Metal joining processes

METAL JOINING PROCESSES

Metal Joining Process is the process of joining of similar or dissimilar metals by the application of heat. It is classified as follows: 1. WELDING 2. SOLDERING 3. BRAZING

 

1. WELDING

Principles of Welding

Welding is the process of joining similar metals by the application of heat. It can be done with or without the application of pressure and with or without the addition of filler metal, called Electrodė. The heat may be developed in several ways for welding operation. The joint obtained is a homogenous mixture of material of composition and characteristics of the two parent metals.

Nowadays, many processes of welding have been developed. There is probably no industry which is not using welding process in the fabrication of its products in some form or the other. The research carried out in this field has given various ways and methods to weld practically all metals. Means have also been found out to weld dissimilar metals.

One beauty of welding in comparison to other processes of joining metals is that by this process we can have 100% strength of the joint.

Applications

Welding is nowadays extensively used in automobile industry, structural work, tanks, boilers, furniture, pressure vessels, building and bridge constructions, railway wagons, aircraft machine frames, ship building, pipe-line fabrication in power plants and refineries, etc. There is a big competition between welding and casting processes nowadays. Many of the cast products are being fabricated nowadays by welding various parts together.

Definitions

1. Parent Metal or Base Metal: The metal to be joined is known as Parent Metal.

2. Filler Metal: It is a metal or alloy used to fill the weld cavity. It has either same composition as the parent metal or added with alloying element.

3. Weld Metal: It is the metal that is solidified in the weld cavity. It may be only metal or a mixture of parent metal and filler metal.

4. Edge Preparation: It is the preparation of the edge of the two metal pieces to be joined..

5. Weld Pass: It is a single movement of the welding torch or electrode along the length of the joint.

 

1. Gas Welding

Principle: Gas welding is a type of fusion, non-pressure welding. In this, the required heat to melt the metal parts is supplied by a high-temperature flame obtained by a mixture of two gases. The gases are mixed in proper proportions in a welding blow pipe called Welding Torch. Gas welding is also called as Oxy-Fuel Gas Welding (OFW), as it uses a fuel gas such as acetylene or hydrogen combined with oxygen to produce a flame.

The principle of Oxy-Acetylene welding is given below.

See Fig. 15. An oxy-acetylene weld is produced by heating with a flame obtained from the combustion of oxygen and acetylene gases.

The temperature of oxy-acetylene flame is 3200°C. The flame will only melt the metal. Hence, additional metal to the weld is supplied by a Filler Rod.

This process is suitable for joining metal sheets and plates of thickness varying from 2 to 50 mm.

In the case of oxy-hydrogen process, the flame temperature is Steel plates of thickness up to 7 mm only can be welded.

Filler Rod or Welding Rod

Filler Rod or Welding Rod is a metal or alloy rod of diameter 0.3 to 12 mm. It is used in gas welding for filling the weld cavity to make the joint. It is melted by the heat of the gas flame and is deposited over the base metals.

Filler metal is added to the weld for joining metal plates of thickness more than 15 mm.

For metal plates of thickness less than 15 mm, filler metal is not used.

Filler rods commonly used are:

- Low carbon steel rods: Used to weld wrought iron steel plates, steel castings.

- Mild steel copper coated rods: Used to weld steel plates and pipes.

- Drawn aluminum rods: Used to weld aluminum sheets, aluminum castings.

Flux

Flux is used during welding to prevent oxidation and to remove impurities. It is used except for mild steel which has more silicon and manganese content that acts as deoxidizing agent.

The molten metal of the weld comes in contact with gases. Hence, oxidation takes place and metallic oxides are formed. The flux should have a melting point lower than the parent metal and filler metal. It readily reacts with metallic oxides so that the oxides are completely dissolved by the time the molten pool solidifies. So-formed slag forms a blanket to protect the metal from atmospheric oxidation.

Advantages

1. Oxy-acetylene flame is versatile, because it can be used for welding, flame cutting, brazing and pre-heating.

2. Temperature of welding can be easily controlled by adjusting the flame. The flame can be controlled by varying the quantity of oxygen and acetylene by means of the two control valves provided on the welding torch.

3. Since the source of heat and the filler rod are separate, the welder has the required control over filler metal deposition rates. Heat can be supplied preferentially to the base metal or to the filler rod.

4. Cost of equipment and also the maintenance cost are reasonably low.

Disadvantages

1. The process is comparatively slower than arc welding. The gas flame takes up a longer time to heat up the metal than an electric arc.

