A designer laying out a new machine frame, enclosure or product often reaches for the joining method they know best. Fabricators weld. Machinery designers bolt. Sheet metal shops rivet or clinch. Product designers glue or snap-fit. Each choice works in its place, but each also commits the product to a particular cost, weight, assembly time, service life, corrosion behaviour and repair method. A frame welded together cannot be dismantled for transport; an aluminium panel bolted to steel without isolation may corrode at every fastener; a bonded joint loaded in peel may let go without warning.
The joining method is one of the earliest and most consequential design decisions, because it shapes the parts themselves: whether they need holes, flanges, overlaps, weld preparations or close-fitting gaps, and what processes and skills manufacture will need. Choosing it deliberately, against the requirements of the joint, often reduces cost and improves reliability at the same time.
This article compares the main joining methods for engineered products, explains how each carries load and fails, sets out the criteria that should drive the choice, and shows how hybrid joints can combine strengths. It is general information for designers, engineers and manufacturers. Structural, pressure, lifting and other safety-critical joints must be designed to the applicable standards by qualified people.
The main methods at a glance
| Method | How it carries load | Main strengths | Main limitations |
|---|---|---|---|
| Bolting | Clamping force (preload) and friction, or the bolt shank bearing in shear | Can be dismantled; joins different materials; no heat | Needs holes and access; can loosen; preload must be controlled |
| Welding | Fused metal continuous with the parts | Strong, rigid, compact; no extra parts | Heat distortion and residual stress; similar metals only; needs qualified welders; permanent |
| Riveting (solid and blind) | Rivet in shear and plate in bearing | Permanent, does not loosen, joins thin sheet and different metals | Needs holes; limited tension capacity; removal destroys the rivet |
| Adhesive bonding | Shear across the whole bonded area | No holes; spreads load; seals; joins almost any material | Needs surface preparation and cure; weak in peel; temperature and environment limits |
| Brazing and soldering | A filler metal, melted below the parts’ melting point, drawn into a close gap | Little distortion; joins dissimilar metals; sealed joints | Needs close fit and clean surfaces; lower strength at temperature; lap joints required |
| Mechanical forming (clinching, seaming, press and snap fits) | Interlocking or interference | Fast, no consumables, suits volume | Limited load and materials; tooling needed |
Bolting: serviceable and versatile
Bolted joints can be taken apart for maintenance, transport, replacement and adjustment, which is often decisive. They join any combination of materials without heat. Their performance depends on clamping: a well-preloaded bolted joint carries load mainly through friction and resists fatigue, while a slack one moves, loosens and fatigues. Bolting needs holes, which weaken the parts and must be accurately positioned, and access for tools on both sides unless threaded inserts, tapped holes or rivet nuts are used. Choose bolting where disassembly matters, where materials differ and where heat is unacceptable.
Welding: strong, permanent and demanding
Welding fuses the parts into one. A properly made full-penetration butt weld can be as strong as the plate itself; fillet welds, the most common type in fabrication, carry load through the throat of the weld, which is about 0.7 times the leg length for a standard fillet. Welded structures are rigid, compact and need no extra parts.
The costs are heat and skill. Welding heat distorts parts and leaves residual stresses; long continuous welds increase distortion, so intermittent welds and balanced sequences are often used. Welded joints are sensitive to fatigue at the weld toe. Only compatible metals can be welded together. Structural and pressure welding requires qualified procedures and welders and inspection to the applicable standards. Welded assemblies are permanent and difficult to modify or transport if large. Choose welding for strong, rigid, permanent assemblies of compatible metals, where the fabrication capability and inspection exist.
Riveting: permanent joints in sheet and plate
A rivet is a headed pin that is formed on assembly so it cannot be removed without destroying it. Solid rivets fill their holes and carry load in shear and bearing without the slip that clearance-hole bolts can show. Blind rivets, which can be set from one side, are widely used for sheet metal assemblies, enclosures and transport bodies. Riveted joints do not loosen and can join different metals and thin sheet that would be difficult to weld.
A riveted joint can fail in four ways: the rivet shearing, the plate crushing at the hole (bearing), the plate tearing between holes, or the plate edge tearing out. Rivet diameter, pitch, edge distance and the number of rows are proportioned so no single mode is much weaker than the others. Rivets carry tension poorly, and the holes reduce the section. Choose riveting for permanent sheet and light plate joints, dissimilar metals and assemblies where heat or loosening is unacceptable.
