question: Why does welding work when the welding “liquid” is a different material to the metal being welded? Does it work like glue in that it gets into crevices and solidifies or is there a chemical reaction between the 2 different metals?
comment: You seem to be referring to "brazing" where a melted metal attaches two other metal surfaces together. As you suspect it is weaker than welding. With welding the "welding liquid" actually includes melting the two metal surfaces such that they mix and join, something which traditional gluing doesn't involve.
passages: Most solids used are engineering materials consisting of crystalline solids in which the atoms or ions are arranged in a repetitive geometric pattern which is known as a lattice structure. The only exception is material that is made from glass which is a combination of a supercooled liquid and polymers which are aggregates of large organic molecules.
   Section::::Welding techniques.:Solvent welding.
  
  In solvent welding, a solvent is applied which can temporarily dissolve the polymer at room temperature. When this occurs, the polymer chains are free to move in the liquid and can mingle with other similarly dissolved chains in the other component. Given sufficient time, the solvent will permeate through the polymer and out into the environment, so that the chains lose their mobility. This leaves a solid mass of entangled polymer chains which constitutes a solvent weld.
   Many thermoplastics can also be welded using chemical solvents. When placed in contact with the plastic, the solvent will begin to soften it, bringing the surface into a thick, liquid solution. When two melted surfaces are pressed together, the molecules in the solution mix, joining them as one. Because the solvent can permeate the plastic, the solvent evaporates out through the surface of the plastic, causing the weld to drop out of solution and solidify. A common use for solvent welding is for joining PVC or ABS (acrylonitrile butadiene styrene) pipes during plumbing, or for welding styrene and polystyrene plastics in the construction of models. Solvent welding is especially effective on plastics like PVC which burn at or below their glass transition, but may be ineffective on plastics like Teflon or polyethylene that are resistant to chemical decomposition.
   Friction welding is also used with thermoplastics, which act in a fashion analogous to metals under heat and pressure. The heat and pressure used on these materials is much lower than metals, but the technique can be used to join metals to plastics with the metal interface being machined. For instance, the technique can be used to join eyeglass frames to the pins in their hinges. The lower energies and pressures used allows for a wider variety of techniques to be used.
  
  Section::::History.
   Section::::Welding techniques.:Laser welding.
  
  This technique requires one part to be transmissive to a laser beam and either the other part absorptive or a coating at the interface to be absorptive to the beam. The two parts are put under pressure while the laser beam moves along the joining line. The beam passes through the first part and is absorbed by the other one or the coating to generate enough heat to soften the interface creating a permanent weld.
   In addition to melting the base metal, a filler material is typically added to the joint to form a pool of molten material (the weld pool) that cools to form a joint that, based on weld configuration (butt, full penetration, fillet, etc.), can be stronger than the base material (parent metal). Pressure may also be used in conjunction with heat, or by itself, to produce a weld. Welding also requires a form of shield to protect the filler metals or melted metals from being contaminated or oxidized.
   The spray-arc transfer variation was developed in the early 1960s, when experimenters added small amounts of oxygen to inert gases. More recently, pulsed current has been applied, giving rise to a new method called the pulsed spray-arc variation.
   The welder must add the filler rod to the molten puddle. The welder must also keep the filler metal in the hot outer flame zone when not adding it to the puddle to protect filler metal from oxidation. Do not let the welding flame burn off the filler metal. The metal will not wet into the base metal and will look like a series of cold dots on the base metal. There is very little strength in a cold weld. When the filler metal is properly added to the molten puddle, the resulting weld will be stronger than the original base metal.
   Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, such as the welding of aluminum with steel in ship hulls or compound plates. Other solid-state welding processes include friction welding (including friction stir welding), magnetic pulse welding, co-extrusion welding, cold welding, diffusion bonding, exothermic welding, high frequency welding, hot pressure welding, induction welding, and roll welding.
  
  Section::::Geometry.
   Glasses and certain types of plastics are commonly welded materials. Unlike metals, which have a specific melting point, glasses and plastics have a melting range, called the glass transition. When heating the solid material past the glass-transition temperature (T) into this range, it will generally become softer and more pliable. When it crosses through the range, above the glass-melting temperature (T), it will become a very thick, sluggish, viscous liquid, slowly decreasing in viscosity as temperature increases. Typically, this viscous liquid will have very little surface tension compared to metals, becoming a sticky, taffy to honey-like consistency, so welding can usually take place by simply pressing two melted surfaces together. The two liquids will generally mix and join at first contact. Upon cooling through the glass transition, the welded piece will solidify as one solid piece of amorphous material.
   "Welding" is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, but sometimes pressure is used in conjunction with heat, or by itself, to produce the weld.
   Titanium alloys are commonly forge welded. Because of titanium's tendency to absorb oxygen when molten, the solid-state, diffusion bond of a forge weld is often stronger than a fusion weld in which the metal is liquefied. 
  
