2 3.1 INTRODUCTIONJoining term covers processes such as welding, brazing, soldering, adhesive bonding and mechanical fastening. These processes are an important for the following reasons:1. The product is impossible to manufacture as a single piece.2. The product is easier and more economical to manufacture as individual components, which are then assembled3. Transporting the product in individual components and assembled them at the customer's plant may be easier and less cost.
4 3.2. ADVANTAGES AND DISADVANTAGES OF WELDING 1. Welding is more economical and is much faster process as compared to other processes (riveting, bolting, casting etc.)2. Welding, if properly controlled results permanent joints having strength equal or sometimes more than base metal.3. Large number of metals and alloys both similar and dissimilar can be joined by welding.4. General welding equipment is not very costly.5. Portable welding equipments can be easily made available.6. Welding permits considerable freedom in design.7. Welding can join welding jobs through spots, as continuous pressure tight seams, end-to-end and in a number of other configurations.8. Welding can also be mechanized.
5 Disadvantages1. It results in residual stresses and distortion of the workpieces.2. Welded joint needs stress relieving and heat treatment.3. Welding gives out harmful radiations (light), fumes and spatter.4. Jigs, and fixtures may also be needed to hold and position the parts to be welded5. Edges preparation of the welding jobs are required before welding6. Skilled welder is required for production of good welding7. Heat during welding produces metallurgical changes as the structure of the welded joint is not same as that of the parent metal.
7 3.3. TYPE OF WELDING JOINTS 1- Butt joints 2- Corner joints: maybe welded from the inside or outside of the corner3- The T-joints: obtain its name from the placement of the base metals to form a T shape.4- Edge joints: may be prepared in a number of ways as seen in Fig. 3.35- Lap joints: the metal which form the lap joint-is seldom altered in preparation for welding
9 3.5. WELDING PROCESSES The welding processes may be classified as: 1. solid state welding,2. resistance welding,3. oxyfuel welding,4. arc welding, and other joining processes such as5. brazing and6. soldering.
10 .6. SOLID STATE WELDINGIn solid state welding, joining is achieved by the application of heat, pressure or both. Unlike the resistance, gas, and arc welding processes, no liquid phase is present in the joint also, in solid state welding, the process is accomplished without fluxes or filler metals.The different welding processes of solid state welding are: Forge welding, cold welding, friction welding, ultrasonic welding, and explosion welding.
11 3.7. THE OXYACETYLENE PROCESS Because steel melts at a temperature above 1500oC, the mixture of oxygen and acetylene (the most common gas welding) is used as it is the only gas combination with enough heat to weld steel. However, other gases such as propane, hydrogen and coal gas can be used for joining lower melting point non-ferrous metals, and for brazing and silver soldering.
13 3.8. ELECTRIC ARC WELDINGThe welding in which the electric arc is produced to give heat for the purpose of joining two surfaces is called electric arc welding.
14 Welding ElectrodesAn electrode is a piece of wire or a rod of a metal or alloy, with or without coatings. An arc is set up between electrode and workpiece.Welding electrodes are classified into following types:-(1) Consumable Electrodes(a) Bare Electrodes(b) Coated Electrodes(2) Non-consumable Electrodes(a) Carbon or Graphite Electrodes(b) Tungsten Electrodes
15 3.9. Shielded Metal Arc Welding (SMAW) or Manual Metal Arc Welding (MMAW)
25 3.16. BRAZINGbrazing is a process of joining metals without melting the base metal. Filler material used for brazing has liquidus temperature above 450°C and below the solidus temperature of the base metal
26 3.17. SOLDERINGSoldering is a method of joining similar or dissimilar metals by heating them to a suitable temperature and by means of a filler metal, called solder, having liquids temperature not exceeding 450°C and below the solidus of the base material.