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Manufacturing · Chapter 7 of 10 · Intermediate

Joining and Welding

Welding fuses metal with a moving pool of melt, but the heat does more than join: it transforms the metal beside the weld and locks in stress. Control the heat and you control the joint.

01

Readiness check

This chapter mixes heat, microstructure, and a little strength of materials. Tick only what you can do closed-notes.

  • Compute electrical power as voltage times current.
  • Recall that fast cooling can form brittle martensite.
  • Use shear stress as force over area.
  • Recall the heat-affected zone idea from materials.
  • Work with J/mm energy per length.
0 or 1 weak itemsContinue with this chapter.
2 weak itemsReview transformations and martensite in Materials Chapter 9.
3 or more weak itemsRevisit cooling and microstructure before continuing.
02

The core idea

A weld is a small, fast casting made in place; the heat input sets the weld pool and, just as importantly, transforms a heat-affected zone and locks in residual stress.

H = ηVI/vthroat = 0.707·leg (fillet)P = τ·(throat·L)

In fusion welding an arc or flame melts the base metal into a pool that solidifies as the joint. The heat input per unit length, H = ηVI/v, governs the pool size and the cooling rate. Beside the fused metal lies the heat-affected zone, base metal that was not melted but was heated enough to change, often hardening and embrittling in steels. Uneven heating also leaves residual stress and distortion. Designing a joint means sizing the weld for strength and choosing heat input for sound metallurgy.

The skill works when: you set heat input for a sound pool and HAZ, and size the weld throat for the load.
The skill breaks down when: heat input is ignored, so the HAZ cracks, or the weld throat is undersized for the force.
The concept. The arc melts a pool that joins the plates; around it the heat-affected zone (HAZ) is transformed without melting. Heat input controls both the pool and the HAZ.
03

The skills, taught in order

Joining spans many methods, with fusion welding the most demanding. Five skills cover the processes, heat input, the HAZ, defects, and joint design.

7.1 Welding and joining processes

Joining methods differ in how they bond and at what temperature.

MethodBond formed byNotes
Fusion weldingmelting the base metalstrongest; arc, gas, resistance
Brazingfiller melts, base stays solidjoins dissimilar metals, above 450 °C
Solderinglow-melting fillerelectronics; low strength, below 450 °C
Adhesive bondingpolymer adhesiveseals, spreads load, dissimilar materials
Mechanical fasteningbolts, rivetsdemountable, no heat

7.2 Heat input

The energy delivered per unit length of weld is H = ηVI/v, where η is the process efficiency, V the arc voltage, I the current, and v the travel speed. More current or slower travel raises H, giving a bigger pool and slower cooling. It is the single most useful welding parameter.

7.3 The heat-affected zone

Next to the fused metal, the HAZ is heated below melting but enough to change its microstructure. In a hardenable steel, fast cooling there can form brittle martensite, the usual cause of weld cracking. Preheating and controlling heat input slow the cooling and keep the HAZ tough.

7.4 Weld quality and defects

Welds fail through characteristic defects, each with a known cause and cure.

DefectCausePrevention
Porositytrapped gasshielding gas, clean joint
Crackinghard HAZ, hydrogen, fast coolingpreheat, low-hydrogen electrodes
Lack of fusiontoo little heat inputmore current, slower travel
Distortionuneven heating and contractionfixturing, weld sequence

7.5 Joint design and weld strength

The fillet weld is the workhorse. Its strength comes from the throat, the narrowest section, equal to 0.707 times the leg for an equal-leg fillet. The capacity is the allowable shear stress times the throat area: P = τ·(0.707·leg)·L. Butt welds carry load through the full thickness.

Engineering connection: structures, pressure vessels, and automotive bodies are welded; the HAZ and residual stress decide fatigue and fracture life, linking back to materials failure.

04

Worked example 1: weld heat input

An arc weld runs at 24 V and 200 A with a travel speed of 5 mm/s and a process efficiency of 0.9. Find the arc power and the heat input per unit length, and say what raising it would do to the HAZ.

Figure 1. Heat input is the arc power spread over the weld length. The same power at a slower travel speed deposits more energy per millimetre, widening the pool and the HAZ.
  1. ProblemFind the arc power and heat input for the weld in Figure 1.
  2. Given / findV = 24 V, I = 200 A, v = 5 mm/s, η = 0.9. Find arc power and H.
  3. AssumptionsSteady arc, constant travel speed, efficiency η accounts for losses.
  4. ModelArc power is VI; heat input is the efficient power divided by travel speed.
  5. EquationsParc = VI H = ηVI/v
  6. SolveParc = 24 × 200 = 4800 W (4320 W effective at η = 0.9). H = 0.9 × 4800/5 = 4320/5 = 864 J/mm.
  7. CheckUnits: (W)/(mm/s) = J/mm, correct. Halving the travel speed would double H to 1728 J/mm, slowing the cooling and widening the HAZ, which softens it but risks distortion.
  8. ConclusionHeat input is the lever for weld metallurgy: enough to fuse and avoid a brittle HAZ, but not so much as to distort or coarsen the grain. It is set by current, voltage, and travel speed together.
Result. Arc power 4800 W; heat input 864 J/mm.
05

Worked example 2: fillet weld strength

An 8 mm leg fillet weld, 150 mm long, joins two plates. The allowable shear stress on the weld throat is 95 MPa. Find the load the weld can carry.