Adhesive bonding: no holes, whole-area load transfer
Structural adhesives spread load over the whole bonded area instead of concentrating it at holes or welds. They join almost any combination of materials, including metals to plastics and composites, seal the joint at the same time, damp vibration and leave clean surfaces with no fasteners.
A bonded joint is a chain of three links: adhesion to each surface and the strength of the adhesive itself. The surface links depend on surface preparation, including cleaning, abrading and sometimes priming, which is the most common cause of bond failure when neglected. As an illustration of capacity, a lap joint 25 mm wide with a 12.5 mm overlap has a bonded area of about 312 mm²; at a modest design shear stress of 15 MPa it would carry about 4.7 kN.
Bonds are strong in shear and weak in peel and cleavage, where load concentrates at one edge. Good bonded joints are designed so the adhesive works in shear, with wide joints rather than long overlaps, and peel edges protected or reinforced. Adhesives also have temperature limits, cure times, shelf lives and handling requirements, and the joint cannot easily be inspected for strength. Choose bonding for thin or dissimilar materials, sealed joints and smooth appearance, with controlled surface preparation and testing.
Brazing and soldering: capillary joints
Brazing and soldering join parts with a filler metal that melts at a lower temperature than the parts, which do not melt. By convention, soldering uses fillers melting below about 450 °C and brazing uses fillers above that temperature. Molten filler is drawn into the joint by capillary action, which needs a close, consistent gap, typically a few hundredths to around a tenth of a millimetre depending on the filler. Too tight a gap starves the flow; too wide a gap loses the capillary pull.
Because the filler is weaker than the base metal, strength comes from overlap area, so joints are designed as laps or sleeves rather than butts. Brazing joins dissimilar metals, such as copper to steel, with little distortion and produces sealed joints, which is why it is common in refrigeration, heat exchangers, tooling and electrical work. Choose brazing or soldering for small, sealed or dissimilar-metal joints with good fit-up and cleanliness.
Mechanical forming and integral joints
Clinching joins sheet by deforming it locally into an interlock, without consumables or heat. Seaming folds edges together. Press fits and shrink fits use interference. Snap fits in plastic parts use flexible features. These methods suit high volumes and automation, but each has limited load capacity and needs tooling and close control of dimensions.
Criteria that drive the choice
Work through these questions for each joint:
- Must the joint come apart? For maintenance, transport, replacement or recycling, favour bolting, rivet nuts or other demountable methods.
- What loads does it carry? Static, fatigue, impact, peel or tension. Welds and bonds need care in fatigue and peel respectively; rivets in tension. The durability is sustainability article explains designing for the loads a product will actually face.
- What materials meet? Dissimilar metals rule out conventional welding and raise galvanic corrosion risk. Thin sheet suits riveting, clinching and bonding.
- What environment will it face? Temperature, moisture, chemicals and outdoor exposure affect adhesives, coatings and galvanic corrosion.
- Must it seal? Bonding, brazing and continuous welds seal; bolts and rivets need sealants or gaskets.
- How many will be made? Volume favours methods that automate well, such as clinching, robotic welding and self-piercing rivets.
- What capability exists? Qualified welders, bonding process control, tooling and inspection must be available, in-house or at suppliers.
- How will quality be checked? Some joints are easy to inspect visually; others, such as bonds and some welds, need destructive testing of samples or non-destructive testing.
- What will it cost overall? Include part preparation, consumables, labour, distortion correction, finishing, inspection and repair, not just the joining operation.
The cost you commit before you spend article explains why these early choices fix so much of a product’s cost.
Watch for galvanic corrosion
When two different metals are in electrical contact in the presence of moisture, the less noble metal corrodes faster. Aluminium joined to steel or stainless steel, and zinc-coated parts joined to stainless steel, are common problems. Isolating washers and sleeves, sealants, coatings, compatible fastener materials and drainage all help. Consider the whole joint, including fasteners, washers and any filler, not just the parent materials.
Hybrid joints
Combining methods often captures the strengths of each. Adhesive with rivets or spot welds gives the load spreading and sealing of a bond with the peel resistance and instant handling strength of the mechanical fasteners. Bolting with a gasket or sealant gives a serviceable sealed joint. Tack welding with bolting can hold alignment while allowing later removal of some parts. Hybrids need careful design so the methods do not interfere, for example by checking that welding heat does not damage nearby adhesive.
