Unit 4 : Solid state Welding Processes
By : Mr. Aniruddh V Kakade
Syllabus Content:
Introduction to Welding�
Definition: Welding is a permanent joining process used to combine two or more metallic components by the application of heat, pressure, or both.
Classification:
I. Fusion Welding:
II. Solid-State Welding:
Importance: Solid-state processes help to achieve high quality welds, often superior to fusion welds in terms of strength and microstructural stability.
Features of Solid-State Welding�
Mechanism of Bonding:
A metallurgical bond is created by diffusion of interface atoms when high pressure brings two surfaces into intimate contact.
Advantages:
Disadvantages:
Types of Solid-State Welding Processes�
Cold Pressure Welding
Principle: Cold welding is a solid-state joining process carried out at room temperature.
The joining is achieved by applying very high pressure on the workpiece surfaces.
At the interface, localized plastic deformation occurs, which removes asperities and allows metallic bonding.
Requirements:
At least one of the materials should be ductile.
Surfaces must be thoroughly cleaned prior to welding.
Cold welding is mainly used for non-ferrous metals such as aluminium, copper, and their alloys.
Typical joint configurations: butt joints, lap joints.
Cold Pressure Welding – Working Process
Surface Preparation:
Surfaces of workpieces are cleaned using wire brushes or chemical cleaning.
Degreasing is performed to remove oil and oxide layers.
Initial Contact:
When two metallic surfaces are placed together, only a small fraction of the total area actually comes into contact due to microscopic roughness.
Application of Pressure:
High pressure is applied using dies, rollers, or mechanical presses.
Plastic deformation of surface asperities occurs, increasing the real contact area.
Bond Formation:
Metallic bonds are established due to intimate contact and diffusion at the interface.
Final joint has strength close to base metal.
Cold Pressure Welding – Advantages
Cold Pressure Welding – Limitations and Disadvantages
Cold Pressure Welding – Applications
Diffusion Bonding
Principle
Diffusion bonding (also called diffusion welding) is a solid-state joining process.
Bonding is achieved by diffusion of atoms across the interface of the two metals.
Process is carried out below the melting point of the materials, usually at 0.5 to 0.7 of the melting temperature (Tm).
Both heat and pressure are applied in a controlled atmosphere to achieve the joint.
The weld strength depends on:
Diffusion Bonding – Working Procedure
Surface Preparation:
Surfaces are cleaned to remove dust, oxide layers, and contaminants.
Grinding or polishing may be used.
Placement in Chamber:
Plates are placed inside a vacuum chamber or inert gas atmosphere to prevent oxidation.
Application of Pressure and Heat:
Moderate pressure (typically 35–70 MPa) is applied.
The parts are heated to 0.5–0.7 Tm using a furnace, autoclave, or electrical resistance heating.
Diffusion Mechanism:
Atoms migrate across the interface under the combined action of temperature and pressure.
Plastic deformation is minimal; joint is mainly due to atomic diffusion.
Optimization:
For ferrous materials, higher pressures are required compared to non-ferrous materials.
Filler foils or interlayers may be added to accelerate diffusion and reduce required temperature.
Characteristics of Diffusion Bonding
Applications of Diffusion Bonding
Types Of Diffusion Welding:
I. Gas Pressure Bonding
Definition and Principle: A diffusion bonding method using inert gas to apply uniform pressure, facilitating atomic diffusion between metal surfaces without melting.
Working Process
II. Vacuum Pressure Bonding
Definition and Principle: A diffusion bonding technique using a vacuum environment and applied pressure to join metal surfaces, relying on atomic diffusion for a strong bond.
Working Process
Setup: Place parts between fixed and moving platens in a vacuum chamber with a thermal barrier.
Vacuum Creation: Use a vacuum pump to eliminate air and oxides from the interface.
Pressure and Heat: Apply 70-90 N/mm² pressure and heat (e.g., 1150°C) via radiation or induction.
Diffusion: Atoms migrate across the boundary, with the foil (if used) melting and diffusing into the base metal.
Final Bond: Prolonged exposure forms a metallurgically sound joint.
III. Eutectic Fusion Bonding
Definition and Principle: A variant of diffusion bonding using a thin eutectic alloy foil between two parts; heat and pressure cause the foil to form a low-melting phase that diffuses, creating a strong bond
Working Process
Setup: Place parts with eutectic foil (thickness 0.005-0.025 mm) between them in a vacuum chamber
Application of Conditions: Apply pressure (70-90 N/mm² via mechanical or hydraulic means) and heat (around 1150°C) through induction, conduction, or radiation
Bonding Mechanism: Foil melts into a eutectic phase, diffuses into base metals, and disappears, forming a welded interface
Final Step: Maintain vacuum to promote diffusion and remove any gases, resulting in a high-strength bond.
