Titanium Welding Technology for Series Production
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Titanium is now gaining importance due to requirements for lighter-weight, lower-emission vehicles. Eberspächer has developed a welding process for titanium materials with which high-quality seams can be reliably produced. The company relies on special glove boxes, which are suitable for use in series production.
Titanium as a Material
Titanium is an interesting structural material, primarily because of its low mass density of 4.5 g/cm3 and very high specific durability. Titanium alloys achieve tensile strengths of above 1000 MPa . The material is also highly resistant to chemicals. Due to its ability to form an alloy, the most desirable properties can also be purposefully optimized. This explains the many titanium alloys available on the market. For a number of years now, titanium materials have been used in the aerospace sector, medical technology, pipework and apparatus construction, as well as in the chemical industry and other industrial branches.
Extreme Toughness and Resilience
Titanium is lighter than steel and, due to its mechanical properties, a suitable substitute. Due to its special properties, titanium is difficult to treat and a much more expensive material than high-alloyed steel. Because of the extreme toughness and resilience, the cold forming processes usually employed in exhaust technology present a challenge for the process chain. Particular attention must be paid here to details such as the layout and design of the effective surfaces in the forming die, the forming speed and the correct choice of lubricant.
Processes for Titanium Fabrication
Titanium is not treated in the same way as conventional materials. The attempt to form titanium sheet metal in tools designed for austenitic stainless steel sheet metal clearly shows that the characteristics of titanium materials definitely need to be taken into account for the tool layout. For the purposes of Simultaneous Engineering, this has resulted in the requirements for the product being reconciled with the process. Today, these experiences are applied in the early development phase in order to reach the goal with only a small number of iterations.
In exhaust technology, welding is the predominant joining process and has been constantly refined over the years. While the welding of steel or stainless steel exhaust systems has been the industry standard for some time and is very well established throughout the world, the welding of titanium must still be regarded as a special process. Good and reproducible results are achieved only with consideration for titanium’s special properties, and this is the reason why titanium welding is practiced only by a small number of exhaust system suppliers. The German Association for Welding and Applied Processes (Deutscher Verband für Schweißen und verwandte Verfahren, DVS) emphasizes in particular the affinity of titanium to reactive gases as a central challenge .
The affinity of titanium to reactive gases is a central challenge.
In light of this circumstance, specific actions are required especially during the welding process. Otherwise, the absorption of the gases quickly leads to material embrittlement, which in turn causes the part to fail under mechanical load. Therefore, having the highest demands for the quality of the welded joints is a basic prerequisite for the successful use of titanium in welded assemblies. The basis for this includes the application of DVS leaflet 2713. The requirements for the operation, the process and also for the quality of the welded joints defined therein also apply in the aerospace sector. These requirements are expanded by other Eberspächer welding standards, some of which are agreed upon in cooperation with the customer on the basis of specific products or projects. Weld seam geometry, permitted irregularities, hardness profile or the use of welding fillers, for example, are also governed by such standards. Regular revisions of these standards are customary and necessary, since both the welding processes and the materials themselves are continually being refined.
Titanium Manufacture in Exhaust Technology
The titanium welding processes in exhaust technology call for special equipment, which has been permanently optimized over the many years of series production. Tests using commercially available equipment, such as local inert gas shielding, drag nozzles and foil domes, show that with such equipment, process-reliable welding that meets the aforementioned requirements for the quality of the welded joints cannot be achieved.
The consequence is the use of professional gloveboxes which are able to guarantee safe welding in an inert atmosphere in a stable manner with a residual oxygen content of just a few parts per million (ppm). These gloveboxes, developed originally for laboratory operation, have been permanently refined — as part of progressive series projects — in terms of size and equipment for purposeful use in the welding of exhaust systems. The gloveboxes each have a dedicated continuously-operating gas cleaning function. This results in the gas quality (Argon 4.6) even being improved during operation of the gloveboxes. In addition, the purity of the atmosphere is monitored by sensors. As a result, exceeding the permitted limit values is detected immediately and indicated via an alarm signal. Welding work in the glovebox presents a particular challenge with respect to the skills and qualifications of the welders. Only specially- trained welders with many years of experience should be deployed here.
Quality Assurance with Robot Gloveboxes
Stable, safe and reproducible processes are an important criterion for quality assurance. The use of robot gloveboxes meets this requirement for the welding processes particularly well. Advantages are consistent welding parameters and a high path and repeat accuracy. Consequently, the production of highly-stressed titanium welded assemblies in robot gloveboxes is a particularly reliable solution, since the possibility of human fluctuations or uncertainties can be eliminated.
