Assembly and forming residual stress on an automotive part can have negative effects on the product’s performance. The above simulation depicts such residual stresses. (Image courtesy of ESI Group.)
When optimizing a product, it’s easy to forget how the manufacturing process can impact the design’s performance.
Therefore, to properly optimize a product’s design, engineers need to understand how that product will be manufactured and how these processes might create unexpected stresses and strains in the final product.
“Virtual Prototyping enables design and process engineers to virtually manufacture and assemble components long before their physical prototypes are built and tested,” said Yannick Vincent, Product Manager at ESI Group. “It facilitates the creation of countermeasures to control distortions and residual stresses in structural assemblies. It also shortens time to market and minimizes the cost of manufacturing planning, tryouts and fabrication validation.”
ESI Group’s SYSWELD is a computer-aided engineering (CAE) platform that studies the effects of stamping, welding and assembly on a manufactured product during the design phase. The solution helps engineers simulate and mitigate the residual stresses, distortions and defects by modeling the chained manufacturing process.
“The goal is to identify the critical joints before a prototype is built while minimizing the cost for manufacturing planning, try-out and fabrication validation and significantly shorten time to market,” explained Saurabh Aggarwal, manager of Worldwide Business Development at ESI Group.
How Manufacturing Can Damage Products
It can be a long, hard and expensive journey designing a product and its production chain. Engineers will need to tweak the product and its production process multiple times before all the residual stresses, distortions and dimensional inaccuracies are within specification.
Welding will distort a product. Clamps can pre-bend a part in the opposite direction so that part geometry will be within tolerance after the welding process. By using simulation, an engineer can determine how much that pre-bend should be, instead of relying on instinct and experience. RPS: Reference Point System. (Image courtesy of ESI Group)
“During the forming process, plastification leads to the material hardening and thinning. Unloading also creates spring-back. These all lead to residual stresses,” explained Aggarwal. “In the case of hot forming of steel alloys, phase transformation takes place. All these effects have a significant influence on the structural behavior of parts during cold and hot joining processes in welding and assembly body shops.”
“In other words, mechanical load and heat effects of the welded assembly process modify material characteristics and introduce residual stresses, which leads to dimensional inaccuracies in the welded assemblies,” clarified Aggarwal.
Traditionally, these structural influences on the product were mitigated with manufacturing and engineering gut-feeling modifications to the production processes. However, this level of trial and error isn’t efficient or accurate.
For instance, engineers who are aware that a part will deform after a weld might set the clamps to bend the part in the opposite direction to compensate, or they might change weld sequences. However, this pre-bending and welding process plan can become an expensive iterative solution.
“The optimization of the clamping tools without simulation is a very difficult task— more clamps mean that more stresses are conserved in the structure, which is completely contrary to the goal,” said Vincent. “The less clamps, the more distortion develops, but the tolerances need to be kept. ESI’s Welding & Assembly Solution helps process engineers find quickly the best compromise.”
CAE tools like Welding & Assembly Solution can iterate through fixes to the production issues faster and without costly physical trials. This helps engineers better predict and correct the deformations. Simulation makes it much easier to sequence welds and pre-bend a clamped part so that the part will fit within specification once the welding is completed.
How to Virtually Model a Production Process with ESI Group’s CAE Technology
First, engineers can use ESI PAM-STAMP to simulate the forming of parts from thin metal sheets. The output from the simulation tool is plastic strain, stresses and variable thickness, along with the deformed mesh. This information and the post-stamping deformation mesh are then imported into the ESI’s Welding & Assembly Solution.
“Sometimes these distortions cannot be reduced as expected and parts have to be compensated. In this case, design engineers must compensate for the deformed shape on the welded assembly side because doing so on the stamping side would be either very costly or highly impractical,” explained Aggarwal.
ESI’s Welding & Assembly Solution simulates the stamping, prepositioning, holding and joining of parts. (Image courtesy of ESI Group)
“ESI SYSWELD imports the deformed mesh after stamping,” said Vincent. “Stamped key data results, such as plastic strain, stresses and variable thickness, are mapped. Then, subsequent simulations for prepositioning, holding and joining are performed. To chain simulations, key data and results must be transferred and mapped from stamping to prepositioning to clamping and finally to the welding simulation model. In order to reduce the time needed to perform such operations, CAT (Control Adapt Transport) is available.”
ESI SYSWELD provides a dedicated workflows to include every design feature of a welding assembly process (pre-positioning, holding, and joining chained manufacturing)
The prepositioning advisor helps engineers create a reference point system (RPS) and ensure that components are placed within the RPS so that gaps are within geometric tolerances. The user begins this process by importing a component and data points (plastic strains, stresses and variable thicknesses). The user can then create guides, locators and clamps that help to map out the RPS.
The next tool is the holding advisor. It is used to define the systems that will clamp components into place. The tool defines how the clamps will close the gaps between the components before they are joined. Engineers have access to various parameters that define the type, shape, number position, offset and sequence of clamps. Engineers can tweak these parameters until they are optimized to limit product stress and deformities.
“Finally, user can export the deformed mesh, residual stresses and plastic strain from stamping, prepositioning and holding processes to simulate the welding operation with the defined weld properties,” said Vincent. “Depending upon the case, seam (laser and arc welding) and spot welding effects can be taken into account.” This simulation determines the final distortions and residual stresses from the chained stamping and welding & assembly processes.
After assessing the manufacturing of the product, the user can employ further simulations to explore the product’s durability and performance in the field.
“In a nutshell, structural distortion, residual stress, plasticity behavior and phase transformation of hot and cold joined assemblies are predicted during chained manufacturing processes,” clarified Aggarwal.
“The change of thermal, metallurgical and mechanical properties in a structure during chained manufacturing is thoroughly analyzed and countermeasures are taken to control the distortions and residual stresses,” he added. “The benefits include reduced costs in both design and manufacturing as physical prototyping and testing is dramatically reduced.”
To learn more about how ESI can incorporate production into simulations, click here.
ESI Group has sponsored ENGINEERING.com to write this article. It has provided no editorial input. All opinions are mine. —Shawn Wasserman