LESSON 10 INTRODUCTION TO WELDED JOINTS
Welding is a process for joining two similar or dissimilar metals by fusion and provides a permanent joint. In welding, the parts are coalesced at their contacting surfaces by a suitable application of heat and/or pressure, with or without the addition of a filler metal. Welding provides a permanent joint but it normally affects the metallurgy of the components. It is therefore usually accompanied by post weld heat treatment for most of the critical components. The welding is widely used as a fabrication and repairing process in industries. Some of the typical applications of welding include the fabrication of ships, pressure vessels, automobile bodies, bridges, welded pipes, sealing of nuclear fuel and explosives, etc.
- Welding is more economical and is much faster process as compared to other processes (riveting, bolting, casting etc.)
- Welding, if properly controlled results permanent joints having strength equal or sometimes more than base metal.
- Large number of metals and alloys both similar and dissimilar can be joined by welding.
- General welding equipment is not very costly.
- Portable welding equipments can be easily made available.
- Welding permits considerable freedom in design.
- Welding can join welding jobs through spots, as continuous pressure tight seams, end-to-end and in a number of other configurations.
- Welding can also be mechanized.
- It results in residual stresses and distortion of the work pieces.
- Welded joint needs stress relieving and heat treatment.
- Welding gives out harmful radiations (light), fumes and spatter.
- Jigs and fixtures may also be needed to hold and position the parts to be welded
- Edges preparation of the welding jobs are required before welding
- Skilled welder is required for production of good welding
- Heat during welding produces metallurgical changes as the structure of the welded joint is not same as that of the parent metal.
10.2 Types of Welding
Welding processes can be broadly classified in two groups: fusion welding and solid-state welding.
10.2.1 Fusion Welding Processes
Fusion Welding processes use heat to melt the base metals. In fusion welding operations, a filler metal is generally added to the molten pool. Fusion welding processes can further be subdivided into following types:
Arc Welding: Arc welding refers to a group of welding processes in which heating of the metals is accomplished by an electric arc.
Resistance welding: Resistance welding achieves coalescence using heat from electrical resistance to the flow of a current passing between the faying surfaces of two parts held together under pressure.
Oxyfuel Gas Welding: These joining processes use an oxyfuel gas, such as a mixture of oxygen and acetylene, to produce a hot flame for melting the base metal.
Other welding processes that produce fusion of the metals joined include electron beam welding and laser beam welding.
10.2.2 Solid-State Welding
Solid-state welding refers to joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting point of the metals being welded. No filler metal is utilized. Some welding processes in this group are:
Diffusion welding: Two surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state fusion.
Friction welding: Coalescence is achieved by the heat of friction between two surfaces.
Ultrasonic welding: Moderate pressure is applied between the two parts and an oscillating motion at ultrasonic frequencies is used in a direction parallel to the contacting surfaces. The combination of normal and vibratory forces results in shear stresses that remove surface films and achieve atomic bonding of the surfaces.
10.3 Types of Welded Joints
Welded joints are primarily of two types: 1. Lap joint or fillet joint, and 2. Butt joint.
10.3.1 Lap Joint
The lap joint or the fillet joint is obtained by overlapping the plates and then welding the edges of the plates. The cross-section of the fillet is approximately triangular. The fillet joints are of three types: - Single transverse fillet, Double transverse fillet and Parallel fillet joints. These are shown in Figure 10.1.
Figure 10.1 Types of Lap or Fillet Joint
10.3.2 Butt Joint
The butt joint is obtained by placing the plates edge to edge as shown in figure. In butt welds, the plate edges do not require bevelling if the thickness of plate is less than 5 mm. On the other hand, if the plate thickness is 5 mm to 12.5 mm, the edges should be bevelled to V or U-groove on both sides. Figure 10.2 shows the types of butt joints.
Figure 10.2 Types of Butt Joint
Corner joint, edge joint and T-joint (shown in Figure 10.3) are some other types of welded joints.
(a) Corner joint (b) Edge joint (c) T - joint
Figure 10.3 Some Other Types of Welded Joints