2. It is not suitable for joining thick plates.

3. Strength of the joint is comparatively less.

4. Storing and handling of gas cylinders need more care.

 

2. Electric Arc Welding

Principle: The source of heat for Electric Arc Welding is an Electric Arc. The arc is produced between an electrode and the work or between two metal pieces to be welded. The electrical energy is converted into heat energy. For arc welding process, filler metal is required and no mechanical pressure is applied. Hence, this is one type of fusion welding. Thus, arc welding is defined as the process of joining two metal parts by melting their edges by an electric arc using filler rod without the application of pressure.

Arc Welding Process [Fig. 16]

Metal parts to be welded are cleaned first using wire brush to remove rust, grease, etc. The electrode is held in the holder. Initially, work and electrode are touched together and then separated leaving a small air gap of 3 mm approximately between the electrode tip and the work. This gap is called arc length.

When current is passed, an electric arc of about 4000°C temperature is produced between the electrode tip and the work. Both the electrode and the works are melted by the arc.

Both the molten pieces of metal become one. The electrode also deposits additional filler metal into the joint. The depth to which the metal is melted and deposited is called Depth of Fusion.

The electrode is kept inclined at 70° with the work to achieve deep fusion. The blast of the arc forces the molten metal out of the pool. Hence, a small depression is formed in the parent metal around which the molten metal is piled up. This is known as Arc Crater.

The distance through the center of arc from the tip of electrode to the bottom of the arc crater is called Arc Length of 3 mm to 4 mm.

The electrode coating gives off inert gases such as CO₂. The inert gases shield the molten metal pool and prevent it from the atmospheric O₂, H₂ and N₂. Thus, contamination of weld metal is reduced.

The coating provides flux to the molten metal pool. Hence, the molten metal mixes with the oxide and other impurities and forms slag. The slag being lighter floats at the top. An A.C. transformer or D.C. generator is used for supplying current. The arc temperature can be increased or decreased by employing high or low current.

Arc Welding Equipment

See Fig. 17. The equipment needed for arc welding process is as follows:

1. D.C. Generator or A.C. Transformer

2. Cables – 2 Nos. (one for the workpiece and one for the electrode)

3. Electrode or filler rod

4. Electrode Holder

5. Safety Devices such as goggles, hand gloves, etc.

A.C. or D.C. Machine

Both direct current and alternating current are used for arc welding, each having its own merits and demerits.

D .C. supply is obtained from a generator driven by an electric motor, or if no electricity is available, by an I.C. engine.

If electricity supply is available, A.C. transformer is used. It steps down the usual supply voltage (220 - 440V) to the normal open circuit welding voltage of 40 to 80 Volts. The output of the transformer can be varied by rotating a hand wheel which alters the air gap and regulates the supply of current.

With A.C., the heat generated at each pole or end (electrode end and workpiece end) is the same because of the reversal of the current. Thus, changing over the connections to the electrode does not have any effect on its performance. In the case of D.C., the arc heat can be regulated by changing the polarity.

 

3. Comparison of D.C. and A.C. Arc Welding

4. Industrial Applications

- Arc welding is versatile, producing high quality welds, depositing metal rapidly.

- It is competitive cost-wise for many industrial applications such as: Fabrication of tanks, boilers, pressure vessels, furniture, etc., including in the structural works of bridges and buildings, automobile industries, aircraft industries, ship building, etc.

- It is preferred for difficult tasks like Overhead Welding. Most metals can be welded by one or more of the forms of arc welding.

 

2. SOLDERING

Soldering and Brazing are processes which unite metáls with a third joining metal which is introduced into the joint in a molten state and allowed to solidify. These processes have wide commercial use in the uniting of small assemblies and electrical components.

 

1. Soldering Process

Principle

Soldering is the process of joining two pieces of metal. This is done by the use of heat and by adding a filler metal, called Solder, of melting temperature 450°C. It is used as a filler rod. The workpieces are not melted in the soldering process. The joint is weak due to the adhesion between the solder and parent metal.

A solder commonly used is an alloy of tin and lead. It has a low melting temperature in the range of 150°C - 450°C. A common proportion of solder is three parts of tin and two parts of lead. A soldering flux is used to prevent oxidation.

Soldering Process: See Fig. 18.

Solder will not stick to metal that is dirty or rusted. The flux used is not intended to remove any appreciable amount of contamination. Hence, the surface of the workpieces is first cleaned well to remove dirt, oil or grease from their surfaces.

Cleaning is done by filing or brushing or using emery cloth. The workpieces are arranged in proper position as shown. Zinc and Ammonia Chloride mixture is used as flux to prevent oxidation of the surfaces to be soldered during the heating process. The flux is spread on the joint using a soldering iron.

Soldering iron:

It is used to heat the workpieces to just above the melting point of solder.

It is a steel rod consisting of a wooden handle at one end and a copper tip pointed at the other end. Copper is used at its end due to its ability to absorb and give up heat. The copper tip of the soldering iron may be heated electrically or in gas flame.

