Setting up a spot welding process

In both automotive production and crash repair, resistance spot welding is the primary process for joining thin sheet steel. Resistance spot welding is a very fast and reliable process that is readily automated and does not require much operator skill. The greatest technical requirement of resistance spot welding is to ensure that the process is correctly set-up in the first instance.

A new resistance spot welding process will be set-up to:

  • Manufacture a component for the first time
  • Manufacture an existing component from a new material combination
  • Repair a component

Despite its apparent simplicity, resistance spot welding is influenced by many local variables, variation in welding machine characteristics and the influence of component geometry and fit-up mean that no universal set of parameters can be applied to a given material combination.

Although many standards exist advising the range of parameters that should usually be applied for specific materials.

When setting-up a new resistance spot welding process optimum parameters must be sought (usually using a standard as a guideline) and the weld quality must be verified, this is done by destructive testing.

Destructive (peel) testing

The main method for verification of spot weld quality is destructive testing, where-by spot welds are torn apart by an externally applied force and the parent metal breaks away from the weld zone. The weld zone is revealed and assessed for it quality, to ensure it is fit for purpose.

Two main methods of destructive testing are commonly applied:

  • Peel test
  • Chisel test

Welds are broken apart in the peel test by clamping one of the welded sheets, typically in a vice. The other welded sheet is gripped by either; pliers, pincers or a roller tool and force is applied in such a way that the weld is loaded perpendicular to its original orientation, thus peeling the weld open (figure 1). A peel test can be performed either manually or be mechanised, in the case of a mechanised test the failure force can be measured.

In a destructive chisel test, a chisel is driven between the sheets separating them and loading the weld. The chisel may have one blade or two separate prongs and the force can be applied manually or via mechanical means. If the chisel test is mechanised the failure force can be measured.

ISO10447 2006 Resistance welding - 'Peel and chisel testing of resistance spot and projection welds' describes the procedure for destructive testing and measurement of resistance spot welds, sections 5.2 and 5.3 give details of how to carry out a peel test and how to measure the weld nugget. The German equivalent is DIN EN ISO 10447:2007-09

Destructive Peel Testing Figure 1
Figure 1. Peel test

Destructive Chisel Test
Figure 2. Chisel test

Assessing weld quality, weld measurement

Destructive chisel or peel testing results in a weld zone that has been torn out of one the metal sheets, the exposed weld area can be measured. This measurement of weld size is typically used as an indication of weld quality, i.e. whether the weld is fit for its intended purpose.

Spot welds are measured using a vernier caliper. As spot welds are not always exactly round two measurements should be taken at 90o to one another. The largest and smallest dimensions of the spot weld should be measured as shown in figure 3. The average weld diameter is calculated as shown below:

Average weld diameter = d1 + d2 / 2
d1 = largest weld dimension
d2 = smallest weld dimension

A minimum average weld diameter is often used for quality control purposes; the weld size is usually related to the sheet thickness. Typical minimum average weld diameters are often 4√t or 5√t, where t = sheet thickness.

Measurement of a spot weld
Figure 3. Measurement of a spot weld

Spot weld failure modes

In conventional low carbon steel most spot welds that are destructively tested will fail as a 'plug' or 'pull out' failure, a plug is characterised by the weld zone tearing clear of one of the sheets leaving a visible protrusion that is easily measured, figure 4.

Higher strength steels and thicker sheets can fail as 'interfaces' or 'partial plugs'.

In the case of an interface failure the weld will shear directly through its centre leaving half of the weld in each sheet and no protruding plug, figure 5.

In the case of the partial plug the weld zone it's self will partially fracture, then the surrounding sheet will also fracture, leaving only a portion of the weld plug protruding from the sheet, figure 6.

An interface failure is also characteristic of higher strength and thicker steels, in the case of an interface failure the weld will shear directly through its centre leaving half of the weld in each sheet and no protruding plug, figure 6.

Figure 4. Plug failure
Figure 4. Plug failure

Figure  5. a: Partial plug failure, b:cross section of partial plug
Figure 5. a: Partial plug failure , b:cross section of partial plug

Figure 6. Interface failure
Figure 6. Interface failure

When measuring the average weld diameter it is important to be sure the correct dimensions are recorded, several factors need to be taken into account.

When measuring a full plug weld failure some of the sheet material may still be attached to the weld zone, this excess material should be folded back to allow accurate positioning of the vernier caliper, figure 7.

Figure 7. a: Cross section showing excess sheet material, b: weld dimensions to be measured, c: correct positioning of the vernier caliper
Figure 7. a: Cross section showing excess sheet material, b: weld dimensions to be measured, c: correct positioning of the vernier caliper

When measuring an interface failure the dimensions of the rough fracture zone should be measured as shown in figure 8.

Figure 8. a: Weld dimensions to be measured, b: correct positioning of the vernier caliper on the rough fracture surface
Figure 8. a: Weld dimensions to be measured, b: correct positioning of the vernier caliper on the rough fracture surface

When measuring a partial plug weld failure, the true weld nugget dimensions are those of the rough fracture surface, this area may be partially covered by the weld plug. In some cases the average diameter of plug area may also be required, this is measured by positioning the vernier caliper at the largest and smallest plug dimensions, in the same manner as for a conventional plug failure, figure 9.

Figure 9. a: Measuring the weld area of a partial plug, b: correct positioning of the vernier caliper on the rough fracture surface, c: measuring the plug area of a partial plug, d: positioning the vernier caliper on the plug area
Figure 9. a: Measuring the weld area of a partial plug, b: correct positioning of the vernier caliper on the rough fracture surface, c: measuring the plug area of a partial plug, d: positioning the vernier caliper on the plug area

Testing frequency

Destructive testing is usually performed when a new welding process is being set-up to verify the weld quality prior to production or repair of a component. In a production line periodic destructive testing of components will always take place. In automotive production lines one auto body will typically be fully destructively tested and all welds measured at a frequency of one car body per week up to one per month, depending upon the individual companies practices.