ME 436: Welding and Joining Processes
Estimated study time: 10 minutes
Table of contents
Sources and References
Primary texts: Welding Metallurgy by Kou (Wiley); Welding Handbook published by the American Welding Society; Principles of Welding by Messler.
Supplementary texts: Welding: Principles and Applications by Jeffus; Introduction to the Physical Metallurgy of Welding by Easterling; Adhesive Bonding: Science, Technology and Applications edited by Adams.
Online resources: AWS D1.1 Structural Welding Code; ISO 3834 welding quality requirements; MIT OpenCourseWare 3.37 Welding and Joining Processes; The Welding Institute (TWI) technical knowledge base; Lincoln Electric Welding Handbook.
Chapter 1: The Role of Joining
1.1 Why Join
Engineering structures are almost always assemblies. Joining enables large structures from manageable parts, allows different materials to be combined, accommodates replacement and repair, and provides service access. Welding and joining accompany every stage from fabrication through maintenance.
1.2 Classification
Joining methods divide into mechanical fastening (bolts, rivets, interference fits), adhesive bonding, soldering and brazing, solid-state welding, and fusion welding. Each class has distinct mechanisms, strengths, weaknesses, and application domains.
1.3 Selection Criteria
Joint selection considers base materials, strength, temperature, environment, geometry, production volume, cost, inspectability, and life-cycle requirements. A joint is a system: its performance depends on base metals, filler, heat-affected zones, residual stresses, and service loads interacting over time.
Chapter 2: Fundamentals of Fusion Welding
2.1 The Thermal Cycle
Fusion welding deposits heat into a moving source, producing a molten pool that solidifies behind it. The Rosenthal thick-plate solution for a moving point source
\[ T - T_0 = \frac{q}{2\pi k r}\exp\!\left[-\frac{v(r + x)}{2\alpha}\right] \]gives the temperature field, with heat input \( q \), travel speed \( v \), thermal conductivity \( k \), and thermal diffusivity \( \alpha \). Cooling rate determines microstructure in the fusion zone and heat-affected zone.
2.2 Heat Input and Efficiency
Heat input per unit length is
\[ H = \frac{\eta V I}{v}, \]with process efficiency \( \eta \) typically 0.6–0.9 for arc processes. Heat input sets pool size, cooling rate, heat-affected zone width, and distortion.
2.3 Weld Pool and Solidification
The pool experiences buoyancy, surface tension, and electromagnetic forces that drive convection. Solidification begins epitaxially from unmelted grains, producing columnar dendrites that tend to grow toward the centreline. Segregation and solidification cracking can occur in susceptible alloys.
Chapter 3: Arc Welding Processes
3.1 Shielded Metal Arc Welding
SMAW (stick welding) uses a flux-coated consumable electrode. The flux generates a shielding gas and a slag that protects the weld pool. Versatility, portability, and modest equipment cost keep SMAW ubiquitous for repair, pipeline, and field work.
3.2 Gas Metal Arc Welding
GMAW (MIG/MAG) feeds a bare wire electrode into the arc with an inert or active gas shield. Short-circuit, globular, spray, and pulsed transfer modes trade spatter, penetration, and positional capability. Modern synergic power supplies automate parameter coordination.
3.3 Gas Tungsten Arc Welding
GTAW (TIG) strikes an arc between a non-consumable tungsten electrode and the workpiece under argon or helium. Filler is added separately. GTAW produces the highest quality welds in thin sections and reactive metals but at relatively low deposition rates.
3.4 Submerged Arc Welding
SAW buries the arc under a granular flux, producing very high deposition rates suitable for long straight welds on thick sections such as pressure vessels, ships, and pipeline girth welds. Mechanisation and tandem arrangements multiply productivity.
Chapter 4: High-Energy and Alternative Fusion Processes
4.1 Electron Beam Welding
An electron beam focused under vacuum delivers power densities up to 10\(^{10}\) W m\(^{-2}\). Deep, narrow welds with minimal heat-affected zone are characteristic. Applications include aerospace, precision instruments, and refractory metals. Non-vacuum and partial-vacuum variants broaden applicability.
4.2 Laser Beam Welding
Solid-state, fibre, and disc lasers at multi-kilowatt power deliver high-quality keyhole welds at high travel speeds. Hybrid laser-arc welding combines laser penetration with filler capability of arc welding, suited to shipbuilding and automotive. Remote-scanner laser welding accelerates automotive assembly.