Process occurs over time under controlled conditions to avoid defects
Explosive Welding (EW)�
Principle
Explosive welding is a solid-state process where two metals are joined by high-velocity impact.
The impact is created by the controlled detonation of an explosive charge, which accelerates one plate (flyer plate) towards the base plate.
The impact pressure and velocity (14,750 – 25,000 ft/s) cause a jetting action that removes surface oxides and contaminants.
This results in intimate metallic contact and formation of a strong metallurgical bond without melting.
Working Mechanism
Stage 1 – Detonation of Explosives
An explosive material such as PETN (Penta erythritol tetranitrate) or TNT(trinitrotoluene) is ignited.
Shock waves travel rapidly, producing very high energy.
Stage 2 – Acceleration of Flyer Plate
The flyer plate is pushed at extremely high velocity towards the fixed base plate.
Impact angle is carefully controlled.
Stage 3 – Collision and Jetting Action
Collision removes oxide layers through jetting.
Clean metal-to-metal contact occurs.
Stage 4 – Bond Formation
Localized plastic deformation ensures permanent weld.
The bond strength is comparable to or greater than the base material.
Materials Used in Explosive Welding
Explosives: PETN (Pentaerythritol Tetranitrate), TNT (Trinitrotoluene), and other high-energy explosives.
Work Materials:
Ductile metals are best suited (Aluminium, Copper, Titanium, Steel).
Can also join dissimilar metals which are difficult to weld by conventional methods (e.g., Al–Cu, Ti–Steel).
Applications of Explosive Welding
Advantages of Explosive Welding
Disadvantages of Explosive Welding
Ultrasonic Welding
Ultrasonic welding is a solid-state welding process that creates a bond between two workpieces by vibrating them at high frequency under pressure. The process involves four stages:
Phase 1: Ultrasonic vibrations with low amplitude are applied to the workpieces, generating heat between the materials in contact, leading to localized heating up to a melting point.
Phase 2: Localized melting occurs due to shear heating, and a layer of molten material forms, initiating the welding process with low vibratory power.
Phase 3: Vibratory power is provided until the thickness of the molten layer becomes constant, and vibrations stop once a steady-state melt flow is achieved.
Phase 4: This cooling stage allows the molten material to solidify, creating the weld joint.
Key components include a power supply, transducer, booster and horn, and fixture or anvil.
Sufficient clamping force ensures intimate workpiece contact;
higher vibratory power suits harder materials; time depends on force and power.
Advantages include welding dissimilar metals, suitability for metal and plastic,
and high weld quality.
Disadvantages include limitation to thin materials, need for a specialized holding device,
and protection of electronic components from vibrations.
Applications include garment manufacturing, electrical connections in aluminum,
and welding thin sheets or wires (e.g., aluminum to steel, tungsten, nickel to brass).
Friction Stir Welding (FSW)�
Principle
Friction stir welding is a solid-state process that joins metals by frictional heat and mechanical stirring, using a non-consumable rotating tool with a shoulder and pin.
Heat from friction softens the material, and the tool’s motion plasticizes and intermixes it to form a seam weld.
Working Mechanism (Stages)
Stage 1 – Tool Engagement: The rotating tool is plunged into the workpiece, generating frictional heat at the interface.
Stage 2 – Material Softening: Heat and pressure soften the material ahead of the tool, enabling plastic deformation.
Stage 3 – Stirring and Mixing: The tool traverses, stirring and forging the material behind the pin to create a solid bond.
Stage 4 – Consolidation: As the tool moves, the stirred material cools and solidifies into a strong weld.
Materials Used
Work Materials: Primarily Aluminum alloys, Magnesium, and Copper; also effective for dissimilar metal joints (e.g., Al–Mg).
Tool: Hardened steel or tungsten-based materials for durability.
Advantages
Solid-phase process with minimal distortion and excellent dimensional stability.
Joins dissimilar metals without melting, reducing defects like porosity.
Environmentally friendly, requiring no shielding gases or filler materials.
Produces fine-grained microstructure, enhancing joint strength.
Disadvantages
Limited to materials with good formability; brittle metals may crack.
Requires robust equipment and precise control of parameters (e.g., rotation speed, tilt angle).
Not suitable for complex geometries due to tool access constraints.
Applications
Aerospace industry for Aluminum structures (e.g., aircraft fuselages).
Shipbuilding for large panel assemblies.
Automotive for lightweight alloy components.
Forge Welding (FW)�
Principle
Forge welding is a traditional solid-state process that joins metals by heating them to a plastic state (below melting point) and hammering or pressing them together.
The process relies on diffusion and mechanical pressure to create a metallurgical bond.
Working Mechanism (Stages)
Stage 1 – Heating: Workpieces are heated in a forge (typically 1000–1300°C for steel) until they reach a ductile, glowing state.
