It’s All About the Combination – Welding and Forming in One

Steinbeis experts develop process technology for the automated arc welding and forming of high-strength steel structural components

Electric vehicle trends, and increasingly stringent requirements regarding the reduction of pollutants and carbon emitted by industrial products, are spawning a growing number of industrial lightweight construction applications. Using high-strength steels is now paving the way for cost reductions, especially compared to fiber-reinforced or metal-based materials. Lightweight and energy-efficient materials – in increasing demand from industrial customers in vehicle, rail vehicle, and container construction – will see the introduction of functionally reliable component configurations. There are now a variety of thermal joining and forming processes available to industry, with every potential to play an important role in achieving functional multi-material designs. As part of a joint R&D project with IWC Engineering, the experts at Intelligent Functional Materials, Welding and Joining Techniques, Implementation, a Steinbeis Innovation Center, have developed combined process technology for the automatic welding and forming of high-strength steel structural components. Welding heat energy generated within components can be harnessed as forming heat to carry out the molding of high-strength steels, with precisely controlled component cooling.

There are a number of ways to manage the weldability of high-strength steel materials these days. When materials heat up to high temperatures, however, they are prone to structural changes, making it much more difficult to carry our subsequent processing. Cooling rates after the welding process must be defined and determined extremely accurately in order to ensure the material properties of the weld seam are similar to those in the rest of the component made from the base material. In particular, martensitic structures dissolve in the zone around the weld seam and the so-called heat-affected zone (HAZ). This results in the formation of brittle carbon accumulations, which also makes the material more brittle and reduces fracture elongation. As a result, engineers often attempt to tailor welding heat energy to the specific component and concentrate energy in order to produce defined seam surfaces and prevent temperatures negatively influencing component properties.

The goal: Reduce costs and process times

The aim of the research project, which was conducted by the Steinbeis experts alongside IWC Engineering and was funded by the Federal Ministry of Economic Affairs and Climate Action (BMWK), was therefore to develop a combined process technology that makes intelligent use, or re-use, of welding heat energy. In addition to defined component properties regarding strength and hardness, the developed technology should produce a fine-grained microstructure zone on components with lower residual stress – and avoid negative influences on welding. This allows welded dual-phase steels to be formed later on in processes, offering a variety of advantages in terms of materials and methods.

A workstation created for the project – which includes optimized component feeding/handling technology, measurement and control technology, and component heating/cooling modules – can be used to combine two aspects of the component production process: welding and forming.

Given rapid growth in the volume of these kinds of high-strength steel materials now in use, there is a strong need to manage and improve weld seam properties by forming the weld seam and its heat-affected zone. To carry out their development work, the project team investigated ways to adjust seam properties and optimize the degree of forming of a complete component, with particular focus on material process technology, before verifying the process by producing application components. To do this, they developed a process technology based on combined arc welding and forming, which was then used to produce finished components with defined mechanical and technological properties. This met several scientific and business objectives:

The combination of technology used in two sub-processes – tungsten inert gas (TIG) welding and active media-based material reforming using welding heat energy – in order to carry out forming in keeping with quality requirements, also according to defined molding temperatures, heat transfer conditions, and component cooling parameters
Reliable modular monitoring of controls and regulation for the entire process based on an efficient component production process chain, including optimized component feeding and handling technology
A forming tool module with localized temperature controls, cooling, and a suitable thermo-mechanical modular control and regulation system, not only for monitoring the active media-based forming process, but also to ensure component properties comply with quality requirements
Achievement of the defined technological and mechanical component properties in high-strength dual-phase steels and overall component structures using the developed process technology and its technical parameters

Determination of process parameters based on optimum welding heat generation and distribution on the overall component in order to define the forming process

The developed process technology was designed so that less effort is required to effectively link individual sub-processes within existing process flows. The aim of the project was to use combined process technology to achieve good formability, not only for the weld seams and the HAZ, but also for overall component structures. This should make it possible to produce high-quality component configurations and thus reduce costs, shorten process times, and minimize the number of process steps required for the thermal joining and forming processes of high-strength steels. In addition to determining the necessary process functionalities, the project team also ascertained the features and data required for the arc welding (TIG welding process) and internal high pressure forming of high-strength steels, and these have now been realized in terms of process technology. In addition, a list of requirements has been created for materials and processes. This is not only applicable to the welding process technology, but also to heat input, heat dispersion within the component, material specifications, and test component geometries.