Design the parts for the joint
Each method asks something of the parts:
- Bolting and riveting: hole sizes, edge distances, pitches, access for tools and flat seating faces.
- Welding: weld preparations, access for the torch, allowance for distortion and fixtures to hold parts.
- Bonding: overlap and width, surface preparation, bond-line thickness control and fixturing during cure.
- Brazing: close-fitting laps or sleeves and clean surfaces.
Decide the joining method early enough that the parts can be designed for it, rather than adapting the method to parts already detailed.
A worked example
This is an illustrative example. A 35-person business builds trailer-mounted pump skids. The current design has welded steel side panels on a painted steel frame. Panels take about six hours of welding, grinding and repainting per skid, add significant weight and cannot be removed for pump servicing. Customers have complained about rust at panel welds and about access.
Requirements. Some panels must be removable for servicing. The skids vibrate during towing and pumping, are used outdoors and must stay watertight. The business wants to reduce weight and assembly time.
Options considered.
- Keep welding: strong, but heavy, slow, prone to corrosion at damaged paint and not removable.
- Aluminium panels bolted to the steel frame: removable and lighter, but galvanic corrosion is a risk and nuts behind panels are hard to reach.
- Aluminium panels riveted and bonded: light, sealed and vibration resistant, but permanent.
Decision. The business uses two methods. Fixed panels are aluminium, attached with structural blind rivets and a flexible structural adhesive that seals and spreads load, with the steel frame painted and the joint isolated from moisture. Service panels are aluminium, fixed with stainless steel bolts into rivet nuts in the frame, with isolating washers and an edge seal, so they can be removed with hand tools from one side.
Verification. Test panels are bonded and riveted following the adhesive supplier’s surface preparation procedure, then subjected to a vibration test and pull-off tests on samples. One adhesive fails the pull-off test because of poor adhesion to the painted frame; a primer is added and the retest passes.
Result. Panel assembly time falls from about six hours to about two and a half hours per skid, weight falls by roughly 40 kg and pump servicing no longer requires cutting or grinding. Corrosion at the panel joints is checked at the first annual inspection of the early units and found to be absent.
Applying this in an Australian business
- Choose the joining method early, before parts are detailed.
- Start from the joint’s requirements: disassembly, load, materials, environment, sealing, volume and capability.
- Use bolting where joints must come apart, with controlled preload.
- Use welding for strong permanent joints, with qualified procedures and distortion control.
- Use rivets, clinching or bonding for thin and dissimilar materials.
- Design bonded joints for shear, and control surface preparation.
- Use brazing for small sealed and dissimilar-metal joints with close fits.
- Prevent galvanic corrosion with isolation, sealing and compatible materials.
- Consider hybrid joints, and test samples before committing.
- Follow the applicable standards for structural, pressure and safety-critical joints.
Where joining choices go wrong
- Choosing by habit rather than by the joint’s requirements.
- Welding assemblies that must later be dismantled or transported.
- Bonded joints loaded in peel.
- Skipping surface preparation for adhesives.
- Mixing metals without isolation.
- Comparing joining operations without including distortion correction, finishing and inspection costs.
- No sample testing before production.
Questions to ask for each important joint
- Does this joint ever need to come apart, and who will do it?
- What kind of load does it carry, and how could it fail?
- Which materials meet, and could they corrode each other?
- Does the joint need to seal?
- Can our people or suppliers make and inspect this joint reliably?
- What does the whole joint cost, including preparation and finishing?
Bringing it together
There is no universally best joining method. Bolting gives serviceability, welding gives strength and rigidity, riveting and clinching give permanent joints in sheet and dissimilar metals, adhesives give sealed, hole-free joints with spread load, and brazing gives small, sealed, low-distortion joints. Choose by working through the joint’s real requirements: disassembly, load, materials, environment, sealing, volume, capability, inspection and total cost. Watch for galvanic corrosion, consider hybrid joints and design the parts for the chosen method from the start. Tested on samples before production, a deliberate joining choice usually makes products lighter, cheaper to build and longer lasting.
Source: KEVOS editorial notes, drawing on earlier KEVOS engineering handbooks on adhesives and sealants, rivets and riveted joints, soldering and brazing, welded joint design and mechanical joint and fastener selection. The worked example is illustrative. This article is general information and does not replace design to the applicable standards by qualified engineers.
