热加工作业考研题目答案分享——Joining processes 1

assignment1

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文章目录

1. Describe fusion as it relates to welding operations.

In welding operation, typical heat is used to melt metals (partial of base metals of two or more parts and filler metal if any), and flux if added to form a welding pool. Then the pool is cooled down (solidified) to coalesce the parts in a welded component.
The heat is usually supplied by chemical or electrical means, includes flame, arc, electron-beam, laser-beam et al.

2. Explain the features of neutralizing, reducing, and oxidizing flames. Why is a reducing flame so called?

在这里插入图片描述
Neutral flame: The neutral flame is composed of an inner cone up to 〖3300〗^0 C and an outer envelope. The ratio of acetylene (C_2 H_2) and oxygen (O_2) is 1:1. It is widely used for OFW of materials.
Reducing flame (or carburizing flame): With a bright luminous inner cone, middle area of acetylene feather, and a long outer blue envelope. The ratio acetylene (C_2 H_2) and oxygen (O_2) is bigger than 1. The temperature of a reducing flame is lower; hence, such a flame is suitable for applications requiring low heat, such as brazing, soldering, and flame-hardening operations.
Oxidizing flame: With the excess oxygen supply, the flame becomes oxidizing flame with small and narrow outer envelope. It is harmful for steel welding, but it is desirable for the welding of copper and copper-based alloys is an oxidizing flame.

3. Explain the basic principles of arc-welding processes.

In arc welding, the heat required is obtained from electrical energy (electric arc strike). The process involves either a consumable or a non-consumable electrode (rod and wire). And arc is produced between the tip of the electrode and the workpiece to be welded by the use of welding power supply. The arc generates temperature up to 30,000°C, which are much higher than those developed in oxyfuel-gas welding. With the action of the high temperature of arc, part of the workpiece and filler metal (is added) are melt and mixed, and then it solidifies as a weld bead.

4. Why is shielded metal-arc welding a commonly used process? Why is it also called stick welding?

The SMAW has the advantages of arc. It is simple and convenient. The equipment is cheap. The welding area is well protected. And it is suitable for welding of many materials in all positions.
Because the electrodes are in the shapes of thin, long rods.

5. Describe the functions and characteristics of electrodes. What functions do coatings have? How are electrodes classified?

An electrode is an electrical conductor (one pole of arc) used to make contact with a non-metallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air), and acts as filler metal (consumable welding).

The coating, which is brittle and takes part in complex interactions during welding, has the following basic functions:
• Stabilize the arc by improving metal transfer mold and weld contour, as well as by reducing spatter.
• Generate gases to act as a shield against the surrounding atmosphere; the gases produced are carbon dioxide, water vapor, and small amounts of carbon monoxide and hydrogen.
• Control the rate at which the electrode melts.
• Act as a flux to protect the weld against the formation of oxides, nitrides, and other inclusions and, with the resulting slag, to protect the molten-weld pool.
• Add alloying elements to the weld zone to enhance the properties of the joint among these elements are deoxidizers to prevent the weld from becoming brittle.

Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding.
Non-consumable electrodes are tungsten electrodes.
Electrodes for consumable arc-welding processes are classified according to the following properties:
• Strength of the deposited weld metal
• Current (AC or DC)
• Type of coating.

6. What are the similarities and differences between consumable and non-consumable electrodes?

Similarities: Act as a pole of arc. Keep the current flowing and the arc stably burn. They are made of metals in shape of rod or wire.

Differences: Consumable electrode melts during welding but non-consumable not. The consumable electrode needs to be continuously feed in.
The composition of consumable electrode is similar to the base metal, but that of the non-consumable one is basically the tungsten.
Some of consumable electrodes are coated (SMAW) or used with flux (SAW, FCAW, ESW). The non-consumable electrode is bare and the arc is protected by insert gas.

8 What is the purpose of flux? Why is it not needed in gas tungsten-arc welding?

In fusion welding, flux is used in OFW, SAW, FCAW, even can be found in coatings (SMAW). Function of the flux (flux 焊剂):
To generate gaseous and slag shield to protect welding area
To remove oxides from the surfaces of workpiece and filler metal
To add some alloy elements effectively to improve the metallurgical reactions and strengthen weld metal
To prevent spatter and sparks and suppresses the intense ultraviolet radiation and fumes characteristic of the SMAW process
To act as a thermal insulator, promoting deep penetration of heat into the workpiece (SAW).

In gas tungsten arc welding, the welding area is protected by the insert shielding gas. By the way, the flux is unfavourable for the tungsten electrode and is inconvenient to add in.

16. Describe the role of filler metals in welding.

Filler metal: Metal added to the weld area during welding.
① to dissolve and remove oxides from the weld zone.
② to retard oxidation of the surfaces of the part being weld by generate gaseous shield around the weld zone.
③ to protect the molten puddle of metal against oxidation as it cools.
④ to fill the gap of joint.

posted @ 2020-08-21 12:47  狂小虎  阅读(103)  评论(0)  编辑  收藏  举报