Resistance Spot Welding

Characterization of Resistance Spot Welding

Resistance Spot Welding: an economical joining process in sheet metal processing

Resistance Spot Welding ( Copyright Fronius International GmbH)

Resistance Spot Welding (often just called spot welding) is a cost-effective joining technology that continues to be very popular in the automotive industry where high reproducibility and automation are essential criteria. An additional advantage is that, with a suitable choice of parameters, a high quality of joints can be obtained without being dependent on a welder’s ability.

Resistance Spot Welding is a pressure welding process. The sheets are pressed together locally with the help of fitted copper electrode welding guns that then have a current applied to them. As a result, Joule heating locally melts the joining partners at the junction resulting in a cohesive composite once the joint has solidified.

Typical Industries and Fields of Application

Spot Welding of a front end (Source: KUKA)

RSW technology is used in following industries:

  • Transportation industry
    • automotive construction industry (for body parts and frames, add-on parts such as doors & tailgates etc.)
  •  Sheet metal forming industry

Trends and Developments

Series production of automobiles, welding robots (Source: KUKA)

For over 150 years resistance spot welding has proved itself as the dominant technology when joining thin metal sheets. In particular, the automotive industry continues to use resistance spot welding as its main joining process, despite strong competition from the processes of beam welding, adhesive bonding and mechanical joining. The main reasons for this is that resistance spot welding offers high productivity at a low cost and is therefore, likely to maintain its position into the future. Major trends and developments in resistance spot welding are:

  • The optimization of welding processes. Also if it comes to welding of aluminum alloys, high strength materials and new coatings
  • Usage of new high-performance robotic welding guns
  • New self-regulating process control, which can also cover an increased range of material thickness combinations
  • The use of control devices for quality assurance and a reduction in the resources needed to inspect parts and processes.
  • Investigations of hybrid welding processes, such as spot-weld bonding
  • Achieving consistent quality by using spot welding systems on a circulating process belt
  • Investigations of spot welding of CF components by the use of local metal structures

The development of welding processes with a low energy input, computational welding simulations and an increase in training using a virtual welding trainers, have all contributed to making joining technologies as energy-efficient and material-friendly as possible.

 One way to preserve the welding expertise within a company and to reduce trial costs is to use a welding simulation software that offers both structure and process simulations. This allows the potential process parameters to be virtually and investigated with appropriate parameters being documented.

 (based on " In Focus - resistance welding in the DVS " German)

Challenges in resistance spot welding

Proving the weldability of structures requires a detailed plan of the weld reliability (construction), the weldability (material selection) and the ability to weld the pieces (manufacturing).

The identification of optimal process parameter windows is particularly important. These often include: optimal current, clamping forces and process times, the interpretation of weld guns, influences of coatings, radii of curvature and influences of weld gun misalignment (e.g. due to the electrode reworking and wear). These parameters are of high importance as they are responsible for quality assurance and enable the process approval.

In addition, the design of the workpiece, the material selection and the manufacturing process also interact with each other - especially with respect to resulting welding distortions that can occur.

Besides reducing the strength of welded assembly, welding distortions are economically the most serious problem in the design of welding processes.

Assemblies often require reworking with straightening operations that not only incur high costs, but are also generally difficult to calculate in advance. These processes can also lead to a high amount, and therefore cost, of defective parts. In addition to this the welding process adversely affects the optimal use of materials. As more material is used, more weight is added, and therefore more space and higher costs are necessary.

What are the causes of welding distortions?

Welding distortions arise:

  1. Because of a shrinkage of melted filler material
  2. Because of an upsetting of material due to heating

Plastic upsetting is a highly non-linear material behavior. Inconsistent thermal expansion of the heat affected zone leads to non-homogeneous plastic deformation through the cross section of a weld. The mechanism behind this effect is used by flame straightening. Thermal expansion and shrinkage lead to residual stresses that cause further distortions and stress redistribution through the cross section. The component takes its initial shape if no plastic deformations are present during this process. Due to changed material properties, such as yield strength, high temperatures and even small stresses lead to plastic deformations. Additional restraints by clamping or the shape of a component itself are able to increase the residual stresses.

