Welded assemblies: resistance, ultrasonic and laser

Three methods, three material families

Resistance welding is a metal process. Current passes through the parts and the electrodes, and the resistance at the interface generates the heat. It is the method behind spot welds on sheet and welds on wire and strip. Ultrasonic welding is mainly a plastics process, using high-frequency vibration to melt a designed feature at the joint. Laser welding works on both: through-transmission welding for plastics and conduction or keyhole welding for metals.

The three are not interchangeable. A method that needs two-sided access cannot weld a closed box. A method that needs a carbon-black absorbing layer cannot weld a clear part. A method that melts a thermoplastic cannot join a thermoset. The choice starts with the material and the access, then moves to the joint.

Resistance welding: current, force and time

A resistance weld is made by clamping the parts between electrodes and passing current for a set time. The heat is highest where the resistance is highest, which should be the interface between the sheets. Current, electrode force and weld time are the three settings, and the squeeze and hold times around them control the cooling and the indentation.

The electrode tip size and the spacing between welds matter as much as the settings. Current can take a shorter path through an adjacent weld and starve the next one, which is why a weld schedule includes a minimum spacing. Coated and galvanised sheet, aluminium and dissimilar metals each need their own schedule, and aluminium is difficult because its oxide layer and high conductivity resist a stable weld.

Ultrasonic welding: the energy director

Ultrasonic welding of plastics uses a horn that presses the part and vibrates at high frequency, usually in the 20 to 40 kHz range. The vibration is concentrated at a moulded feature, the energy director, which melts first and flows into the joint. The result is a bond that forms in under a second, with no consumable and no added heat.

The energy director is a design feature, not a process detail. A triangular ridge on one face, a matching flat on the other, a controlled interference and a flash trap around the joint are the usual elements. A joint with no director welds poorly, and a joint with too much interference pushes molten material outside the seam. Near-field and far-field welding differ in how far the horn is from the joint, and that distance limits the wall thickness you can join.

Laser welding: transmission and conduction

Through-transmission laser welding joins two plastics by passing the beam through a transmissive top layer and absorbing it in a dark bottom layer. The interface melts and the parts bond under clamp pressure. It gives a clean seam with little flash and low stress, which suits sensors, medical parts and sealed housings. It needs one transmissive layer, one absorbing layer, good contact and a clamp that holds the interface closed.

For metals, laser welding runs in conduction mode for a shallow seam or keyhole mode for deeper penetration. It is fast and precise, and it can be automated, but the fit-up has to be tight and the gap at the joint has to be small. A laser will not bridge a gap the way a resistance weld can tolerate a poor fit.

The comparison on one page

The table sets out the practical differences. The joint preparation and access rows are the ones that decide whether a method is possible at all.

Joint design for each method

For resistance welding, the joint is an overlap with enough flange for the electrode and enough spacing between spots. For ultrasonic welding, the joint is a designed interface: energy director, interference, flash trap and a support behind the horn. For laser welding, the joint is a clamped interface with controlled gap and, for plastics, the right optical pair.

The common mistake is to design the part and then ask the process to fit. A boss that is too close to a wall gives the ultrasonic horn nowhere to press. A flange that is too narrow gives the electrode no room. A clear plastic cover with no absorbing layer cannot be laser welded without an absorbing additive. Each of those is a drawing change, not a machine setting.

Inspection and qualification

Welded joints are usually inspected by a mix of appearance, dimension and destructive test. For resistance welds, check the nugget size on a peel or a macro section, the indentation and the spacing. For ultrasonic welds, check the melt flash, the joint width and a burst or peel result. For laser welds, check the seam width, the leak rate if the part is sealed and a section at the agreed frequency.

Set the sample frequency and the acceptance values before production. A weld that looks good can be cold, and a weld that looks rough can be sound. The destructive result is the evidence, and it should be recorded against the lot and the machine settings.

• Welding method and machine settings recorded per lot
• Joint dimensions checked on the first part of each shift
• Destructive test at the agreed frequency with a stated value
• Leak test for any sealed housing
• No burning, flash or distortion on the visible face
• Material pair and additive confirmed for laser welds