Fundamentals of Welding
Joints
Tension Joint
Shear Joints
Old technology
Introduction to Welding Process
Welding process very common. is a consolidation process to facilitate the joining or assembly. Application of welding processes Automotive / auto industry Aerospace / air plane/ship Train carriage / rail Electrical / electronics Radiator / container Domestic hardware Medical instruments Nuclear equipment Food and drink, etc. Bridge, crane, structure
Welding Process Mechanism
Welding is the permanent joining of two materials, usually metals, by coalescence, which is induced by a combination of temperature, pressure and metallurgical conditions. Coalescence between two metals requires sufficient proximity and activity between the atoms of the pieces being joined to cause the formation of common crystals. Welding process is a kind of casting process
Requirements for Welding Process
The production of a high-quality weld requires (1) a source of satisfactory heat and/or pressure (2) a means of protecting or cleaning the metals to be joined, and (3) caution to avoid, or compensate for, harmful metallurgical effects. (4) Proper design of joining
Classification of Welding
Welding processes
Oxyfuel gas welding(OFW)
Refers a group of welding processes that use the flame produced by the combustion of fuel gas and oxygen as their heat source.
Arc welding
Refers a group of welding processes that use arc between two electrodes as heat source.
Resistance welding
Refers a group of welding processes that both heat and pressure are used to induce coalescence. Electrode contacts with the metal.
Unique processes
Friction welding, ultrasonic welding etc
Basic Joint Design
Five basic joint configurations which can be made with the use of bead, groove and fillet welds In selecting the types of joint to be used, a primary consideration should be the type of loading that will be applied. Cost and accessibility for welding are other important factors. Welding joint design is overlooked. A crack occurring in a small piece may result in a complete failure for a large structure.
Welds Design
For fusion welding, a pool of molten metal is created, with the molten metal coming from either parent plate alone or mixture of parent and filler material. Heating and cooling are essential and integral components, which tend to produce metallurgical changes that often are undesirable. Butt weld The molten pool is an actually a complex alloy of all four materials. The molten material Filler metal is held in place by a metal mold formed by surrounding metal metal solids. It can be viewed as casting a small amount of molten metal Backing plate of metal into a metal mold.
Heat Effects
Thermal Induced Residual Stresses
The stress is the result of restraint to thermal expansion and contraction offered by the pieces being welded. The effect can be observed in the form of dimensional changes, distortion or cracking.
Shrinkage of A Typical Butt Weld
Thermal contraction occurs both parallel (longitudinal) and perpendicular (transverse) to the weld line. The width of the welded assembly simply becomes less than that of positioned components at the time of welding. Components being joined during fabrication typically have considerable freedom of movement, but welds made on a nearly completed structure that somewhat constrained, additional stresses will be induced.
Effect of Thermal Stresses
No specific rules can be used to avoid warping; Tips:
Total heat input should be minimized; Welds should be made with the least; Welding sequences should be designed to use as few passes as possible. The base material should be permitted to have a high freedom of movement.
Top contracts more than the bottom
One side contracts
Weld Defects
Oxyfuel Gas Welding
Using the flame produced by the combustion of a fuel gas (e.g: acetylene) and oxygen as the heat source.
Oxyfuel Gas Welding
Oxyacetylene flame and associated temperature distribution
Oxyfuel gas welding is fusion welding. If there is gap between two base metals, a consumable filler metal is required. filler
Arc Welding
Features: 1) the most common process due to its versatile; low cost equipment and portable;
Submerged Arc Welding
A thick layer of granular flux is deposited just ahead of a solid bare-wire consumable electrode, and an arc is maintained beneath the blanket of flux. High quality welds, require backing plates
Resistance Welding
Using the electric resistance heating to form the joint. Welding processes create joints without any melting of the workpiece or filler material, known as solid-state welding processes. In resistance welding, both heat and pressure are used to induce coalescence. Electrodes are placed in contact with the material and electrical resistance heating is used to raise the temperature of the workpieces and the interface between them.
Heating
The heat for resistance welding is obtained by passing a large electrical current through the workpiece for a short period of time. The amount of heat input can be determined by the basic relationship.
H=I2 R t
H=total heat input in joules I=current in amperes R=electrical resistance of the circult in ohms t= length of time during which current is flowing in seconds
Total Resistance
Total resistance consists of three components
The resistance of the workpiece themselves The contact resistance between the electrodes and the workpieces The resistance between the surfaces to be joined, known as the faying surface
Note the workpieces actually form part of the electrical circuit. High resistance in faying surface is desirable, which depends on the surface roughness and presence of contamination, pressure and contact area.
Figure 38-1
Figure 38-2
Resistance Spot Welding
economical simple means for thin sheet metal join 2000 ~ 5000 spot welds in an automobile.
Resistance Butt Welding
Fabricate the pipes and tubes or other simple structural shapes. Material from 0.1 mm to more than 20 mm Speed up to 80 m/min High speed and high-frequency current produces a very narrow heat-affected zone.
Advantages and Limitation of Resistance Welding
Rapid The equipment can often be fully automated They conserve material, since no filler metal and shield gas is required There is minimal distortion of the parts being joined. Skilled operators are not required. Dissimilar metals can be easily joined. A high degree of reliability and reproducibility can be achieved. The equipment has a high initial cost. There are limitation to the thickness of material that can be joined ( less than 6 mm) and type of joints that can be made (mostly lap joints). Skilled maintenance personnel are required to service the control equipment Surface may receive specifical preparation prior to welding
Friction Welding Processes
In friction welding, the heat required to produce the joint is generated by friction heating at the interface.
Friction Welding Equipment
Friction Stir Welding
The heat is generated by a nonconsumable probe that is rotated at high speed between the abutting edges of rigidly clamped plates. Lower melting metal can be welded. 50 mm thick plates have been welded successfully. Used for improving the surface properties.
