Factors affecting weld quality

The following factors can have a large influence on

the final outcome of the spot weld:

1 The type of metal being welded (for example,

coated metals)

2 Joint configuration

3 Current and timing settings

4 The type of welding equipment being used

5 Electrode maintenance.

Clearance between welding surfaces

Any clearance between the surfaces to be welded

causes poor current flow. Even if welding can be

made without removing such gaps, the welded area

would become smaller, resulting in insufficient

strength. Flatten the two surfaces to remove the gaps,

and clamp them tightly with a clamp before welding.

Metal surface to be welded

Paint film, rust, dust, or any other contamination on

the metal surface to be welded causes insufficient

current flow and poor results. Remove such foreign

matter from the surface before commencing welding.

Corrosion prevention on metal surfaces

Coat the surface to be welded with anti-corrosion

agent that has a high conductivity (e.g. zinc primers).

It is important to apply the zinc primer uniformly

between the panel surfaces to be welded.

Minimum welding pitch

The strength of individual spot welds is determined

by the spot weld pitch (the distance between spot

welds). The bond between the panels becomes

stronger as the pitch is shortened, but if the pitch

becomes too small and the spot welds are too close

together this can allow current shunting, which in

effect drains away the weld current towards the

previous spot weld (Figure 10.15).

Positioning of welding spot from the edge

and end of the panel

The edge distance is also determined by the position

of the welding tip. Even if the spot welds are

normal, the welds will not have sufficient strength

if the edge distance is insufficient.

Table 10.1Recommended tip pressures according

to the gauge of steel

Gauge of panel Pressure Length of arm

(mm) (kg) (mm)

0.55 _ 0.55 50 250

0.80 _ 0.30 60 250

0.95 _ 0.95 70 250

1.00 _ 1.00 80 250

1.25 _ 1.25 100 250

1.25 _ 2.00 120 250

These pressures relate to HS steels: a reduction in pressure may be

necessary for conventional steels

Figure 10.15Current shunting in resistance welds

positioned in close proximity

Inspection of spot welds

Spot welds are inspected either by outward appearance

(visual inspection) or by destructive testing. A

visual inspection is used to judge the quality of the

outward appearance (tip bruises, pin holes, spatter,

number of spots, spot positions), while destructive

testing is used to measure the strength of a weld.

Most destructive tests require the use of sophisticated

Electric resistance welding 287

Figure 10.17Modes of spot weld failure (Motor

Insurance Repair Research Centre)

equipment which most body shops do not possess.

Consequently a simpler method called a peel test has

been developed by general use in the workshop.

Destructive testing: peel test

After setting up the equipment a test weld should

be made, using sheet metal of the same thickness

and condition as the job to be done. When the test

piece is completed, it should be torn apart. If the

settings are correct, the joint will ‘unbutton’, that is

the weld nuggets will be on one sheet and the holes

on the other (Figure 10.16(a)).

Weld tests for shear and peel for HSLA steels

When shear testing spot welds produced when welding

together two HSLA steels, partial spot-weld failure

may occur. If the partial failure does not exceed

20 per cent of the total nugget area, the strength of

the weld should not be impaired (see Figure 10.17).

10.9 ARO Spotrite Pulsa resistance

welding system

For resistance spot welding in vehicle body repair

there is now a British invention called the Spotrite

(made by ARO Machinery Co. Ltd). This is an

adaptive (self-setting timing) in-process control

unit which operates in several stages. Stage 1 is the

monitoring stage for recognizing the material (lowcarbon

or HSLA steel) and for sensing through any

Figure 10.16(a) Peel test (b) destructive test (Motor

Insurance Repair Research Centre)

protective coatings, whilst judging how closely the

clamped materials are fitting between the electrodes

which will form the joint. Stage 2 is automatically

linked and self-setting to provide a burst of current

to produce the weld nugget. Stage 3 (and 4 if

needed) will provide further precise timed pulses

to bring the weld to the desired size, taking into

account the number of sheets of metal and their

thickness and the distance of the weld from the edge

of the workpiece. The adaptive part of the control

decides the number of stages needed and their

duration. A weld cycle which includes all these

stages added together, will take less than 0.75 s.

288Repair of Vehicle Bodies

With this equipment much of the technology is

transferred from the operator to the controls, only

because the operator has no means of knowing which

type of material he is welding. The control monitors

what the welding gun is doing, and uses the information

to take the decision by computing variables such

as electrode tip wear, spot-weld pitch, voltage fluctuation,

changes in metal thickness, and the presence of

scale, dirt and rust. Consequently quality control is

no longer the operator’s responsibility.

Audiovisual signals warn the operator if there is

overheating, if the electrode tip needs reshaping or

cleaning, and if the weld arms are open too early.

For difficult or unusual work, the experienced

body repairer can switch to manual setting to control

the unit’s weld heat and time tolerance controls

(see Figure 10.18).

Figure 10.18CEBORA spot welder (Mig Tig Arc)

Figure 10.19Pincer welding gun used in different

locations in repair work (Motor Insurance Repair

Research Centre)

A pincer-type welding gun with a powerful weld

transformer (to maintain the very short weld times)

is fitted with a strong pressure system to deal with

the majority of work, which can be approached

from two sides in pincer-type fashion. A range of

interchangeable weld arms and electrodes of different

shapes and lengths can be used to convert

the standard gun instantly into the special tool

that a job may require (see Figure 10.19). Special

Electric resistance welding 289

5 Test earth leakage unit by pressing test button.

6 Set spot size to zero.

7 Make splashless welds by selecting the lowest

heat and the longest time setting to make the

desired size of weld nugget.

8 Make two welds approximately 51 mm apart.

Peel test the second weld, i.e. tear the test piece

apart and check whether the nugget is the size

approximately to the weld strength needed. The

nugget size should be 4.5_t.

9 Increase the spot size setting from zero in single

steps, and make five trial welds in each setting.

Figure 10.20Electrode arm sets for different makes

of vehicles (SIP (Industrial Products) Ltd )

sets of arms with electrodes, specifically designed

for use on individual makes of vehicles, have been

designed in collaboration with most car manufacturers

(Figure 10.20).