Based on materials engineering design studies on process development, the welding process technology and subsequent forming was developed and implemented. “We’ve produced and studied welded components with a defined weld seam geometry under different component heat inputs and cooling conditions. We took the component heating procedure resulting from the arc welding technology and used that for the subsequent forming of welded components,” explains Steinbeis Entrepreneur Dr.-Ing. habil. Khaled Alaluss. Dual-phase steel, DP600, was used to produce component assemblies as a finished product. This was based on temperatures ranging from 723 to 880 °C. To conduct thin-to-thick, thin-to-thin, and thick-to-thick welding, the experts used the welding technology to join pipes of different sizes (diameter 40 mm, wall thickness 1.5 and 2 mm), and these were then formed under varying component cooling conditions and heat energy outputs. Finally, the properties of the welded component assemblies were characterized using metallographic analysis and mechanical material testing, looking at criteria such as hardness, microstructures, component shape changes, the absence of cracks, and material strength. The achieved results met both the defined component requirements and technical parameters.

Demonstration parts pass the test

Property characterization of welded/formed component assemblies in the welding seam and HAZ area: a) Verification of the achievable strength of welded/formed component assemblies b) Verification of the absence of cracks on component assemblies using the red-white method

The project team examined the produced component geometries for external defects, dimensional/form accuracy, cracking, and mechanical/technical properties. They also extracted segments from samples by making cuts along two axes. Measurements were taken in different areas of the surfaces cut out of the demonstration parts to assess the thickness and radii of tube walls. The wall thicknesses at both measuring points correspond to that in the area of the forming. “With wall thickness, you have to take into account that dimensions can change due to forming, and there might be scale formation due to the molding temperature. We found that the strong flow properties achieved at the defined molding temperature resulted in higher levels of malleable deformation, with a high degree of reshaping and a component design free of cracks,” confirms Alaluss. This has a positive influence on the remolded radii of the component, and a precise geometry could be mapped for component dimensioning and forming. There were small variations in radii, not only depending on pressure levels (500/700 bar), but also as a function of the reference component or tool shape. There were also minor variations in wall thicknesses along the entire length of components, and this decreased slightly with increasing pressure. Subsequently there were no component cracks with only minor variations in component thickness.

Property characterization of welded/formed component assemblies in the welding seam and HAZ area: a) Hardness plots b) Microstructure formation. The results show that the Vickers hardness values measured over the entire component depend on component thickness and forming parameters (pressures: 500/700 bar) – and that they are high. In the HAZ and base material area, typical martensite fractions were formed on all of the investigated samples, although their morphology depends on cooling conditions. Fine or coarse microstructures (second phase as structures like islands [martensite/bainite]) were formed at the grain boundaries.

The component strength values achieved with welded and formed parts using tube-to-tube welding and subsequent molding were excellent. Yield values of 694 to 882 MPa were achieved on formed pipe parts with different wall thicknesses based on maximum stress levels of 18 to 20 kN. The fractures occurred in the base material area of all tested samples. The formation of microstructures (martensite/bainite) at higher temperatures leads to an increase in the yield strength of materials compared to lower temperatures. The hardening behavior of the high-strength steel material has a significant influence on the formation of component structures and material flows. Finally, the tests also showed that unlike samples shaped at room temperature or at lower forming temperatures (under 400 °C), there were no cracks or fractures in the weld seam area, in the HAZ, or in the base material area.

The project was a success for the team: The produced demonstration samples were of sufficiently high quality and thus confirm the suitability of both the developed process technology and the forming process for the production of high-strength structural elements to be used as finished components. The process technology also proved its suitability in practical terms. The mechanical and technological properties of the produced component assemblies were free from errors, reproducible, and of the required quality.

This project is funded by the Federal Ministry for Economic Affairs and Climate Action following a resolution passed by the German Bundestag.