Asignment 4

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S2. Explain the difference between resistance seam welding and resistance spot welding.
5. What is the basic principle of (a) ultrasonic welding and (b) diffusion bonding?
7. Describe the principle of resistance-welding processes.
12. Explain the similarities and differences between the joining processes described in this chapter and those described in Chapter 10.
18. Explain the sources of heat for the processes described in chapter11.
23. Explain the significance of the magnitude of the pressure applied through the electrodes during a spot-welding operation.

S2. Explain the difference between resistance seam welding and resistance spot welding.

RSEW (resistance seam welding) is a modification of RSW (resistance spot welding).
The electrodes of RSEW are replaced by rotating wheels or rollers. Rotating rolls act as electrodes.
Using a continuous AD or DC power supply, the electrically conducting rollers produce a spot weld whenever the current reaches a sufficiently high level. The spot welds actually overlap into a continuous seam rather than a spot nugget. The resistance seam welding has higher nugget formation frequency. Because of the shunt of the overlap nuggets and heat accumulation, the seam welding process needs a programmed current.

5. What is the basic principle of (a) ultrasonic welding and (b) diffusion bonding?

(a) In USW, the faying surfaces of the two components are subjected to a static normal force and oscillating shearing (tangential) stresses. The shearing stresses cause plastic deformation at the interface of the two components, breaking up oxide films and contaminants and thus allowing good contact and producing a strong solid-state bond. In certain situation, the temperature generated can be sufficiently high to cause metallurgical changes in the weld zone. With the temperature rise and accelerated diffusion at the interface, a strong solid-state bond is formed.

(b) DFW is a process in which the strength of the joint results primarily from diffusion (movement of atoms across the interface) and secondarily from plastic deformation of the faying surfaces. The parts are combined with a fixture which assures the position and proper pressure, sometimes with filler metals to promote the bonding. The combinations are placed in a furnace to allowing a long period of diffusion. This process requires temperatures of about 0.5T_m (where Tm is the melting point of the metal on the absolute scale) in order to have a sufficiently high diffusion rate between the parts being joined. Its strength depends on (a) pressure, (b) temperature, © time of contact, and (d) how clean the faying surfaces are.

7. Describe the principle of resistance-welding processes.

The category of (resistance welding) RW covers a number of processes in which the heat required for welding is produced by means of electrical resistance across the two components to be joined.
H=I^2 Rt
Considering the heat lose, the effective heat becomes
H_ε=I^2 RtK
The resistance includes (a) electrode resistance (b) workpiece resistance © electrode-workpiece contact resistances, and (d) workpiece-workpiece contact resistance. With a high current pass through, the resistance heat generated makes the welding interface melting or softening. After cooling down, a sound joint is formed.
Pressure is applied all the time in resistance welding.

12. Explain the similarities and differences between the joining processes described in this chapter and those described in Chapter 10.

Similarities:
Two pieces are joined together by application of heat. Every welding process in the two chapters is used to permanently join the separated parts together. The interface of mating components undergo deformation. Both of them consist of many types of welding methods.

Differences:
All the fusion welding processes join the components by melting part of the base metal, but the solid-state welding will not. Mechanical energy (friction) and resistance heat are commonly found in chapter 11, and the arcs and high energy beam are mainly used in chapter 10.
In solid-state welding, workpieces do not need filler metal and special protection are used; if heat is used, it is not externally applied, but instead is generated internally-for example, with friction. Pressure should also be applied on the mating components.
In fusion welding processes, workpiece undergo a phase change and in most of fusion welding process, filler metal and special protection are used. The heat is externally applied by gas, arc and so on. Usually, the process goes on without additive pressure.
(… do not require, whereas … requires)

18. Explain the sources of heat for the processes described in chapter11.

In CW and EXW, the source of heat comes from the plastic deformation.
In USW and FRW, the heat is generated by friction and plastic deformation.
In RSW, RSEW, HFRW and PEW, the heat required for welding is produced by means of electrical resistances across the two components to be joined.
In FW and SW, the source of heat is from the combination of arc and resistance heat.
In DFW, the parts are usually heated in a furnace or by electrical resistance.
The source of heat come from plastic deformation, friction, electrical resistance or arc in this chapter.

23. Explain the significance of the magnitude of the pressure applied through the electrodes during a spot-welding operation.

The pressure applied through the electrode is very significant during RSW. The pressure is applied to keep the workpieces in appropriate contact to ensure the stable heat generation. If the pressure is too low, the contact area will be smaller. It would result in an over high current density and consequently a splash. On the other hand, an over pressure will lead to the lower current density and a weak joint. By the way, sufficient pressure is also important to follow the nugget shrinkage in solidification. Accurate control and timing of the electric current and of the pressure are essential in RSW.
Under the action of the pressure, an interface corona-bond around the melting region is formed to hold the melt metal inside.

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