Stage 2 – Surface Preparation: Oxides and scale are removed (e.g., by flux or scraping) to ensure clean contact.
Stage 3 – Joining: The heated pieces are hammered or pressed together, forcing atomic bonding.
Stage 4 – Consolidation: Continued pressure refines the joint, eliminating voids for a solid weld.
Materials Used
Work Materials: Traditionally steel, iron, and wrought iron; also applicable to Copper and Bronze.
Tools: Forge, hammer, anvil, and flux (e.g., borax) to prevent oxidation.
Advantages
Cost-effective using simple tools and minimal energy for small-scale work.
Produces strong, homogeneous joints comparable to the base metal.
Highly flexible for manual or artisanal applications.
Disadvantages
Labor-intensive and skill-dependent, requiring experienced craftsmen.
Limited to simple shapes; complex geometries are difficult to achieve.
Risk of oxidation or inclusions if surface preparation is inadequate.
Applications
Blacksmithing for tools, weapons, and decorative ironwork.
Restoration of historical metal artifacts.
Small-scale fabrication of custom metal parts.
Roll Welding (RW)�
Principle
Roll welding is a solid-state process that joins metals by passing them through rolling mills under pressure, often with heat to enhance diffusion.
The continuous pressure and deformation create a metallurgical bond without melting.
Working Mechanism (Stages)
Stage 1 – Material Alignment: Two or more metal sheets are stacked and aligned for rolling.
Stage 2 – Heating (Optional): Materials may be preheated (below melting point) to improve ductility and bonding.
Stage 3 – Rolling: The stack passes through rollers, applying high pressure to deform and weld the surfaces.
Stage 4 – Bond Refinement: Repeated rolling ensures a uniform, strong bond across the length.
Materials Used
Work Materials: Steel, Aluminum, and Copper; often used for cladding (e.g., Stainless Steel on Carbon Steel).
Equipment: Rolling mills, preheat furnaces (if used).
Advantages
Ideal for producing long, continuous welds in sheet or strip form.
Efficient for cladding and joining dissimilar metals with varying thicknesses.
Offers excellent surface finish and mechanical properties.
Disadvantages
Limited to flat or slightly curved surfaces; complex shapes are unfeasible.
Requires precise alignment and consistent material thickness.
Initial investment in rolling equipment can be significant.
Applications
Manufacturing of clad plates for corrosion-resistant structures.
Production of bimetallic strips for electrical and thermal applications.
Construction industry for large-scale metal sheets.
Hot Pressure Welding
Principle
Hot Pressure Welding (HPW) is a solid-state welding process where flat sheets are joined by applying heat and high pressure simultaneously.
The applied heat softens the material and makes it more plastically deformable.
Under high pressure, surface asperities and oxide films are broken down, producing fresh metallic contact.
This action increases the clean surface area, allowing diffusion and atomic bonding across the joint.
Unlike fusion welding, there is no melting, only localized plastic flow of metals.
The process is usually conducted in a vacuum chamber or inert gas atmosphere to prevent oxidation.
Heating methods include oxy-fuel torch, induction heating, or eddy current.
Pressure can be applied mechanically or using hot inert gas in a closed chamber.
HPW is considered a subcategory of forge welding, but performed in controlled conditions.
Working :
Preparation of Workpieces
Surfaces of flat sheets are cleaned and aligned.
Oxide layers and contaminants are minimized.
Application of Heat
Heat is applied to bring the material to high temperature below its melting point.
Sources: Oxy-fuel, induction heating, eddy currents.
Application of Pressure
A controlled high pressure is applied simultaneously.
Methods: Mechanical presses, hydraulic presses, or hot inert gas chamber.
Plastic Deformation and Bond Formation
Material undergoes plastic deformation, creating surface cracks and fresh metallic exposure.
Diffusion begins across surfaces, leading to strong bond.
Cooling and Solid Bond
The joint cools under pressure, forming a permanent weld.
Advantages of Hot Pressure Welding
i. Ability to Join Diverse Materials
Can weld both similar and some dissimilar flat sheet metals effectively.
ii. Reduced Heat-Affected Zone (HAZ)
Since temperatures are below melting point, thermal damage is minimal.
iii. Good Mechanical Properties
Weld strength is nearly equal to the base metal.
iv. Less Distortion
No melting means less residual stress and dimensional changes.
v. No Filler Required
Unlike fusion welding, the bond is formed by direct atomic contact.
vi. Suitable for Thin Sheets
Provides reliable joining without burn-through problems.
vii. Better Surface Bonding
Plastic deformation enhances diffusion and metallurgical integrity.
