Welded assembly design
Define joints, access, distortion and final inspection before welding begins. Keep the load-bearing design and the delivered assembly requirements connected.
Use welding where it serves the assembly
Welding joins shapes that may be difficult to form as one piece. It also adds heat, local material changes and a process that needs access and acceptance criteria.
| Requirement | Route to consider | Question to resolve |
|---|---|---|
| A uniform-thickness shell | Forming with fewer joints, where geometry permits. | Can the required shape be bent with available tooling and access? |
| A frame or reinforced support | Joined stock sections, plates and gussets. | How will the load travel through the joints, and how will the frame be held during welding? |
| A serviceable assembly | Bolted or otherwise removable connections. | Which components need replacement, adjustment or separate finishing? |
| A sealed volume | A continuous joint designed and verified for containment. | What medium, pressure, leak-rate limit and test method define acceptance? |
| A precise mounting interface | Fabrication followed by planned final machining. | Which surfaces need machining stock, datum access and inspection after welding? |
Specify the weld size without confusing leg and throat
Leg and throat are different dimensions
For an ideal flat-faced, equal-leg 90° fillet, throat a = z / √2, about 0.707 × leg z. This geometry does not establish the required weld strength.
State the acceptance question
Dimensions, surface-breaking defects and leakage are different checks. Select inspection methods and acceptance criteria for the actual joint and service.
For an ideal flat-faced, equal-leg 90° fillet, throat a = z / √2 ≈ 0.707z. A 4 mm leg therefore gives a theoretical throat of about 2.83 mm. This is a geometry example, not a rule that a 4 mm weld is adequate for a particular load.
| Joint detail | State explicitly | Why it matters |
|---|---|---|
| Size convention | Leg or throat dimension using the applicable drawing-symbol standard. | The same number attached to different conventions defines a different weld. |
| Extent | Weld length, location, side, continuity and intermittent pitch when applicable. | A small symbol should not leave the fabricator guessing which edges are joined. |
| Preparation | Root gap, edge preparation, backing or access requirements where needed. | Fit-up and access affect whether the intended penetration and quality can be achieved. |
| Design basis | Required loads, fatigue, service environment and governing code. | Weld size is a design calculation; it is not established by the thinner sheet alone. |
| Final contour | As-welded, blended or machined appearance on identified faces. | Grinding a weld flush can remove required material; preserve the design section. |
An intermittent weld may suit some non-sealing connections, but it is not interchangeable with a continuous weld. Fatigue, corrosion, load transfer and sealing requirements decide whether it is appropriate.
Choose the procedure for the joint and material
Process selection includes fit-up, access, thickness, filler, heat input and inspection. “TIG for quality, MIG for speed” is too broad to define a weldment.
| Route | Selection consideration | Confirm |
|---|---|---|
| GTAW / TIG | Control of the weld pool and filler addition on accessible joints. | Access, shielding, root protection, procedure and cycle time for the actual geometry. |
| GMAW / MIG / MAG | Wire-fed deposition for compatible materials and production arrangements. | Transfer mode, heat input, access, spatter and joint quality under the chosen procedure. |
| Resistance spot welding | Overlapping sheet joints with suitable electrode access. | Material/coating compatibility, joint spacing, electrode access and qualification of the spot pattern. |
| Laser welding | A candidate where concentrated heat and suitable fit-up support the design. | Gap control, joint tracking, material response, safety setup and the inspection plan. |
- Protect material properties. For heat-treatable aluminum such as 6061-T6, the heat-affected zone can govern joint strength. Do not apply unwelded T6 properties throughout the finished assembly or assume a simple aging step restores them.
- Define stainless cleanup. Agree treatment of weld oxides, contamination and any passivation requirement. A cosmetic blend and a corrosion-restoration process are different operations.
- Identify coatings before welding. Pre-existing coatings affect preparation, process safety and final corrosion protection. State the incoming surface condition rather than specifying only the base alloy.
Plan the sequence and the release from the fixture
The useful question is how the assembly measures after welding, cooling and unclamping. A rigid fixture alone does not guarantee the final shape.
Design
Use the required weld volume
Avoid adding weld metal beyond the design requirement. Consider joint placement and balanced welds where they suit the load path and access.
Build
Agree the sequence
Plan tack locations, welding sequence and handling with the fabricator. Trial builds may be needed to establish repeatable compensation and fixturing.
Measure
Check the released assembly
Inspect the final, cooled condition using defined datums. Record whether the part is free, supported or restrained during measurement.
- Use locating features carefully. Tabs and slots can aid assembly, but their fit and clearance must permit the intended weld and shrinkage. They do not replace dimensional inspection.
- Leave access for the whole sequence. A reachable first weld can block the torch, clamp or inspection probe needed for the next one. Review the assembly order in CAD.
- Treat correction as a planned operation. If straightening or heat treatment is proposed, confirm its effect on material properties, dimensions and the governing specification.
Separate fabrication accuracy from final machining
Measure from the functional interface
Locate critical holes from the mounting face and a shared origin. Long chains of intermediate dimensions accumulate variation.
