IDB-WAJ-037
Joining · welds · brazing · adhesives
Welded & adhesive joints
Permanent joining methods — fusion welds and their symbols and sizing, brazing/soldering, and adhesive bonding — with how to size each and the failure modes to design out.
Abstract
When a joint must be permanent, you weld, braze/solder, or bond it. Each suits different materials, loads and tolerances: welding fuses like metals at full strength, brazing and soldering join (including dissimilar metals) with a filler, and adhesives bond almost anything — including plastics — if the joint is designed for shear, not peel.
Section 1 compares the methods. Section 2 is weld joints and symbols. Section 3 is weld strength sizing. Section 4 is brazing and soldering. Section 5 is adhesive bonding. Section 6 is selection and failure modes.
1.Joining methods
| Method | Heat | Dissimilar metals | Strength | Gap fill | Notes |
|---|---|---|---|---|---|
| Fusion welding (MIG/TIG/spot/laser) | very high | same / similar only | full base-metal | none | distortion + heat-affected zone (HAZ) |
| Brazing | 450–900 °C | yes | high (joint) | small (capillary) | dissimilar metals, less distortion |
| Soldering | < 450 °C | yes | low–moderate | fills | electrical, sealing, electronics |
| Adhesive bonding | low / RT cure | yes (incl. plastics) | moderate (area-set) | fills gaps | design for shear; cure time |
| Riveting / bolting | none | yes | mechanical | — | serviceable (see Bolted joint ref) |
Choose welding for full-strength metal structures, brazing/soldering for dissimilar or delicate joints, adhesives for mixed materials / large thin areas / sealing, and fasteners when it must come apart.
2.Weld joints and symbols
- Joint types: butt, fillet (T / lap / corner), edge. The fillet weld is the workhorsea triangular bead in a corner.
- Weld symbol (ISO 2553 / AWS A2.4): an arrow points to the joint; a reference line carries the weld symbol. The fillet triangle sits on the line with the leg size before it; symbol below the line = arrow side, above = other side. Extra flags: length and pitch (intermittent welds), a circle at the elbow = weld all-around, a flag = field weld.
A complete callout tells the welder the type, size, side, length and location — put it on every structural weld.
3.Weld strength sizing
A fillet weld fails across its throat, not its leg:
- Throat
a = 0.707 × z(z = leg size) for an equal-leg fillet. - Capacity ≈ throat area × allowable shear:
F = a · L · τ_allow(L = effective weld length). - Size by throat; a bigger leg adds heat and distortion faster than strengthoften two passes or a longer weld beats one huge fillet.
- Balance the weld about the load's neutral axis (and the part's centroid) so it doesn't twist.
- Fatigue lives at the weld toeit's a severe stress raiser. For cyclic loads use fatigue-classified joint details (lower allowable stress ranges), and grind or peen the toe. See the Fatigue primer.
4.Brazing and soldering
The filler melts, the base metal doesn't — so there's no HAZ and dissimilar metals can be joined.
- Joint design: rely on capillary action into a small, controlled gap (typically 0.05–0.20 mm) and use lap (overlap) joints, not buttstrength comes from bonded area, not filler thickness.
- Brazing (>450 °C) gives strong, sealed joints (tube fittings, carbide tips, assemblies). Soldering (<450 °C) is lower strengthelectronics, sealing, light mechanical.
- Clean, flux and control the gap; too-wide gaps starve the capillary and weaken the joint.
5.Adhesive bonding
Adhesives bond almost anything — metals, plastics, composites, glass — and spread load over area, but the joint geometry decides success:
- Load it in shear or compression, never peel or cleavage. Use generous overlap (lap joints), and avoid thin edges that pry the bond apart.
- Surface prep is everything: clean, degrease, abrade/etch, sometimes prime. A bond is only as good as the surface it grips.
- Cure & temperature: respect open time and full cure before loading; check the service-temperature limit.
| Adhesive | Strength | Gap | Service temp | Use |
|---|---|---|---|---|
| Epoxy (2-part) | high, structural | gap-filling | ~120–180 °C | metal/composite structure |
| Acrylic / MMA (2-part) | high | tolerant of oily/rough | ~120 °C | structural, fast, less prep |
| Polyurethane | tough, flexible | gap-filling | ~80–100 °C | dissimilar, large panels, sealing |
| Cyanoacrylate (CA) | rigid, brittle | thin only | ~80 °C | fast fixturing, small parts |
| Silicone (RTV) | low, flexible | gap-filling | ~200–300 °C | sealing, thermal, vibration |
| Anaerobic | medium–high | close metal fit | ~150 °C | threadlock, retaining (see Bolted joint) |
6.Selection and failure modes
| Mode | Cause | Fix |
|---|---|---|
| Weld distortion / HAZ cracking | heat input, restraint, fast cooling | smaller/balanced welds, preheat, sequence, correct filler |
| Weld toe fatigue | stress raiser under cyclic load | fatigue detail, grind/peen toe, lower stress range |
| Adhesive peel / cleavage failure | joint loaded the wrong way | redesign for shear, add overlap/fillet, mechanical backup |
| Adhesive bond-line failure | poor surface prep / wrong adhesive | clean+abrade+prime, match adhesive to substrate |
| Braze starved joint | gap too wide / dirty | control capillary gap, flux, clean |
| Galvanic corrosion (dissimilar) | electrolyte at the joint | seal, isolate, matched metals (see Galvanic chart) |
Checklist: permanent? → weld (same metals, full strength) / braze (dissimilar, sealed) / adhesive (mixed materials, area). Size welds by throat and balance them; design adhesive joints for shear with overlap and surface prep; check fatigue at weld toes and peel at bond edges.