Solid State (Solid Phase) Welding
Application of P alone or combination of heat and pressure results in join
- If heat is used, the T in process is <m. pt.
No filler metal added
Suitable for joining refractory metals at Ts that do not affect their metallurgical properties
Used in aerospace, automotive and electronics
Principles involve: diffusion, pressure, relative interfacial movements
- Movement between surfaces helps the process, e.g. heat can be generated by friction or ultrasonic vibration
Surface preparation is critical, typically require:
- Chemical cleaning - degreasing, chemical etching
- Mechanical abrasion - erosion, abrasion, lapping, polishing using sand, diamond powder, silicon carbide
Advantages of SSW
(3)
- No melting - no heat affected zone (HAZ) so metal around joint retains original properties and little chance for formation of defects associated with FW
- Many SSW processes produce welded joints that bond entire contact interface between two parts rather than at distract spots or seams
- SSW processes can be used to bond dissimilar metals without concerns about relative m. pts., thermal expansion and other problems that arise in FW
Disadvantages of SSW
(3)
- Expensive equipment
- Involve significant preparation of the parts to be welded
- Most SSW are limited to certain joint designs and thin materials
Roll Welding (Cladding)
Pressure applied to two or more sheets through rollers until sufficient plastic deformation has occurred to produce solid state welds
- Both cold (without external heat) and hot (external heat)
- Equivalent to forming processes - material must be ductile
- Similar metals work best
Surface preparaion - degreased, wire-brushed and cleaned to remove oxide smudges
Roll Bonding
- Introduces nascent surface - new metal-to-metal surfaces formed during plastic deformation
- Asperities are collapsed and oxides broken up
- Increased nascent surfaces results in increased weld strength
- This theory describes the main objective of most SSW processes
Friction Welding (FRW)
Coalescence is achieved by frictional heat combined with P
- When properly carried out - no melting occurs at faying surfaces (also means no chance for solidification-related defects)
- No filler metal, flux or shielding gases normally used
- Can be used to join dissimilar metals
- Widely used commercial process - can be automated
Weld Zone in Friction Welding
- Weld zone is confined to a narrow region
- High P and rotational symmetry of parts (at least one) required
- Surfaces extruded out leaving clean surfaces at high Ts which bond together
- Excess material (flash) usually removed after welding
- P force and rotational speed applied is critical to the shape of the final weld zone
Friction Stir-Welding (FSW)
A third body is rubbed against the two surfaces to be joined called rotating non-consumable probe (D5-6mm)
- Contact Ps causes frictional heating to reach 230-260 degrees
- Al-alloy plates up to 75mm thick can be welded by this process
Industries: auto, aircraft, farm equipment, petroleum and natural gas - shafts and tubular parts
Advantages of FSW
(6)
- No melting means no chance for solidification-related defects
- No filler materials are needed
- Bery few process variables result in a very repeatable process
- Can be used in a production environment
- Fine grain structure of friction welds typically exhibits excellent mechanical properties relative to the base metal, especially when welding Al
- No special joint preparation or welding skill required
Disadvantages of FSW
(5)
- Equipment is very expensive
- At least one of the parts must be rotational
- Flash must usually be removed (extra operation)
- Upsetting reduces the part lengths (which must be taken into consideration in product design)
- Limited joint designs and in the case of Continuous Drive and Inertia Friction Welding, parts must be symmetric
Diffusion Welding
Relies on diffusion to create a weld through combo of heat and P
Diffusion bonding depends on:
- Absence of contamination and adequate surface finish
- Ability of at least one component to undergo sufficient plastic flow to contact interface
- Sufficient time for diffusion to occur in interface region for microstructural stability
Strength depends on P, T, time of contact and cleanliness of surfaces
Joining of high-strength and refractory metals in aerospace and nuclear industries
Can be used to join either similar and dissimilar metals
- For joining dissimilar metals, a filler layer of different metal is often sandwiched between base metals to promote diffusion
Advantages of Diffusion Welding
(5)
- High strength joints
- Versatility - can be used with dissimilar metals, ceramics and composites
- Good for reactive metals - Ti, Mg, Be, Zr
- Minimal degradation to the base metal
- No distortion or deformation
Disadvantages of Diffusion Welding
(4)
- High cost
- Extremely long weld times
- Significant surface preparation required
- Does not work well with all metals (e.g. Ni)
Ultrasonic Welding (USW)
- Produces a weld through the localised application of high-frequency vibratory E combined with moderate static P
- Static normal force and oscillating shearing (tangential) movement creates heat at interface between work pieces
- T can reach 0.3-0.5 Tm
- Works for dissimilar metals (bimetallic strips)
- Wire terminations and slicing in electrical and electronics industry - eleminates need for soldering, assembly of Al sheet metal panels, welding of tubes to sheets in solar panels, assembly of small parts in automotive industry
Advantages of USW
(5)
- Fast welding speeds
- Very low heat input and minimal part distortion (other than surface marking)
- No consumables
- Good for automation
- Versatile
Disadvantages of USW
(4)
- At least one of the parts being welded must be very thin
- Mostly limited to soft metals (e.g. Cu, Al)
- Limited to lap joints
- Equipment cost
Explosion Welding (EXW)
Two metallic surfaces welded by accelerating one of the components at extremely high velocity by the E of a detonated explosive
- Explosive is distributed over top surface of prime component
- Upon detonation, high velocity collision occurs between prime and base component
- Joining happens continuously by local plastic deformation
Features of EXW
- No filler metal
- No external heat applied
- No diffusion occurs - time is too short
- Bonding is metallurgical, combined with mechanical interlocking that results from a rippled or wavy interface between metals
- Can produce almost any dissimilar metal combo e.g. to clad one metal on top of a base metal over large areas
- Cald plate is typically used in the chemical and petrochemical industries as tube sheet for heat exchangers
Advantages of EXW
(4)
- Versatile - can bond with wide range of dissimilar metals in many applications
- Strong bonding
- Cost-effective - does not require expensive equipment
- Reduced HAZ
