Solid State Welding (SSW) – Types Of Solid State Welding Process 

Solid State Welding is a welding process, in which two work pieces are joined under a pressure providing an intimate contact between them and at a temperature essentially below the melting point of the parent material. Bonding of the materials is a result of diffusion of their interface atoms.

Advantages of Solid State Welding:

  • Weld (bonding) is free from microstructure defects (pores, non-metallic inclusions, segregation of alloying elements)
  • Mechanical properties of the weld are similar to those of the parent metals
  • No consumable materials (filler material, fluxes, shielding gases) are required
  • Dissimilar metals may be joined (steel – aluminum alloy steel – copper alloy).

Disadvantages of Solid State Welding:

  • Thorough surface preparation is required (degreasing, oxides removal, brushing/sanding)
  • Expensive equipment.

The following processes are related to Solid State welding:

  • Forge Welding (FOW)
  • Cold Welding (CW)
  • Friction Welding (FRW)
  • Explosive Welding (EXW)
  • Diffusion Welding (DFW)
  • Ultrasonic Welding (USW)

Forge Welding (FOW)
Forge Welding is a Solid State Welding process, in which low carbon steel parts are heated to about 1800°F (1000°C) and then forged (hammered).
Prior to Forge Welding, the parts are scarfed in order to prevent entrapment of oxides in the joint.

Forge Welding is used in general blacksmith shops and for manufacturing metal art pieces and welded tubes.

forge welding process
forge welding process

Advantages of Forge Welding:

  • Good quality weld may be obtained;
  • Parts of intricate shape may be welded;
  • No filler material is required.
  • Disadvantages of Forge Welding:
  • Only low carbon steel may be welded;
  • High level of the operators skill is required;
  • Slow welding process;
  • Weld may be contaminated by the coke used in heating furnace.

Cold Welding (CW)
Cold Welding is a Solid State Welding process, in which two work pieces are joined together at room temperature and under a pressure, causing a substantial deformation of the welded parts and providing an intimate contact between the welded surfaces.

As a result of the deformation, the oxide film covering the welded parts breaks up, and clean metal surfaces reveal. Intimate contact between these pure surfaces provide a strong and defectless bonding.

Cold welding in steel and indium
Cold welding in steel and indium

Aluminum alloys, Copper alloys, low carbon steels, Nickel alloys, and other ductile metals may be welded by Cold Welding.

Cold Welding is widely used for manufacturing bi-metal steel – aluminum alloy strips, for cladding of aluminum alloy strips by other aluminum alloys or pure aluminum (Corrosion protection coatings). Bi-metal strips are produced by Rolling technology. Presses are also used for Cold Welding.

Cold Welding may be easily automated.

Friction Welding (FRW)
Friction Welding is a Solid State Welding process, in which two cylindrical parts are brought in contact by a friction pressure when one of them rotates. Friction between the parts results in heating their ends. Forge pressure is then applied to the pieces providing formation of the joint.

Carbon steels, Alloy steels, Tool and die steels, Stainless steels, Aluminum alloys, Copper alloys, Magnesium alloys, Nickel alloys, Titanium alloys may be joined by Friction Welding.

Explosive Welding (EXW)
Explosive Welding is a Solid State Welding process, in which welded parts (plates) are metallurgically bonded as a result of oblique impact pressure exerted on them by a controlled detonation of an explosive charge.

One of the welded parts (base plate) is rested on an anvil, the second part (flyer plate) is located above the base plate with an angled or constant interface clearance.
Explosive charge is placed on the flyer plate. Detonation starts at an edge of the plate and propagates at high velocity along the plate.
The maximum detonation velocity is about 120% of the material sonic velocity.
The slags (oxides, nitrides and other contaminants) are expelled by the jet created just ahead of the bonding front.

explosive welding diagram
explosive welding diagram

Most of the commercial metals and alloys may be bonded (welded) by Explosive Welding.

Dissimilar metals may be joined by Explosive Welding:

  • Copper to steel;
  • Nickel to steel;
  • Aluminum to steel;
  • Tungsten to steel;
  • Titanium to steel;
  • Copper to aluminum.

Advantages of Explosive Welding

  1. Large surfaces may be welded;
  2. High quality bonding: high strength, no distortions, no porosity, no change of the metal microstructure;
  3. Low cost and simple process;
  4. Surface preparation is not required.

Disadvantages of Explosive Welding:

  • Brittle materials (low ductility and low impact toughness) cannot be processed;
  • Only simple shape parts may be bonded: plates, cylinders;
  • Thickness of flyer plate is limited – less than 2.5” (63 mm);
  • Safety and security aspects of storage and using explosives.

Explosive Welding is used for manufacturing clad tubes and pipes, pressure vessels, aerospace structures, heat exchangers, bi-metal sliding bearings, ship structures, weld transitions, corrosion resistant chemical process tanks.

