Manufacturing Process

Welding

焊接

Joining sheet, tube and machined parts by TIG, MIG/MAG and resistance spot welding, with joint preparation, distortion control and inspection.

Process Steps

Joint and process selected
Fit-up and tacking
Weld sequence set to control distortion
Weld checked and dressed

What welding covers

Direct answer: welding melts the two parts, and sometimes a filler, into one joint. A fabrication shop needs more than one process to do that well: TIG where the joint is thin, visible or has to be leak-tight; MIG/MAG where the joint is thicker and speed matters; resistance spot welding where two sheets are joined in a stack without filler; and brazing where the joint should hold without melting the base metal at all.

Choosing the process

Process	Typical material and thickness	Where it is used	Trade-off
TIG (GTAW)	Stainless and thin sheet, roughly 0.8–3 mm	Visible welds, tube joints, thin sheet that would burn through elsewhere	Slower than MIG on long joints
MIG/MAG (GMAW)	Mild steel 1–6 mm, aluminium	Frames, brackets, larger assemblies and anything with long weld runs	More spatter; dressing may be needed
Resistance spot	Sheet stacks, coated or uncoated, 0.5–3 mm	Enclosures, brackets to panels, parts that must stay flat	Access to both sides, and a witness mark on one face
Projection / stud	Fasteners and studs onto sheet	Weld studs, standoffs and threaded hardware	Needs the right stud grade for the sheet
Brazing	Copper, brass and stainless tube and fittings	Capillary joints, coils, manifolds and multi-joint assemblies	Joint strength and service temperature differ from a fusion weld

Brazing is a joining route rather than a welding process — the filler melts, the base metal does not — so it has its own page with its own numbers: metal brazing services. Fusion welding for production parts sits on metal welding services.

Distortion is what decides whether the part is usable

Heat shrinks as it cools, so every weld pulls its own joint. Four controls keep a welded part inside tolerance:

Sequence. Weld from the middle outwards, or back-step, so the shrinkage runs cancel instead of accumulating at one end;
Heat input. Smaller passes, shorter runs and letting the part cool between welds all cost less distortion than one heavy pass;
Fixturing and tacking. Tack in the right order, in the right places, and the part stops moving before the main welds go in;
Machining after welding. Where a face has to be flat, the flat cut is taken after the weld, not before.

What the drawing should carry

A weld symbol, or a written note, for every joint — a drawing with "weld all round" on it and nothing else will be priced three different ways by three suppliers;
Whether the joint is structural or sealing, because that decides which process is used;
The finish expectation: as-welded, spatter-free, dressed flush, or dressed and polished for a visible face;
Any inspection requirement — visual, dimensional, or a leak test on a tube or pressure-tight assembly.

Inspection

Welded parts are checked visually against the drawing, then dimensionally, because the weld is where the two parts actually meet. Where a joint has to seal, the assembly is leak-tested by the method agreed at order stage. First-article inspection documentation ships with the pre-production sample, as it does for the rest of the shop.

Frequently asked

MIG or TIG for stainless?

TIG for thin sheet, visible joints and anything that has to be leak-tight; MIG/MAG where the section is heavier and the joint is long. Both are run to a qualified procedure, and the choice is quoted from the joint, not from habit.

Can a welded assembly still be powder coated?

Yes, and welding comes first. Spatter is removed, the weld is dressed where the finish is visible, and the part is cleaned before coating. Weld dressing is part of the finishing route, not an afterthought: see metal finishing and secondary operations.

How do you stop a welded frame from twisting?

By planning the sequence before the first arc: balanced welds, back-stepping, tacks in the corners that move least, and letting the frame cool between passes. Where the drawing still asks for a flat face, it is machined after welding.

Is brazing as strong as welding?

For a lap or capillary joint with the right overlap, brazing gives a joint that is strong enough for most tube, coil and manifold work, and it avoids the distortion a fusion weld would put into thin tube. Where the joint has to carry the full section in tension, welding is the answer.

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