Key takeaways
- The fixture, not the welder, sets the position of a welded joint, so fixture design is a quality decision.
- Cross wire resistance welding suits similar wire diameters and clean, flat contact; a large ratio needs a different joint.
- Welding softens the heat affected zone on cold drawn wire, so keep the joint away from the highest stress.
- Inspect position, nugget, strength and distortion as separate checks rather than one visual pass.
A welded assembly is not a wireform plus a weld
A single formed wire part is defined by its bends. A welded assembly is defined by the relationship between its parts. Two wires can each be inside their own tolerance and still produce a frame that is twisted, because the weld pulls the joint and the fixture allowed it to move.
That is why a welded assembly has a different drawing and a different first article. The drawing needs a datum scheme for the assembly, not just for each wire. The first article needs the joint positions, the squareness of the frame and the weld result, not only the bend dimensions of the components.
Joint types and where each fits
Cross wire resistance welding is the default for cages, grids and frames. Two wires cross at right angles, the electrodes press them together and a current pulse forms a nugget at the contact. It is fast, it needs no filler and it suits similar wire diameters.
A T joint welds the end of one wire against the side of another. It is common in frames and baskets, and it is less forgiving than a cross joint because the current path and the contact area are less symmetrical. A projection or a formed flat on the end improves the contact and the repeatability.
Parallel lap joints, where two wires run alongside each other, need a wider electrode or a projection. A simple cross wire setup will not make a consistent nugget along a parallel contact. For thicker wire or a structural joint, MIG or TIG welding with a filler may be the better route, at the cost of heat input and distortion.
- Cross wire resistance weld: grids, cages, frames with similar wire sizes
- T weld with a formed end: frame corners and basket rims
- Parallel lap with projection: rails and stiffening wires
- MIG or TIG: thicker wire, structural joints, dissimilar sections
- Brazed or crimped: heat sensitive parts and serviceable joints
Design rules for the joint
Keep the wire diameters close. A cross wire weld is happiest when the two wires are within about a 2:1 ratio. Beyond that the thinner wire reaches welding temperature first and the nugget forms mostly in the thicker wire, which weakens the joint. If the design needs a large ratio, use a T joint with a formed end or a different process.
Put the weld on a straight run, not on a bend. A bend already carries forming strain, and the weld adds a heat affected zone on top of it. Keep at least one wire diameter of straight between the bend and the edge of the nugget, and more if the joint carries load.
The contact faces need to be clean and flat. Mill scale, drawing lubricant, plating and oxide all raise the resistance and make the weld inconsistent. If the wire is galvanised, the zinc has to be removed at the joint or the weld will be porous and the fumes need extraction. Specify the condition of the wire at the joint, not just the finish on the finished part.
Think about electrode access before the geometry is fixed. An electrode needs a clear path to both sides of the joint, and a wire that crosses close to another feature can block it. A joint that cannot be reached by the electrode will be made with a longer arm, a higher current and a worse result.
Fixture strategy
The fixture holds the wires in the relationship the drawing asks for, and it has to survive the welding current without becoming part of the circuit. Locating pins and clamps set the position. Copper or brass backup bars carry current away from the joint and stop the part from marking.
Design the fixture around a clear datum. The first wire should sit against a fixed stop, the second should be clamped to it, and the weld should be made without the operator holding either wire.
Allow for thermal expansion and weld shrinkage. The joint contracts as it cools, and a frame with several welds can pull out of square if the sequence runs from one end to the other. Weld in a balanced sequence, from the centre out or in a pattern that distributes the shrinkage, and check the first off part before running the batch.
Setting and monitoring the weld
A resistance weld is set by three variables: current, time and electrode force. The force has to be applied before the current and held until the nugget has formed. A machine that drops the force too early produces a weld that looks acceptable and fails under load.
Set the weld on the actual wire lot, not on a generic chart. Surface condition, plating and wire diameter all change the resistance, and a setting that works on bright wire may not work on a coated lot. Record the settings on the setup sheet and keep them with the fixture.
