Metal Fabrication Design Guide

THE CENTRAL IDEA
The final geometry of a weldment is created by the entire manufacturing route—not by the welding operation alone. Design the route, then design the weldment.

1. Think in Manufacturing Routes, Not Individual Welds

A fabricated assembly passes through multiple geometry-changing operations: material preparation, cutting, forming, fit-up, tack welding, welding, straightening, heat treatment where required, machining, inspection and surface protection. Each stage can influence the next.

A design that looks simple in CAD can become difficult when a welder cannot reach the joint, a fixture blocks the torch, a machined face distorts after welding, or a closed section cannot be coated or inspected. DFM therefore begins by mapping the complete route.

FABRIOX FABRICATION FRAMEWORK
FUNCTION → LOAD PATH → MATERIAL → JOINT → PROCESS → WELD SEQUENCE → DISTORTION CONTROL → FINAL MACHINING → INSPECTION → PROTECTION

2. Material Selection for Fabricated Structures

Material selection should begin with structural and service requirements, then consider weldability, thickness, delivery condition, forming, machining, coating and commercial availability. Two steels with similar yield strength may require different welding controls because chemistry, carbon equivalent, heat-treatment condition or thickness changes cracking risk and procedure requirements.

Specify the material completely

A purchase or drawing specification should identify the applicable material standard, grade, delivery condition and any required impact, through-thickness, traceability or certification conditions. Do not approve an 'equivalent' grade from chemistry alone when mechanical properties, delivery condition, toughness, weldability or product standard are part of the requirement.

3. Design the Load Path Before the Weld Detail

A weld is a load-transfer feature, not decoration. Joint geometry should be developed from the forces the assembly must transmit and the applicable design rules. The manufacturing team should not be expected to choose weld size or length solely from shop habit.

The engineering design should distinguish structural welds, sealing welds, attachment welds and cosmetic requirements. This prevents a common problem: increasing weld volume in areas where it adds heat and distortion but little functional value.

IMPORTANT
More weld is not automatically safer. Weld size and length should come from engineering design and the governing specification. Excess weld can add filler, arc time, heat input, shrinkage and distortion.

4. Joint Design and Welding Access

A joint may be theoretically weldable but industrially poor if the torch angle is restricted, the welder cannot see the root, slag cannot be removed, a robot cannot reach the path, or NDT cannot access the examination surface. These constraints should be resolved during design review.

5. Choose the Welding Process Around the Joint and Production System

There is no universal 'best' welding process. Selection depends on material, thickness, position, joint type, access, required properties, production volume, equipment and applicable procedure qualification.

6. WPS, Procedure Qualification and Welder Qualification

Production welding should be controlled by the applicable welding procedure system. A Welding Procedure Specification (WPS) gives the welder/operator the approved production variables. Procedure qualification demonstrates that the proposed welding procedure can achieve the required result under the governing standard. Welder or welding-operator qualification demonstrates personnel capability within the applicable qualification range.

Terminology and qualification rules differ between ISO/EN and AWS/ASME systems. PQR and WPQR are related concepts but should not be presented as universally interchangeable documents. The contract should define the governing system.

ISO 3834-1:2021 provides criteria for selecting among comprehensive, standard and elementary quality-requirement levels in the ISO 3834 series for fusion welding. It applies in workshop and field manufacturing, but it is not itself a complete quality-management system.

7. Fit-Up Is a Manufacturing Process

Welding quality begins before the arc starts. Gap, alignment, edge preparation, cleanliness, tack quality and component location determine whether the WPS can be executed consistently.

• Create functional locating datums and repeatable stops in the fixture.

• Control root gaps and alignment where the joint design requires them.

• Place tack welds so they stabilize the assembly without creating inaccessible defects or excessive restraint.

• Check that clamps do not block the welding torch, cleaning or inspection.

• Plan how the assembly is released from the fixture without springing into an uncontrolled shape.

• Control drawing revision and component identity before fit-up.

FIXTURE DESIGN QUESTION
Can this be located, clamped, welded, inspected and removed from the fixture repeatedly?

8. Welding Distortion: The Core DFM Problem

Welding introduces concentrated heat. The heated metal expands, then contracts as it cools. Because heating and restraint are nonuniform, the final assembly can shrink, rotate, bow, buckle or twist. Distortion is therefore a predictable manufacturing phenomenon—not evidence by itself that a welder has performed poorly.

Design it out

• Use only the weld volume required by design.

• Keep stiffness and weld placement as balanced as practical.

• Avoid abrupt thickness changes and heavily restrained local details where possible.

• Place critical machined interfaces so they can be finished after welding when necessary.