4.3 Plasma Arc and Oxyfuel
Plasma arc welding constricts the arc through a water-cooled nozzle for stable, high-energy density welds on thin and moderately thick sections. Oxyfuel welding, once dominant, persists for braze welding, light repair, and heating operations.
Chapter 5: Resistance Welding
5.1 Resistance Spot Welding
RSW passes current through clamped sheets. Joule heating \( P = I^2 R \) concentrates at the interface where contact resistance is highest, forming a molten nugget that solidifies on current removal. Weld quality depends on current, time, electrode force, and electrode condition. RSW dominates automotive body-in-white assembly.
5.2 Other Resistance Processes
Seam welding produces continuous welds through rotating wheel electrodes. Projection welding concentrates heat at raised features. Flash and upset welding butt-join bar and wire at very high currents. Each process matches particular geometries and materials.
5.3 Process Control
Resistance welding benefits from real-time monitoring of current, voltage, displacement, and acoustic emission. Adaptive control compensates for electrode wear and sheet surface condition, maintaining weld quality across long production runs.
Chapter 6: Solid-State Joining
6.1 Friction Welding
Conventional friction welding rotates one part against another under axial force, generating heat through interfacial sliding. After a forge stroke, the components bond without melting. Variants include inertia, continuous-drive, and linear friction.
6.2 Friction Stir Welding
FSW employs a rotating, profiled tool traversed along the joint line. Plasticised material flows around the tool pin, producing a solid-state bond. FSW excels for aluminium alloys difficult to fusion weld (2xxx, 7xxx), including automotive, marine, and aerospace applications.
6.3 Ultrasonic, Explosion, and Diffusion Bonding
Ultrasonic welding applies high-frequency vibration under pressure, used for thin metals and thermoplastics. Explosion welding bonds dissimilar metals through controlled detonation, creating metallurgically bonded cladding. Diffusion bonding joins materials at elevated temperature and pressure over long times, producing bonds indistinguishable from base metals.
Steels with CE above about 0.45% typically require preheat and controlled cooling to avoid cracking.
Chapter 7: Soldering, Brazing, and Adhesive Bonding
7.1 Soldering and Brazing
Soldering (melt below 450°C) and brazing (melt above 450°C) use a filler metal that wets the base materials without melting them. Wetting is characterised by contact angle; fluxes remove oxides and promote wetting. Capillary action fills narrow joints, making gap design critical.
7.2 Adhesive Bonding
Adhesives distribute load over larger areas, join dissimilar materials without galvanic corrosion, and produce sealed joints. Epoxy, polyurethane, acrylic, silicone, and cyanoacrylate adhesives span rigid to flexible performance. Surface preparation—cleaning, abrasion, primers—is the decisive variable for durable bonds.
7.3 Joint Design
Brazed, soldered, and adhesive joints perform best under shear and compression; they are weak in cleavage and peel. Lap joints, scarf joints, and tongue-and-groove configurations maximise shear area. Stress analysis accounts for thermal mismatch between adherends and adhesive.
Chapter 8: Weld Metallurgy, Quality, and Design
8.1 Microstructural Zones
A weld comprises the fusion zone (melted and resolidified metal), heat-affected zone (solid but metallurgically altered), and unaffected base metal. Peak temperature, cooling rate, and composition determine the microstructure in each region: martensite, bainite, acicular ferrite, polygonal ferrite, or grain-coarsened structures.
8.2 Defects
Common defects include porosity (gas entrapment), slag inclusions, lack of fusion, incomplete penetration, undercut, cracking (hot, cold, reheat, lamellar), and distortion. Root causes trace to heat input, consumables, fit-up, cleanliness, and thermal cycle. Appropriate procedure, welder skill, and consumable handling prevent most defects.
8.3 Inspection and Codes
Visual, liquid penetrant, magnetic particle, ultrasonic, radiographic, and phased-array ultrasonic testing assess weld quality at increasing levels of resolution. Acceptance criteria follow ASME, AWS, ISO, and CSA codes. Procedure and welder qualification by destructive and non-destructive testing establish that the proposed combination of materials, filler, and parameters reliably produces compliant welds.
8.4 Design of Welded Structures
Weld design considers joint geometry, residual stresses, distortion, fatigue, fracture, corrosion, and inspectability. Fatigue of welds, governed by geometric notches and residual stresses, is typically the limiting criterion in cyclically loaded structures; code-based detail categories simplify life assessment. Weld distortion is managed through fit-up, sequence, clamping, and post-weld straightening. Thoughtful design extends beyond the weld itself to the structural system that the weld serves, returning the discipline to its engineering purpose.