Disadvantages of Hot Pressure Welding
i. High Equipment Cost
Requires facilities like vacuum chamber, high-pressure systems, and heating units.
ii. Requirement of High Pressure
Equipment must withstand heavy compressive forces.
iii. Limited to Flat Sheets
Not suitable for complex geometries or curved components.
iv. Slow Process
Requires sufficient time for heating, pressurizing, and cooling.
v. Surface Preparation is Critical
Contamination or improper cleaning may lead to weak joints.
vi. Not Suitable for Brittle Materials
High pressure may cause cracks in brittle alloys.
Applications of Hot Pressure Welding
i. Aerospace Industry
Used for joining titanium, aluminum, and high-strength alloys.
ii. Electronics
Bonding of thin foils and conductive sheets.
iii. Nuclear Industry
Fabrication of reactor components requiring high reliability.
iv. Defense Applications
Manufacture of lightweight yet strong joints.
v. Cladding of Sheets
Used where two different metals need to be bonded in sheet form.
Laser Cladding�
Cladding means depositing a layer of one material onto another to improve surface properties.
Laser Cladding (LC) uses a laser beam as the heat source to melt and deposit metallic powder on a substrate.
The powder is carried through a nozzle by a gas stream.
When it enters the laser’s focal zone, it melts and forms a molten pool on the base surface.
Upon cooling, this molten pool solidifies as a dense coating layer.
The process produces a metallurgical bond between the coating and the substrate.
Laser cladding improves corrosion resistance, wear resistance, hardness, and thermal properties of the surface.
Working of Laser Cladding
Powder Injection: Metallic powder is fed into the laser beam zone through nozzles.
Laser Interaction: The laser melts both the incoming powder particles and a thin surface layer of substrate.
Melt Pool Formation: A localized pool of molten metal is created.
Deposition: The molten material spreads and adheres to the substrate.
Solidification: Rapid cooling forms a dense, defect-free coating.
Layers: Process can be repeated layer by layer for thick coatings or 3D printing applications.
Heat-Affected Zone (HAZ) is minimal compared to conventional fusion cladding due to localized heating.
Advantages of Laser Cladding
i. High-Quality Coating
Strong metallurgical bond with good mechanical integrity.
ii. Low Heat Input
Reduced HAZ, less distortion compared to arc-based methods.
iii. Flexibility
Can use a wide range of powders (alloys, composites, carbides).
iv. Precision Control
Laser power and powder feed rate can be finely adjusted.
v. Multi-Layer Capability
Suitable for building thick coatings or near-net-shape parts.
vi. Improved Surface Properties
Provides excellent corrosion, oxidation, and wear resistance.
Vii .Compatibility with Additive Manufacturing
Plays a key role in repairing components and 3D printing.
Disadvantages of Laser Cladding
i. High Initial Cost
Requires expensive lasers, powder feed systems, and protective enclosures.
ii. Surface Roughness
Coated layer may require post-processing (grinding or polishing).
iii. Cracking and Delamination
Improper parameter selection can cause coating defects.
iv. Process Complexity
Requires skilled operators and precise control of powder flow and laser energy.
v. Limited Deposition Speed
Compared to thermal spraying, deposition rates are slower.
vi. Powder Loss
Not all powder is melted and deposited; some is wasted.
Applications of Laser Cladding
Aerospace Components
Turbine blades, shafts, and structural parts requiring wear-resistant coatings.
Automotive Industry
Coatings on engine parts, crankshafts, and gears.
Tooling and Dies
Surface reinforcement for high wear resistance.
3D Printing / Additive Manufacturing
Building complex parts layer by layer.
Energy Sector
Coating of turbine parts, pumps, and valves for high corrosion resistance.
Repair and Refurbishment
Restoring worn-out components to original dimensions.
Adhesive Bonding�
Adhesive Bonding is a joining process where a filler material (adhesive) is used to bond two surfaces.
Adhesives are generally polymer-based materials, which form a bond after curing.
The adhesive may join metal, ceramic, plastic, wood, or composites.
Unlike welding, adhesives are non-metallic, so bonding depends on mechanical interlocking and surface adhesion forces.
Curing is the process where the adhesive changes from liquid/semi-solid to a solid, providing strength.
Curing can be achieved by heating, using catalysts, or UV light exposure.
Adhesive bonding is widely used in packaging, automotive, aerospace, electronics, and construction industries.
Types of Adhesives
I. Natural Adhesives
Derived from plants and animals (gum, starch, dextrin, collagen, soya flour).
Applications: bookbinding, plywood, cardboard cartons, furniture.
II. Inorganic Adhesives
Based on compounds like sodium silicate, magnesium oxychloride.
Properties: low cost, low strength, used in non-critical applications.
III. Synthetic Adhesives
Made from thermoplastic and thermosetting polymers.
Most widely used in industries.
Cure by catalysts, heating, radiation (UV), or solvent evaporation.
Can be applied as films, sprays, or pressure-sensitive tapes.
Advantages & Limitations of Adhesive Bonding
Advantages
Limitations
Applications of Adhesive Bonding