• Use subassemblies to create manageable welding and fixture sequences.

Control it during fabrication

• Develop a defined tack and weld sequence.

• Use balanced, skip or back-step concepts only where validated for the joint/process.

• Control heat input and interpass conditions through the qualified procedure.

• Use fixtures, strongbacks or temporary restraint strategically—not simply maximum clamping force.

• Use presetting or pre-camber only where engineering/process experience supports the compensation.

• Map first-article dimensions to understand actual movement before serial production.

Correct only what remains

Straightening, thermal correction and post-weld machining can be valid production operations, but they should not compensate for a fundamentally unstable design. The preferred hierarchy is:

DISTORTION HIERARCHY
DESIGN IT OUT → CONTROL IT DURING FABRICATION → MACHINE / CORRECT ONLY WHAT REMAINS

9. Tolerancing a Welded Structure

A large weldment should not be toleranced like a small machined block. Weld shrinkage, plate flatness, profile variation, forming and fixture release all contribute to final geometry. The drawing should distinguish general fabricated tolerances from individually critical functional characteristics.

ISO 13920:2023 is the current second edition for general tolerances of welded constructions. It defines four tolerance classes for linear/angular dimensions and shape/position based on customary workshop accuracy, with class selection driven by functional requirements. ISO explicitly states that the applicable tolerances are those stated on the drawing and that complex structures may require special provisions.

TOLERANCE PRINCIPLE
Do not force machined-part thinking onto every dimension of a large weldment. Control the dimensions that drive assembly and function, and choose the manufacturing sequence that can actually achieve them.

10. Machining Before or After Welding?

Machining sequence is one of the most important weldment decisions. Features machined before welding may move because of welding heat; features machined after welding require enough stock, machine access and a stable datum strategy.

Large fabricated bases, bearing housings, pivot interfaces and aligned bores may require face milling, line boring or large-envelope machining after welding. Design should reserve access and stock for that operation from the beginning.

11. Weld Quality Levels and Acceptance

ISO 5817:2023 is the current fourth edition specifying quality levels for imperfections in fusion-welded joints in steels, nickel, titanium and their alloys within its stated scope. It defines levels B, C and D; level B corresponds to the highest requirement on the finished weld.

That does not mean every industrial weldment should automatically specify level B. The appropriate acceptance requirement depends on design, load, service, applicable code and contract. A higher workmanship requirement can add preparation, process control, inspection and repair cost without adding value if the product does not require it.

Imperfection is not automatically a reject

Terms such as porosity, undercut, reinforcement, lack of fusion or profile describe conditions that must be evaluated against the applicable acceptance criteria. The existence of a visible imperfection and the decision to reject the weld are not the same thing.

12. Inspection and NDT

Inspection should be selected from the defect mechanism, material, joint, thickness, access, service risk and governing specification. No single NDT method finds every relevant discontinuity.

The extent, technique and acceptance criteria for NDT should be stated by the applicable product/code/customer specification. Inspection should not be added as a generic percentage without understanding the risk it is intended to control.

13. Surface Protection Is Part of the Design

Closed hollow sections sent for hot-dip galvanizing require correctly engineered venting and drainage for process safety and coating quality. Do not publish generic vent-hole sizes without validating the part geometry, galvanizer requirements and applicable guidance.

14. Fabrication Cost Drivers

PROCUREMENT INSIGHT
The cheapest €/kg fabrication quote is not necessarily the lowest-cost assembly. Dimensional instability, rework, coating repair, late inspection failure, packaging damage and assembly problems can dominate the economics.

15. 20-Point Metal Fabrication DFM Checklist

• Is the structural load path clear?

• Are material grade, standard and delivery condition unambiguous?

• Are thickness changes and high-restraint details justified?

• Can every weld be physically accessed?

• Can the joint be cleaned and inspected after welding?

• Is weld size/length defined from engineering requirements?

• Are unnecessary sealing/cosmetic welds avoided?

• Can the parts be located repeatably in a fixture?

• Does the fixture allow torch and inspection access?

• Has a weld/tack sequence been considered?

• Are distortion-sensitive interfaces identified?

• Are functional dimensions separated from general fabricated dimensions?

• Is post-weld machining planned where necessary?

• Is sufficient machining allowance provided?

• Can the completed weldment fit the required machine envelope?

• Are NDT method, extent and acceptance criteria defined where required?

• Are material and weld traceability requirements defined?

• Is surface protection compatible with closed sections, threads and fits?

• Are lifting, packing and transport constraints designed into the assembly?

• Have manufacturing and quality teams reviewed the drawing before serial release?

16. What to Include in a Metal Fabrication RFQ