Diffusion Welding (DFW)
Diffusion Welding is a Solid State Welding process, in which pressure applied to two work pieces with carefully cleaned surfaces and at an elevated temperature below the melting point of the metals. Bonding of the materials is a result of mutual diffusion of their interface atoms.

In order to keep the bonded surfaces clean from oxides and other air contaminations, the process is often conducted in vacuum.
No appreciable deformation of the work pieces occurs in Diffusion Welding.

Diffusion Welding is often referred more commonly as Solid State Welding (SSW).

Diffusion Welding is able to bond dissimilar metals, which are difficult to weld by other welding processes:

  • Steel to tungsten;
  • Steel to niobium;
  • Stainless steel to titanium;
  • Gold to copper alloys.

Diffusion Welding is used in aerospace and rocketry industries, electronics, nuclear applications, manufacturing composite materials.

Advantages of Diffusion Welding:

  • Dissimilar materials may be welded (Metals, Ceramics, Graphite, glass);
  • Welds of high quality are obtained (no pores, inclusions, chemical segregation, distortions).
  • No limitation in the work pieces thickness.

Disadvantages of Diffusion Welding:

  • Time consuming process with low productivity;
  • Very thorough surface preparation is required prior to welding process;
  • The mating surfaces must be precisely fitted to each other;Relatively high initial investments in equipment.

Ultrasonic Welding (USW)
Ultrasonic Welding is a Solid State Welding process, in which two work pieces are bonded as a result of a pressure exerted to the welded parts combined with application of high frequency acoustic vibration (ultrasonic).
Ultrasonic vibration causes friction between the parts, which results in a closer contact between the two surfaces with simultaneous local heating of the contact area. Interatomic bonds, formed under these conditions, provide strong joint.

Ultrasonic cycle takes about 1 sec. The frequency of acoustic vibrations is in the range 20 to 70 KHz.
Thickness of the welded parts is limited by the power of the ultrasonic generator.

Ultrasonic Welding is used mainly for bonding small work pieces in electronics, for manufacturing communication devices, medical tools, watches, in automotive industry.

Advantages of Ultrasonic Welding:

  • Dissimilar metals may be joined;
  • Very low deformation of the work pieces surfaces;
  • High quality weld is obtained;
  • The process may be integrated into automated production lines;
  • Moderate operator skill level is enough.

Disadvantages of Ultrasonic Welding:

  • Only small and thin parts may be welded;
  • Work pieces and equipment components may fatigue at the reciprocating loads provided by ultrasonic vibration;
  • Work pieces may bond to the anvil.

PROCESS VARIATIONS

  • CW: process is performed at room temperature using high forces to create substantial deformation (up to 95 per cent) in the parts to be joined. Surfaces require degreasing and scratch-brushing for good bonding characteristics.
  • Cold pressure spot welding: for sheet-metal fabrication using suitably shaped indenting tools.
  • Forge welding: the material is heated in a forge or oxyacetylene ring burners. Hand tools and anvil used to hammer together the hot material to form a solid state weld. Commonly associated with the blacksmith’s trade and used for decorative and architectural work.
  • Thermocompression bonding: performed at low temperatures and pressures for bonding wires to electrical circuit boards.
  • USW: hardened probe introduces a small static pressure and oscillating vibrations at the joint face disrupting surface oxides and raising the temperature through friction and pressure to create a bond. Can also perform spot welding using similar equipment.
  • Ultrasonic Seam Welding (USEW): ultrasonic vibrations imparted through a roller traversing the joint line.
  • Ultrasonic soldering: uses an ultrasonic probe to provide localized heating through high frequency oscillations. Eliminates the need for a flux, but requires pre-tinning of surfaces.
  • Ultrasonic insertion: for introducing metal inserts into plastic parts for subsequent fastening operations.
  • Ultrasonic staking: for light assembly work in plastics.
  • FRW: the two parts to be welded, one stationary and one rotating at high speed (up to 3000 rpm), have their joint surfaces brought into contact. Axial pressure and frictional heat at the interface create a solid state weld on discontinuation of rotation and on cooling.
  • Friction stir welding: uses the frictional heat to soften the material at the joint area using a wear resistant rotating tool.
  • EXW: uses explosive charge to supply energy for a cladding sheet-metal to strike the base sheetmetal causing plastic flow and a solid state bond. Bond strength is obtained from the characteristic wavy interlocking at the joint face. Can also be used for tube applications.
  • DFW: The surfaces of the parts to be joined are brought together under moderate loads and temperatures in a controlled inert atmosphere or vacuum. Localized plastic deformation and atomic interdiffusion occurs at the joint interface, creating the bond after a period of time.
    • Superplastic diffusion bonding: can integrate DFW with superplastic forming to produce complex fabrications.
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