As a result of the temperature field, there is asymmetric behavior during heating and cooling triggering the development of plastic deformations. Three main mechanisms are to be noted:

  1. Plastic deformations during heating are of a local nature. During cooling, the  surrounding material experiences effects of heating by heat conduction from the weld pool, generates pressure towards shrinking material, and permits a partial regeneration of plastic deformations
  2. The elasticity of surrounding material is reduced
  3. Plastic deformations during heating appear at positions where tensile strength is reduced and thermal expansion is increased. Compressive stresses are reduced during cooling, which leads to plastic deformation of highly shrunk material at lower temperatures and higher tensile strength. After heating and cooling, if there are plastic deformations observable, residual stresses remain in the structure. Those residual stresses lead to reduced fatigue strength.
Distortion during resistance spot welding - simulated with Simufact.welding

Typical tasks of resistance spot welding simulation

Until recently, mathematical heat source descriptions were used in Simufact Welding. This type of welded structure simulation is used to minimalize the amount of prototypes needed when examining the potential welding distortions in more complex geometries. For the automotive industry in particular, resistance spot welding is a central joining processes along with laser welding. Both welding processes have been taken into account in Simufact.welding version 5.0 and higher. In order to guarantee Simufact’s typical user-friendly handling while resistance spot welding several welding points of complex geometries at once, the necessary complex weld gun kinematics have been integrated for both the C- and X - welding gun in a robot definition.

Pain Points - welding distortions

Quality assurance of individual joining points

Another typical task for resistance spot welding is the quality assurance of individual joining points. A car usually contains between 4000-5500 weld points with between 200 to 400 different material thickness combinations. Process windows can be determined and approved regarding the weld gun power, current and process time (among other parameters) for each material thickness when combined with the corresponding coatings in a production process. Other influences such as the radii of curvature for complex geometries and shunting effects can also be investigated with Simufact.welding.

How can Simufact.welding provide support during the design of a welding process?

Understanding of a process as well as reduction of development loops by

  • Visualization of values influencing the process, especially temperature distribution, residual stresses, and deformations
  • Virtual try-out of clamping, welding sequences, intervals, unclamping times, effects of preheating, as well as variation of materials

Welding sequences and intervals are crucial if temperatures between layers are important. For mass production, short cooling times before unclamping are desirable in order to increase the output. The influence on welding distortions is usually a matter of seconds.

A further advantage compared to experimental investigations is the possibility to study different processing setups before there has been money invested in welding equipment.

Our Welding Simulation Solution

The product line Simufact.welding

Simufact.welding offers the possibility to calculate welding stresses, distortions, and the evolution of material properties from a graphical user interface. This means:

  • You can investigate possible problems up front
    • Identification of critical distortions, i.e. with respect to assembly, bulging, imbalances, and clearances
    • You gain confidence about maintaining tolerances
  • More insight into welding processes
    • You can create a basis for  construction design and development of welding processes and manufacturing, which together lead to optimized construction
    • You have a tool which supports you during planning of welding processes
    • You are able to gather and preserve experience and results from real and virtual try-outs, as well as retain corporate knowledge
    • Make use of a powerful tool for development and training
  • Methodical optimization of processes
    • You can plan the position of welding seams as early as during the design phase, which will lead to minimization of distortions based on construction design. Thus, you can also minimize the influence of a welding operator as well as welding equipment on distortions
    • You can investigate and optimize clamping tools even before an investment in tools has been made
    • You are able to identify optimized welding directions and welding sequences
    • You can investigate the influence of unclamping on welding distortions and residual stresses
    • Use a tool which supports you during planning of welding processes
    • Use virtual try-outs which would be very expensive in reality
    • Investigate the behavior of new materials during welding
  • Verification of quality of welding seams
    • Use a tool which helps you to verify the quality of welding seams, i.e. by calculation of brittle metallurgical phases, hardness, and effects of preheating.
    • Gain knowledge about the development of the heat affected zone
    • Draw conclusions about several properties of a welding seam (i.e. residual stresses that have effect on fatigue strength and bulging), convert your results to an open format (Universal File Format), and use them for further calculations in other FE simulations
  • Investigate process chains that appear during manufacturing

 

Economic benefits of faster welding process design

  1. High efficiency of the development process due to a reduced number of expensive failed attempts
  2. Decreased expenses of manufacturing of prototypes
  3. Reduction of machining and straightening costs
  4. Reduction of development times which leads to shorter time-to-market
  5. Decrease of material and energy consumption for experimental investigations
  6. Reduction of manpower needed for experiments
  7. When bidding on a project, efficient feasibility studies lead to winning offers

Please take a look at the product description for an overview of functionalities of Simufact.welding:

Product description Simufact.welding