Figure 2. A fillet weld carries load across its throat, the narrowest plane at 45° (0.707 of the leg). Strength is the allowable shear stress times the throat area.
  1. ProblemFind the load capacity of the fillet weld in Figure 2.
  2. Given / findleg = 8 mm, L = 150 mm, allowable τ = 95 MPa. Find P.
  3. AssumptionsEqual-leg fillet, failure on the throat plane, uniform shear.
  4. ModelThroat is 0.707 of the leg; capacity is the allowable shear stress times the throat area.
  5. Equationsthroat = 0.707·leg P = τ·(throat·L)
  6. Solvethroat = 0.707 × 8 = 5.66 mm. Throat area = 5.66 × 150 = 848 mm². P = 95 × 848 = 80 600 N = 80.6 kN.
  7. CheckCapacity scales with leg size and length, so doubling either roughly doubles the load. The throat, not the leg, is the strength dimension, the point students most often miss.
  8. ConclusionSizing a fillet weld is throat shear, not leg size directly. Oversizing the leg wastes weld metal and adds heat and distortion, so welds are sized to the load, not larger.
Result. Throat 5.66 mm; the weld carries about 80.6 kN.
06

Misconceptions and diagnostics

MistakeSymptomDiagnostic questionCorrection
The weld is the only concernHAZ cracks despite a sound bead"What happened beside the weld?"The HAZ can harden and crack; manage cooling and preheat.
Sizing on the legWeld capacity overestimated"Did I use the throat, 0.707·leg?"Strength is the throat area times the allowable shear.
Ignoring distortionAssembly warps after welding"Is heating symmetric and fixtured?"Use fixturing and a balanced weld sequence to limit distortion.
More heat is always better fusionCoarse grain, soft HAZ, distortion"Is the heat input within the procedure?"Match heat input to thickness and material; excess harms metallurgy.
07

Practice ladder

Level 1 · Direct skill

A weld runs at 28 V, 250 A, travel 6 mm/s, efficiency 0.85. Find the heat input.

Show answer

H = ηVI/v = 0.85 × 28 × 250/6 = 5950/6 = 992 J/mm. Higher current raised it above the worked example.

Level 2 · Mixed concept

For the Worked Example 2 weld, what leg size is needed to carry 120 kN over the same 150 mm length?

Show answer

Required throat = P/(τL) = 120 000/(95 × 150) = 8.42 mm, so leg = 8.42/0.707 = 11.9 mm, round up to 12 mm. Capacity scales with throat, hence with leg.

Level 3 · Independent problem

Why does a high-carbon steel need preheating before welding, while mild steel usually does not?

Show answer

High carbon makes the HAZ hardenable, so fast cooling forms brittle martensite that cracks. Preheating slows the cooling rate, avoiding martensite. Mild steel has too little carbon to harden significantly, so it tolerates faster cooling.

Level 4 · Transfer to real engineering

Find a welded structure (a bicycle frame, a railing, a pressure vessel). Identify the joint type and process, estimate a weld size or heat input, and note where HAZ or distortion would matter.

What good work looks like

The process and joint identified, a heat-input or throat estimate, and a comment on HAZ embrittlement or distortion control.

08

Working with AI, and proving it yourself

Use AI as an examiner, not a solver

"Check that I sized the weld on the throat, not the leg."
"Give me five joints; I will choose welding, brazing, adhesive, or fastening for each."
"Compute the heat input." Combining V, I, and travel speed yourself is the skill.
"Will this weld crack?" Reasoning from HAZ and cooling is the point.

Portfolio task

Analyse one welded joint: choose a process and joint type, compute the heat input, and size the weld throat for the load, noting any HAZ or distortion risk.

Must include: a heat-input figure, a throat-based strength check, and an HAZ or distortion note.
09

Retrieval and spaced review

Closed notes. Answer out loud, then reveal.

1. Write the weld heat input.

H = ηVI/v, the efficient arc power per unit travel.

2. What is the heat-affected zone, and why does it matter?

Base metal heated below melting but transformed; it can harden and crack in steels.

3. Give the throat of an equal-leg fillet weld.

throat = 0.707·leg; strength is τ times throat area.

4. Name two welding defects and their causes.

Porosity (trapped gas) and cracking (hard HAZ, hydrogen, fast cooling).

5. When would you braze rather than weld?

To join dissimilar metals or thin parts without melting the base metal.

TodayFinish this quiz and Levels 1 and 2 of the ladder.
+1 dayRe-derive the heat input and weld capacity from a blank page.
+3 daysOne heat-input and one weld-strength problem.
+7 daysMove to polymers and additive, Chapter 8.
+30 daysConnect the HAZ to fatigue life from materials failure.
10

Textbook mapping

ItemMapping
Primary sourceKalpakjian and Schmid, Manufacturing Engineering and Technology, Chapters 30 to 32 (joining and welding)
Cross-referenceGroover, Ch. 28 to 31 · DeGarmo, joining chapters
Core topics7.1 Processes · 7.2 Heat input · 7.3 Heat-affected zone · 7.4 Defects · 7.5 Joint design
Engineering connectionStructures, pressure vessels, and vehicle bodies, where HAZ and residual stress set fatigue life.
Read nextChapter 8: Polymer Processing and Additive Manufacturing.