Weld Designing

< Key Principle > To make weld designs and choose material combinations to balance the heat generation with a focus at the weld interface.

< Real Problem > Impossible to see heat generation inside the weld, thus difficult to make good weld designs to obtain the best heat balance.

< Digital Solution > Visualize heat generation and predict weldability by simulations with all weld design options and material combinations.

In weld designing, the focus is on the geometric design options and selection of materials for each weld part. Very often, the positions of weld points and also the distance between weld points are also important to be optimized and determined in order to obtain the best quality of welded structures.

By welding simulations with SORPAS, the geometric designs of weld parts can be modeled with flexible selection of material for any part with any material freely picking from the list of all materials in the integrated material database. In this way, all possibilities of geometric designs and material combinations can be simulated virtually on the computer. The optimal solutions of weld designs and material combinations can be obtained based on the results of simulations before running any weld tests with real materials and real parts.

We describe below in further details on the Weld Designing solutions for spot welding, projection welding and mechanical joining applications.

Spot welding design factors and weldability of materials

For resistance spot welding, the most decisive weld design factors are the following:

  • Thickness ratio of the sheets
  • Strength ratio of the sheet materials
  • Resistance ratio of the sheet materials
  • Surface coating properties and conditions
  • Position of the weld close to edge or corner

Every time before a new material is introduced to production with welding and joining, a lot of investigations have to be carried out to evaluate the weldability of the material and feasibility of implementing the material in production. From time to time, material suppliers may be changed even for on-going production at the assembly plants. This often needs evaluation of the weldability of materials from the new suppliers in order to maintain weld quality and production stability.

SORPAS 2D.welding and SORPAS 3D.welding have been used as powerful tool to evaluate the weldability of materials with various ratios of sheet thickness and materials.

resistance spot welding design principle on thickness ratio
The ratio of sheet thickness (thickest to thinnest) needs to be less than 3:1 in resistance spot welding.

One of the most commonly encountered weldability problem in automotive industry is the three-sheet spot welding with a thin low carbon steel sheet and two thicker higher strength steels.

Publication on Welding journal 2011: Three-Sheet Spot Welding of Advanced High-Strength Steels.

SORPAS 2D simulations for weldability of material
Experimental and numerically calculated cross-sectional views of center-point welds in three-sheet spot weld

In recent years, SORPAS has been used to improve the resistance spot weld performance of 3rd Generation of advanced high strength steels (AHSS).

 

Conference paper on 11th International Seminar on Numerical Analysis of Weldability (2015):

Improved Resistance Spot weldability of 3rd Generation AHSS for Automotive Applications

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Overview of the SORPAS thermal-mechanical model showing the sample and electrode geometries, and a close-up of the mesh

SORPAS has been also used to understand liquid metal embrittlement(LME) crack in welding of Zn-coated high strength steel. Many researchers use SORPAS to investigate the factors affecting LME during spot welding such as, temperature and stress at LME sensitive area. It also used to analyze the impact of electrode geometry and different weld schedule to prevent LME crack.

Related publications:

Current density in dome and radius type electrodes by SORPAS 2D.welding
Current density in dome and radius type electrodes in which the current bending is explained (figure from *)

Projection welding design factors and weldability of materials

Unlike in spot welding where the design of electrodes is important for concentrating the weld current to weld zone, in projection welding the design of projections on workpieces is essential to localize the current. The electrodes only supply the weld force and conduct the weld current to the workpieces.

The local projections on workpieces can be designed specially on purpose to concentrate weld current to the weld zone, such as embossment on flat sheet, ring projection on round nut or corner projections on square nut etc. The natural shapes of the workpieces can also be used as projections to concentrate weld current to the weld zone, such as cross wire welding and fusing of round wire to flat connector.

The design factors in projection welding are the shape, angle and height of the projections. The design principle of projection welding is that the design of projections shall be effective to concentrate current, but be stable and not to collapse too fast or cause severe expulsions.

By effective concentration of current with projections on the workpieces, projection welding can be used to weld largely dissimilar materials, which may be impossible to weld by spot welding. By applying strategic surface coatings, it is even possible to weld extremely difficult materials by projection welding, such as copper alloy to steel.

Generally speaking, to obtain the heat generation with the best heat balance at the weld interface, the projection shall be placed on the side which is more difficult to produce heat:

  • In case of similar materials, the projection shall be placed on the thicker workpiece due to larger volume and more mass to be heated.
  • In case of dissimilar materials, the projection shall be placed on the workpiece of material with lower resistance.
  • Likewise, the strategic surface coating usually with higher resistance shall be placed on the material with lower resistance.

The optimal designs of projections and the best choices of strategic surface coating materials, best fitting individual material combinations to be welded, can be simulated with SORPAS2D.welding for projection designs with symmetry, and more effectively simulated with SORPAS 3D.welding for more complex projection designs. The best projection designs can be obtained by comparing the simulated welding results with different designs and material choices.

projection design variations for effective heat generation and stability
The projection design should be effective to concentrate current and not collapse too fast

Mechanical joining design factors and quality of the joints

Mechanical joining is widely used for joining dissimilar materials, and probably the only effective method for joining carbon fiber reinforced polymers to metals. Self-piercing riveting (SPR) and clinching are most commonly used for joining sheet materials in automotive industry.

To achieve the best joint quality, rivet shape and material, die design and material, and tool force should be optimized appropriately. The quality of the SPR joint can be evaluated by the key joint factors, for example, the interlocking length (rivet flaring), minimum thickness of bottom sheet, rivet head height, and effective length of the rivet in the bottom sheet.

SORPAS® 2D.joining can be used to simulate the mechanical joining with various designs and materials of rivets and dies for all combinations of sheet thickness and materials to be joined.

Self-piercing rivet joint diagram
Quality factors of self-piercing rivet joint: interlock distance, bottom thickness, and head height