Inspect the delivered condition
A welded, coated or hardware-installed assembly can differ from its cut components. Mark which dimensions apply after the final operation.
Choose machining allowance from the expected fabrication variation, geometry and planned finishing route. A universal extra millimeter is not a reliable rule for every frame. See CNC machining design for accessible datums, tool reach and final features.
| Requirement | Define on the drawing | Plan before fabrication |
|---|---|---|
| General fabrication tolerance | Applicable standard, edition and tolerance class, where used. | Do not cite a standard without selecting the class and checking its scope. |
| Critical mounting faces | Flatness, parallelism, datum system and final surface condition. | Machining stock, fixture support and access after welding. |
| Bearing or locating bores | Final size, position and axis relationship. | Whether these are machined in one setup after the assembly is stable. |
| Long frame or shell | Overall dimensions, squareness, twist and measurement condition. | Supports used during inspection and any agreed correction sequence. |
ISO 13920 addresses general dimensional and geometrical tolerances of welded constructions. It does not establish weld strength or replace individually specified functional tolerances.
Separate weld quality, leakage and appearance
For weld-imperfection quality levels, ISO 5817 covers relevant fusion-welded steel, nickel and titanium joints, excluding beam welding; ISO 10042 addresses arc-welded aluminum and its alloys. Their quality levels do not by themselves prove fitness for the intended service. Use the standard and acceptance class required by the design, contract and material.
| Question | Potential method | What to specify |
|---|---|---|
| Are dimensions and weld geometry correct? | Dimensional and visual examination. | Features, weld-size convention, final condition and acceptance limits. |
| Are there surface-breaking discontinuities? | A suitable penetrant or magnetic-particle method for the material. | Procedure, access, extent and acceptance criteria. These are not leak tests. |
| Is internal weld quality acceptable? | A suitable ultrasonic or radiographic technique where geometry permits. | Technique, coverage, code requirements and interpretation criteria. |
| Is the assembly leak tight? | A separately defined pressure or leak test. | Medium, pressure, duration, permitted leak rate and a safe test procedure. |
| Is the cosmetic surface acceptable? | Comparison to an agreed visual reference. | Visible zones, blending, colour, texture and allowable witness marks. |
Specify required inspection, qualification records and reports before ordering. Listing a method here does not mean that it is included by default in every FabDigit quote.
Make the weldment package complete
- Assembly definition. Assembly STEP, part drawings, BOM, material grades and conditions, revision and all purchased interfaces.
- Joint definition. Weld symbols and convention, sizes, lengths, sides, preparation, required procedure or code, and access constraints.
- Final operations. Machined faces, stock allowance, any heat-treatment sequence, finish, masking and protected threads or bores.
- Acceptance and records. Datum-based final inspection, measurement condition, weld-quality criteria, any leak or NDT requirement, and deliverable reports.
- First-article decision. Identify what must be approved on the first assembly before repeat production, especially fit, distortion and finished interfaces.
Welded assembly questions
Is fillet weld size 0.7 times the sheet thickness?
No. For an ideal equal-leg 90-degree fillet, the theoretical throat is about 0.707 times the weld leg. That geometric relation does not select the required weld size from sheet thickness; sizing depends on the joint, loads, material and applicable design rules.
Does TIG always cause less distortion than MIG?
No. Distortion depends on the joint, heat input, travel speed, weld volume, sequence and restraint. Compare procedures for the actual geometry rather than assigning a universal distortion ranking to the process names.
Can a welded 6061-T6 assembly use the original T6 strength?
Not throughout the weld region without supporting qualification. Welding changes the local material condition, and the heat-affected zone can govern strength. Design using appropriate welded properties and any qualified post-weld treatment.
Is dye penetrant inspection a leak test?
No. Penetrant examination detects suitable surface-breaking discontinuities. A leak-tightness requirement needs its own defined test medium, conditions and acceptance limit.
Should critical holes be machined before or after welding?
If welding movement would violate their final alignment or size, plan final machining after welding and any required stabilization. Define machining stock, datums, access and the delivered condition before fabrication.
Which tolerance standard should a weldment use?
Select one appropriate to the contract, material and application. ISO 13920 can define general fabrication dimensions and geometry when its class is specified; functional features may need individual tolerances. Weld-imperfection standards answer a different question.
Sources and further reading
- TWI — fillet weld design and throat geometryDistinguishes leg length, throat and the actual weld cross-section.
- TWI — distortion prevention by designWeld volume, placement and balanced sequence affect distortion.
- TWI — restraint and distortionRestraint, release and residual-stress considerations.
- ISO 13920:2023 — welded construction tolerancesPublic scope; use the contractually applicable edition and class.
- ISO 5817:2023 — weld-imperfection quality levelsFusion-welded steel, nickel and titanium; excludes beam welding and is not an aluminum acceptance standard.
- ISO 10042:2018 — aluminum weld-imperfection quality levelsProduction quality levels do not themselves establish fitness for purpose.
- Lincoln Electric — aluminum welding guideAlloy, filler and welded-property considerations for aluminum.
Quote the complete fabrication route
Include the joints, final machining, finish and inspection requirements so the quotation covers the delivered assembly.