weld and welded joints in detail.

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WELDED JOINTS

Welding is a method of joining two parts by melting and/or pressing them together.
Welds are permanent joints of metals (iron, steels, aluminum alloys, titanium alloys) or plastic materials.
Aluminum and steel cannot be melted together since they have different melting points (temperatures).


There are the following types of welds: 
A. butt-weld 
B. corner weld
C. T-weld
D. lap weld


Static and fatigue strength is highest for a lap-weld in comparison with other joints from the list.

n forge welding, A, for steel chain manufacturing, two parts are heated and then hammered together.
Gas welding, B, uses an oxy-acetylene flame to heat the metal and a rod of metallic filler material.
In electric-arc welding, C the filler rod forms one electrode and the metal itself another. Electric current passes across the gap between the electrodes by arcing or sparking and melts the surfaces together. The electric current (ac or dc, alternating current or direct current) is stable with an amperage of 150 - 500 Amperes. Industrial power sources usually work with voltages between 22 - 36 Volts.
Contrary to gas welding, electric-arc welding is used for thick pieces of metal and high temperature.
If an electric current passes through two metal surfaces in close contact the temperature rises and melts the surfaces together known as spot welding or seam welding, D. This method is used in mass production.


There are the following types of butt-welds:
A. without a gap
B. with a gap
C. with one-sided bevel
D. with two-sided bevel

A butt-weld without a gap is used if there is a guarantee of full melting. A butt-weld with a gap is used for thin-walled structures. 

Edge preparation guarantees full melting and improved quality of the joint. There are Y-, U- and X-shaped edge preparation. U-shaped edge preparation is used instead of X-shaped edge preparation for thick parts if it is not possible to weld from two sides. Joints can be welded in a single pass or by few passes.


Weld joining of thick tubes also involves edge preparation, B in contrary to thin-walled tubes, A. Additional casing, C can be used.
Welds with a В«smoothВ» transition correspond to a stronger structure.
A great deal of skill is required to produce a reliable weld.


Arc heat is expended during the melting of metal electrodes as it is in the heating of base parts. Approximate values of arc heat expended in shielded metal-arc welding: 

A. Dissipation into the neighboring environment - 20%
B. Transition with molten drops - 26%
C. Vaporization of electrode metal - 24%
D. Absorption by base metal - 30%

Bharatiyulam

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