpiece becomes controllable. This results in a reduced heat input in the weld and in the welded material itself, and so reduces component distortion and the problem

› The different gap widths of punched, complex manifold half shells can be reliably welded by the cold process

processes, Mr Oppermann states that “ this is only possible with CP. ” The new process achieves welding speeds of 2.2m/min, even with complex PWS components which have welds with numerous bends and narrow radii. The Z2 compact production cell features two stationary work tables. While a finished component is unloaded from the first work table and two new half shells are loaded, the Romat ® robot welds at the second work place. In an area of only 3.90m x 2.25m, the Z2 contains all the necessary components. These include the Romat 320 robot with

of crack formation. CP welds are nearly spatter-free, so rework on components is also reduced. Whether high-strengthsheet steel, aluminium or thin stainless steel plate, the cold process can join materials of up to 3.5mm wall thickness with low heat expenditure. In the pulse phase there is maximum spatter-free deposition from the wire. During the negative phase the arc surrounds the wire end, directing the heat into the wire and cooling the weld pool. Penetration is minimised due to the lower heat input with the cold welding process, resulting in a reduction in surface discolouration. Even in the case of coated plate, the surface remains in good condition. “ Another benefit is the improved gap bridging ability which enables our punched parts to be welded ,” explained Mr Tim Oppermann, who is in charge of welding technology at PWS. The Cloos cold process also makes higher welding speeds possible, resulting in shorter cycle times in production. At present, PWS manufactures approximately 15,000 manifold components a month. After many tests and optimisation trials with other

› CP welding provides high welding speeds with low heat input into the component

Producing punched parts, the company found that welding technology requirements increased so considerably that the required quality and production throughput could not be achieved using the conventional process. According to Mr Michael Jud, technical managing director, “ These three- dimensional punched parts with plate thicknesses of around 1mm naturally show very different gap dimensions. In addition, the automotive manufacturers demand spatter-free components, because under the extremely hot conditions spatter can be released in the exhaust and destroy the catalysers, for example. ”

fi The almost spatter-free welds achieved by the cold process bridge high gap widths

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S eptember /O ctober 2007