  Forge welding between similar materials is caused by solid-state diffusion. This results in a weld that consists of only the welded materials without any fillers or bridging materials. Forge welding between dissimilar materials is caused by the formation of a lower melting temperature eutectic between the materials. Due to this the weld is often stronger than the individual metals.
  
  Section::::Processes.
   Early work on the mode of material flow around the tool used inserts of a different alloy, which had a different contrast to the normal material when viewed through a microscope, in an effort to determine where material was moved as the tool passed.
   These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct current (DC) or alternating current (AC), and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.
  
  Section::::Processes.:Arc.:Power supplies.
   In the case of webs and films a filler rod may not be used. Two sheets of plastic are heated via a hot gas (or a heating element) and then rolled together. This is a quick welding process and can be performed continuously.
  
  Section::::Welding techniques.:Hot gas welding.:Welding rod.
  
  A plastic welding rod, also known as a "thermoplastic welding rod", is a rod with circular or triangular cross-section used to bind two pieces of plastic together. They are available in a wide range of colors to match the base material's color. Spooled plastic welding rod is known as "spline".
   Section::::Vibration welding process.
  
  The vibration welding process consists of four steps: solid friction, transient flow, steady state flow, and solidification.
  
  Section::::Vibration welding process.:Solid friction.
  
  In this first stage, vibration is commenced between two cold parts pressed together at a constant pressure. The frictional energy causes the polymers to heat. In this stage there is no weld penetration as melting has not yet occurred.
  
  Section::::Vibration welding process.:Transient flow.
   When designing a weld joint, multiple factors are considered.. Some examples of those factors include: desired weld strength, geometry of the parts, material being welded, cosmetic of the joint, whether post processing is an option or not. It is important to balance all of these factors to achieve the optimal final part.
   Intermediate film welding is a method to join incompatible plastic components by using an intermediate film between them. Similar to transmission welding, laser radiation passes through the transparent components and melts the intermediate layers to create a joint. This film can be made of an opaque thermoplastic, solvent, viscous fluid, or other substances that heat up upon exposure to laser energy. The combination of intermediate films and adhesion promoters is able to join incompatible thermoplastics together. The thin layer then generates the heat required to fuse the system together.
  
  Section::::Applications.
  
  Section::::Applications.:Automotive applications.
   Section::::Welding techniques.:Friction welding.:Spin welding.
   Section::::Production.:Thermal spraying.
  
  Thermal spraying techniques are another popular finishing option, and often have better high temperature properties than electroplated coatings.Thermal spraying, also known as a spray welding process, is an industrial coating process that consists of a heat source (flame or other) and a coating material that can be in a powder or wire form which is melted then sprayed on the surface of the material being treated at a high velocity. The spray treating process is known by many different names such as hvof, plasma spray, flame spray, arc spray, and metalizing.
  
  Section::::Microstructure.
   Depending on different interactions between laser and thermoplastics, four different laser welding techniques have been developed for plastic joining. CO lasers have good surface absorption for most thermoplastics, hence they are applied for direct laser welding and laser surface heating. Through transmission laser welding and intermediate film welding require the deep penetration of laser beam, so YAG lasers and diode lasers are the most common sources for these techniques.
  
  Section::::Laser welding techniques.:Direct laser welding.
   Like the first welding process, forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.
   Section::::Metal transfer modes.:Short-circuiting.:Cold Metal Transfer.
  
  For thin materials, Cold Metal Transfer (CMT) is used by reducing the current when a short circuit is registered, producing many drops per second. CMT can be used for aluminum.
  
  Section::::Metal transfer modes.:Spray.
   Welding processes work around the melting point of the base metal and require the base metal itself to begin melting. They usually require more precise distribution of heat from a small torch, as melting the entire workpiece is avoided by controlling the distribution of heat over space, rather than limiting the maximum heat. If filler is used, it is of a similar alloy and melting point to the base metal.
   Section::::Welding techniques.:Laser welding.:Effect of Constituent Properties on Weldability.:Matrix.:Chemical bonding.
  