Chapter 4: Classification of welding methods¶
Introduction¶
The process of welding has been evolved in recent years with the advent of technology. Welding can be classified into several categories:
However, fusion welding and pressure welding is mostly used classification as it covers all processes in both the categories irrespective of heat source and welding with or without filler material. In pressure welding, molten metal solidifies under pressure or semi-solid metal cools under pressure while in fusion welding, molten metal solidifies freely and external application of pressure is not required for this process except resistance welding group where substantial contact pressure needs to be maintained during welding for the sound weld.
- All the welding processes can be categorized under two broad heading:
Plastic or Pressure Welding
Fusion or Non-Pressure Welding
Plastic or Pressure Welding¶
In plastic welding, the metal pieces to be joined are heated to a plastic state and forced together by external pressure. For example, making of gold ornaments by goldsmith.
Fusion or Non-Pressure Welding¶
In fusion welding, the metal pieces to be joined are heated to a molten state and are allowed to solidify together into a single inseparable joint. For example, joining of iron rods to create an Iron railing
Classification on the basis of fusion and pressure welding is further illustrated in given tree diagram.
Figure. Classification on the basis of fusion and pressure welding¶
Classification on the basis of Metal to be Welded¶
Autogeneous Welding
Homogeneous Welding
Heterogeneous Welding
Autogenous Welding:¶
It is the one way of performing fusion welding without applying any filler material externally (base material can be melted to use as filler material.) For instance, Tungsten Inter Gas (TIG) and oxy-fuel welding processes can be performed in autogenous welding.
Advantages:
Fusion welding on thin sheets is possible
Cost effective since no extra cost for filler material.
No post grinding is necessary
Weld appearance is good.
- Disadvantages:
Only butt join with no root gap between parent parts is possible.
Thickness of sheet is limited to 2- 3 mm for sound welding.
Strength of joint is less than base plates.
Oil, rust or scale should be removed completely for sound weld
Homogenous Welding:¶
It is another way of performing fusion welding with the application of external filler material whose metallurgical composition is almost same as that of base metals.
For instance, Manual Metal Arc Welding and Gas Metal Arc Weld use consumable filler material so they are Homogenous Welding.
- Advantages:
Thick plates can be welded with proper edge preparation
Joints are reliable and strong.
Low chances of crack formation due sufficient supply of molten metal.
Joints like butt, lap, tee and corner are possible to perform while wielding.
- Disadvantages:
Thin sheets cannot be joined.
Grinding is necessary to remove excess filer material which could cause reinforcement.
Weld bead appearance is not lovely.
Expensive since extra cost is required for filler material and maintenance.
Heterogeneous Welding:¶
It is another way of performing fusion welding with the application of external filler whose metallurgical composition is substantially different than that of the base metals. Using this process, we can join two different kinds of metals. Filler material must have low melting point than that of base or parent metal. Initially, base metal should be heated near melting point and then welding must be started immediately using filler rod. That filler rod melts before base metal and spread around joint. Finally, strong joint is achieved once it cools down.
For instance, Brazing and soldering are common heterogeneous joining.
- Advantages:
Thick plates can be welded efficiently with proper edge preparation
Joints like butt, lap, tee and corner are possible to perform while wielding.
Joints are reliable and strong.
Stress and corrosion resistance can be enhanced by choosing appropriate filler material.
- Disadvantages:
Expensive due to costly filler material and maintenance.
Appearance of weld bead is not good.
Sputter may produce if process materials are not chosen properly like metal compatibility.
Grinding is necessary to remove excess filer material which could cause reinforcement.
Classification of welding and allied process¶
As we know there are different welding, brazing and soldering methods are being used in industries today. The general classification of welding and allied process is given below:
A. WELDING PROCESSES
1. Oxy-Fuel Gas Welding Processes
a) Air –Acetylene Welding
b) Oxy –Acetylene Welding
c) Oxy-Hydrogen Welding
d) Pressure Gas Welding
2. Arc Welding Processes
a) Carbon Arc Welding
b) Shielded Metal Arc Welding
c) Submerged Arc Welding
d) Gas Tungsten Arc Welding
e) Gas Metal Arc Welding
f) Plasma Arc Welding
g) Atomic Hydrogen Welding
h) Electro-slag Welding
i) Stud Arc Welding
j) Electro-gas Welding
3. Resistance Welding
a) Spot Welding
b) Seam Welding
c) Projection Welding
d) Resistance Butt Welding
e) Flash Butt Welding
f) Percussion Welding
g) High Frequency Resistance Welding
h) High Frequency Induction Welding
4. Solid-State Welding Processes
a) Forge Welding
b) Cold Pressure Welding
c) Friction Welding
d) Explosive Welding
e) Diffusion Welding
f) Cold Pressure Welding
g) Thermo-compression Welding
5. Thermit or Thermo Chemical Welding Processes
a) Thermit Welding
b) Pressure Thermit Welding
6. Radiant Energy Welding Processes
a) Laser Welding
b) Electron Beam Welding
B. ALLIED PROCESSES
1. Metal Joining or Metal Depositing Processes
a) Soldering
b) Brazing
c) Brazing Welding
d) Adhesive Bonding
e) Metal Spraying
f) Surfacing
2. Thermal Cutting Processes
a) Gas Cutting
b) Arc Cutting
Forge welding¶
It is one of the oldest welding techniques which is also famously called as Black Smith welding. This process involves of joining two metal pieces to increase the length by pressing or hammering them when they are at the forging temperature (or plastic temperature). Using this technique, we can weld Mild Steel, Carbon Steel as well as Wrought Iron.
In this process two metal pieces to be joined are heated in a forge or furnace to a melting temperature and they are united together by pressure which involves hammering, rolling, drawing or squeezing. It is limited to light work because all welding is performed by hand sledge and also a slow manual process. There is also a probability of an oxide scale formation on tile surface. This oxidization process can be counteracted by covering the surfaces with a fluxing material. Borax combined with salt ammoniac is commonly used a flux. The forge welding is highly recommended for such metals, which have a large welding temperature ranges like low carbon steel and wrought iron. The more carbon content in the metal the more care should be given to it.
Use of flux in forge welding¶
Oxidization occurs when a metal combines oxygen which makes atoms of metal loses its electrons as a result a layer of scale is formed in metal surface. For example, when iron is exposed to moisture environment scaling (or rusting) occurs in iron surface. Flux is used to remove dirt and prevent metal surface from direct environment exposure. Applying flux on iron pipe prevents it from getting rust due to contact of rain and heat from environment.
- Mainly used Fluxes are:
Silicon or Pure Sand
Borax
Salt ammoniac
Sodium Chloride
- On the basis of pressure, forge welding can be categorized in three types:
Hammer Welding
Die Welding
Roll Welding
- We should carefully follow following tasks for forge welding a good joint:
Keep area around furnace or Hearth clean and tidy
Use Hard Coke for fire
Right amount of air should be supplied in furnace to minimize chances of oxidization
Be careful while making edges of metal
Metal should not have scale and other dirt in its surface
We should choose right Flux material
Do not waste time after taking metal out of furnace
Clean the surface of Anvil before hammering and make sure it has firm position