Monitoring is what separates a controlled process from a hopeful one. Weld current and time monitoring on the machine catches a drift before the parts reach inspection. A destructive peel or torsion test on a sample from each shift proves the nugget, and the failure should be in the parent wire, not at the weld interface. A weld that peels cleanly away from the wire has not formed a nugget.
Distortion, straightening and finishing
Welding heat pulls a frame. The amount depends on the number of joints, the wire size and the sequence. A light frame may need straightening after welding, and straightening is a manual skill that adds cost and variation. Design the frame so that the critical faces are set by the fixture and the non critical faces can be adjusted.
Stress relief after welding can recover some ductility in the heat affected zone, but it also softens the cold drawn wire and can change the spring rate of any formed feature. If the part is a spring and a weldment at the same time, the heat treatment needs to be agreed with the load requirement in view.
Finish after welding, not before. Plating a wire and then welding burns the coating at the joint and leaves an unprotected area. For a coated assembly, weld first, clean the joint and then coat, with a mask on any thread or contact surface. If the parts must be plated before assembly, the joint needs a touch up that is defined and inspected.
What to inspect
A welded assembly needs four kinds of check: position, nugget, strength and distortion. They answer different questions and none of them replaces another. A visual pass can confirm that a weld exists, but not that it holds or that the frame is square.
| Check | Method | When |
|---|---|---|
| Joint position | Fixture gauge or vision measurement against the assembly datum | First off and at each setup |
| Nugget size | Section a sample and measure the nugget | At the trial and on a periodic sample |
| Joint strength | Peel or torsion test to failure, with the failure location recorded | Each shift or as agreed |
| Distortion | Squareness and flatness on a surface plate or fixture | First off and at the end of the run |
| Heat affected zone | Visual check for discolouration and cracking at the bend | Every part, or a defined sample |
| Coating coverage | Thickness and visual check at the joint and masked areas | After finishing |
| Fixture wear | Pin and clamp check against the wear limit | At each setup |
The frequency should be agreed at the first article and written into the control plan, not left to the operator.
What to put on the drawing
Give the assembly a datum scheme and dimension the joint positions from it. Identify each joint with a number so that the inspection report and the weld map use the same reference. State the joint type, the wire diameter at the joint, the finish condition and any strength requirement.
Ask for a weld map on the first article: a drawing marked with the joints that were tested, the test method and the result. For a frame with twenty joints, a single test result does not prove the assembly, and a map shows which joints were sampled and which were not.
- Assembly datum and the dimensions that locate each wire
- Joint number, type and wire diameters at the joint
- Minimum nugget size or strength, with the test method
- Wire condition at the joint, including any coating removal
- Post weld treatment, straightening allowance and finish
- Squareness or flatness call on the faces that mount
Frequently asked questions
Can galvanised wire be resistance welded?
It can, but the zinc has to be treated as a process problem. Zinc has a low melting point and contaminates the electrode and the nugget, and the fumes need extraction. Remove the coating at the joint, use a suitable electrode and check the joint strength more often than on bright wire.
What wire diameter ratio is suitable for a cross wire weld?
Keep the two wires within about a 2:1 ratio as a starting point. A larger ratio concentrates the heat in the thinner wire and makes the nugget uneven. A T joint with a formed end, or a different welding process, is usually better beyond that.
How do I inspect a weld without destroying the part?
Use a fixture gauge for position, a visual check for the nugget and discolouration, and weld current monitoring for consistency. Strength is proved by a destructive peel or torsion sample from the same setup. Non destructive strength testing of a small wire joint is limited, so the sample has to represent the batch.
Does welding weaken the wire?
The heat affected zone is softer than the cold drawn wire around it, and it can be more brittle if the cooling is fast. Keep the joint away from the highest stress and consider a stress relief if the part is cycled. A bend next to a weld is the usual place a crack starts.
Sources and further reading
- ISO 14554-1, Resistance welding, quality requirements for welding of metallic materials
- EN 10270-1, Steel wire for mechanical springs, patented cold drawn unalloyed spring steel wire
- Qumek fixture design and weld inspection notes for wire assemblies
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