热加工作业考研题目答案分享——Joining processes 3

Assignment 3

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文章目录

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10. How is weldability defined?
12. Describe the common types of discontinuities in welded joints.
15. Explain why some joints may have to be preheated prior to welding.
21. Describe the methods by which discontinuities in welding can be avoided.
29. Explain the significance of residual stresses in welded structures.
1. Explain what is meant by solid-state welding.
2. What is cold welding? Why is it so called?

Chapter 10

10. How is weldability defined?

The weldability of a metal is usually defined as its capacity to be welded into a specific structure that has certain properties and characteristics and will satisfactorily meet service requirements. It is decided by metals, design and manufacture. The influence factors of weldability are the material characteristics of base metal and the filler metal, the mechanical and physical properties of materials, the effects of melting as well as solidification, consequent micro-structural changes, surface preparation, welding process and welding parameters et al.

12. Describe the common types of discontinuities in welded joints.

• Porosity: a cavity type discontinuity formed by gas entrapment during solidification.
• Slag inclusions: compounds such as oxides, fluxes and electrode-coating materials that are trapped in the weld zone.
• Incomplete fusion and penetration: a discontinuity in a groove weld in which the weld metal doesn’t extend through the joint thickness, which produces poor weld beads.
• Weld profile: important for effects on the stress and appearance of the weld.
• Cracks: occur in various locations and direction.
• Lamellar tears
• Surface damage
• Residual stresses
• Stress relieving of welds
Cracks usually presented as longitudinal, transverse, crater, underbead and toe types are most dangerous for weld joint. The concentrated or elongated porosity, slag inclusion, incomplete fusion and penetration may significantly reduce joint strength. All discontinuities should be carefully avoided during welding practice.

15. Explain why some joints may have to be preheated prior to welding.

Reduce the risk of hydrogen cracking
Reduce the hardness of the weld heat affected zone
Reduce shrinkage stresses during cooling and improve the distribution of residual stresses.
If preheat is locally applied it must extend to at least 75mm from the weld location and be preferably measured on the opposite face to the one being welded.

Thermal cycling has significantly impact on weld quality. Preheating can adjust the thermal process of welding to certain extent.
For some easy harden materials, preheating can avoid unexpected microstructures by reducing cooling speed.
On welding of a complex structure or thick workpieces, preheating can help to reduce the residual stress or distortion by decreasing the temperature gradient. This is also beneficial to limit the generation of cracks.
Preheating is also helpful to eliminate porosity in welding.

21. Describe the methods by which discontinuities in welding can be avoided.

Porosity
Slag inclusions
Incomplete fusion and penetration
Underfilling
Undercutting
Overlap
Cracks
Lamellar tears
Surface damage

Welding design 2,3,7,8
Selection of electrode, filler metal, flux/gas 1,2,3,7
Preheating 1,7,8
Increasing the heat input 1,3
Reducing travel speed during the welding 1,3
Cleaning 1,2,3
Ensuring workpiece to be fitted properly 3
Welding operation 5,6,7,9
The amount of filler metal 4

29. Explain the significance of residual stresses in welded structures.

Because of localized heating and cooling during welding, expansion and contraction of the weld area causes residual stresses in the workpieces. Residual stresses can lead to the following defects:
• Distortion, warping, and buckling of the welded parts
• Stress-corrosion cracking.
• Further distortion if a portion of the welded structure is subsequently removed, such as by machining or sawing.
• Reduced fatigue life of the welded structure.
Thus, the residual stresses developed can be very harmful, and it is not unusual to further treat welds in highly stressed or fatigue-susceptible applications.

Chapter 11

1. Explain what is meant by solid-state welding.

In solid-state welding processes, joining takes place without fusion at the interface of the two parts to be welded. Solid-state welding does not have liquid or molten phase is present in the joint.
If two clean surfaces are brought into atomic contact (by diffusion) with each other under sufficient pressure, they form bonds and produce a joint. Applying additional heat improves the bond diffusion. Heat may be generated internally by friction, electric-resistance heating, chemical reaction, or externally by induction heating.

2. What is cold welding? Why is it so called?

Cold welding or contact welding is a solid-state welding process in which joining takes place without fusion/heating at the interface of the two parts to be welded. Unlike the fusion-welding processes, no liquid or molten phase is present in the joint.
The welding process is completed by only applying pressure. In cold welding (CW), pressure is applied to workpieces through either dies or rollers. Because of the plastic deformation involved, it is necessary that at least one, but preferably both, of the mating parts be ductile.
Without additional heat input, the welding process is carried out with tiny temperature rise. So, it is named cold welding.