Tungsten Inert Gas (TIG) welding has become an established standard for the manual welding of titanium pipes and sheet metals. This is a fusion welding process, which involves igniting an extremely focused light arc between a tungsten electrode and the joint. Also during the welding process, additional cold or hot wire is fed in and fused to the weld metal, which ultimately joins the parts. This is an extremely delicate process with low fusing deposition rate. Although this does result in a correspondingly high weld seam quality without weld spatter, the process is comparatively slow and hence expensive.
The search for even higher effectiveness inevitably leads to Metal Inert Gas (MIG) welding. MIG welding is a light arc welding procedure that involves fusing an endless wire electrode in the light arc and connecting the joints to one another. The fusing deposition rate with MIG welding is many times higher than that of TIG welding.
Even if the outer dimensions of the units to be welded exceed the dimensions of the gloveboxes, the parts can still be welded in a process-reliable manner. Orbital welding, for example, which permits the joining of pipes outside of gloveboxes, is used here. The closed orbital welding clamps are precisely matched to the geometry of the parts to be welded and form a local inert gas chamber, a 360° miniature glovebox, so to speak. This allows weld seams to be produced under similar conditions to those in the regular glovebox. In this case too, the atmosphere is monitored by sensors to guarantee high-quality welded joints.
Resistance welding is also well-suited to titanium materials. As a rule, spot, roll seam and stud welding processes can be employed even without complex shielding against atmospheric oxygen. The narrow air gaps between the parts allow hardly any oxygen to be fed to the weld, thereby ensuring an adequately-protected atmosphere around the weld point. However, due to the numerous different applications, this statement is not universally valid and should be reviewed on a case-by-case basis.
Special Series Production
Due to the high cost of titanium exhaust systems, the use of this material has been limited in recent years mainly to the motorsport sector or to exclusive, high-price vehicles. Due to the task of permanently and continuously reducing the emissions of vehicles with combustion engines, and the associated importance of lightweight design, titanium is now gaining importance for other vehicle series too. The demand for titanium exhaust systems is steadily increasing in the high- performance sector.
The activities being employed to forge ahead with titanium materials in exhaust technology are, therefore, going beyond the development of welding processes and the associated equipment. The better understanding of material properties and the purposeful optimization thereof for application in exhaust technology play an important role here. Consequently, in-depth knowledge of material technology is one of the keys to an efficient development process and a comparatively cost-effective product design for titanium applications.
Welding technologies for titanium go hand in hand with material development. The differences in material properties, such as strength, formability and temperature resistance, major in some cases, must be taken into account. At the same time, it is necessary to verify which material combinations can be joined via which welding procedures with complete process reliability.
A proprietary method for material characterization was developed.
Titanium materials for exhaust systems present an additional challenge in this respect due to the task of combining higher oxidation resistance with high-temperature durability and cold formability. These materials are manufacturer-specific and as such not standardized. Consequently, the data basis is not entirely comparable. Eberspächer has therefore developed, together with internal material specialists, a proprietary method for material characterization and examined all high-temperature capable sheet metal products on the titanium market. As part of this characterization process, weldability as well as physical and mechanical-technological properties are examined and compared with previous experiences. In recent years, various suppliers have brought some potentially suitable titanium materials onto the market and so the activities for material characterization have further intensified.
Eberspächer is working with various titanium material manufacturers, the aim being to qualify more materials for future series applications. The data basis created during this process also serves as the basis for all simulations. Nowadays, these are grouped together under the concept of virtual Product and Process Development (vPPD). Simulations of forming processes can now also take place at component level, just like calculations of thermo- and vibration mechanics at system level to welding distortion simulation of the subsequent production assemblies.
Outlook for the Future
The welding of titanium materials will continue to be refined in the future. The focus will be less on optimizing weld seam quality — as this is already good — and more on increasing effectiveness and achieving favorable production costs. There is, hence, potential for making titanium products in exhaust technology even more interesting and appealing in the future. For complex parts in special series, the additive production of titanium may be an option in the future. Eberspächer is planning appropriate activities in this direction. The production machines required for this purpose are already in place at the German Schwäbisch Gmünd site.
The challenge of titanium welding
The German Association for Welding and Applied Processes describes the challenge of welding titanium in the leaflet DVS 2713  as follows: “Titanium has a very high affinity for the atmospheric gases oxygen, nitrogen and hydrogen. Titanium absorbs these gases, especially in its molten state. As a result, the ductility properties in the solid state at room temperature [...] to complete embrittlement can be reduced and the durability can be increased. During the cooling-down process (from the welding heat, for example), titanium reacts at its surface, even in the solid state, to oxygen and still leads to the known temper colors at around 300 °C. The hazard, besides the change to the surface, lies particularly in the diffusion of gases into the material.”