Tinning:

In order to have solder cling to the soldering iron, its tip must be tinned. The heated tip is dipped in flux and then rubbed on the solder to pick up a thin film of solder with it. This is called tinning of the tip.

The solder is melted and the molten solder is picked up by the tip of the soldering iron and is deposited along the joint. The molten solder joins the two workpieces and solidifies. A good joint is characterized by a small amount of solder and perfect adhesion, rather than by large unsightly masses of solder.

Soldering Fluxes:

The flux used for soldering is usually in the form of liquid or paste. The flux fills the space between the soldering iron and the work. Thus, it enables better heat flow from the iron to the work. The soldering flux may be corrosive (Zinc Chloride) or non-corrosive (Rosin in alcohol).

 

2. Types Of Soldering

1. Soft Soldering: If the melting point of the filler metal is very much lower than the melting point of metals to be joined, it is called soft soldering. The solder is composed of lead and tin with a low melting point of 150°C – 350°C.

2. Hard Soldering: The hard solders are alloys of copper and zinc to which silver is also added. The hard solders melt above 900°C. Hard soldering produces stronger joints than soft soldering Dip Soldering:

3. Dip Soldering is based on the method of heating the joints. It is used for soldering electronic appliances.

3. Applications of Soldering

The soldering joints are of low strength. Hence, they should be designed so that the soldered joint is not relied upon to carry much load. The solder should serve primarily as a filler material to stop leakage and to seal the joints against corrosion and also to carry electricity.

Soldering is hence used for joining wires, repairing radiators, thin sheet metal work, etc. Soldering provides positive and dependable electrical connections.

The different compositions of solder for different purposes are:

i) Soft solder – lead 37 %, tin 63 %

ii) Plumber's solder – lead 70% , tin 30 %

iii) Electrician's solder - lead 58%, tin 42%

 

3. BRAZING

1. Brazing Process

Principle: Brazing is the process of joining two similar or dissimilar metals by the use of heat and a filler metal called Spelter (filler rod). It is similar to soldering except that spelter is used instead of solder.

Spelter, a harder filler rod, is having a melting temperature of about 500°C which is below the melting point of the work metals. Two classes of filler metals used for most work are copper alloys and silver alloys. Copper alloys made of copper, zinc and tin are mostly used for brazing ferrous metals. Silver alloys made of silver and copper with a melting range of 600°C – 800°C are used for brazing any metals.

Workpieces are not melted in the brazing process. The flux used for brazing is borax powder. The molten filler metal (spelter) flows into the joint clearance by capillary action. The molten filler metal flows between the grain boundaries of the workpiece metal structure.

Brazing Process: See Figs. 19(a) and 19(b).

The metal parts to be brazed should be cleaned first to remove dirt, oil, grease and rust from their surfaces. Cleaning may be done by brushing, filing or grinding. Chemical cleaning may also be done using acids. The parts are assembled and clamped properly in position. The parts are so joined that a gap should exist between them so that the filler metal may flow inside the joint.

After cleaning, a small amount of flux (borax powder) is mixed with water to form a paste. It is evenly applied to the surfaces to be brazed before they are heated.

Then, the surfaces to be brazed are heated to a temperature below their melting point. Heating may be done by oxy-acetylene flame (neutral flame) or in a furnace. The flux applied on the work surfaces melts and flows in the gap between the surfaces. When spelter is applied to the joint, it is melted. The molten spelter flows along the joint, i.e., in the gap between the workpieces and solidifies, forming a hard brazed joint.

Flux: The primary function of the flux is to dissolve and absorb oxides which are formed during heating. It controls the fluidity of brazing metal. The most common brazing fluxes are borax, fluorides, chlorides and boric acid.

 

2. Types of Brazing Based on the Method of Heating

1. Torch Brazing

Oxy-acetylene gas flame is widely used for heating the metal parts for brazing. Neutral flame is applied over the parts by a special torch called brazing torch. Controlling the flame temperature is difficult. Many types of ferrous and non-ferrous assemblies and repair works are done.

2. Dip Brazing

The parts to be brazed are assembled and dipped into a bath of molten filler metal. The filler metal flows into the joint. This method is used for joining small components of metal strips. Suitable holding fixtures are necessary for holding the parts. Current temperature of the bath can be maintained.

3. Furnace Brazing

In this, the parts to be brazed are assembled and kept in a furnace maintained at a temperature that may melt the filler metal. Brazing temperature can be easily controlled. Furnace brazing is used for mass production.

 

3. Brazing Joints [Fig. 20]

The basic types of joints done by brazing are lap, butt, and scarf designs are shown. Lap joint and butt joint are used in sheet metal work. Scarf joint is used for rod or pipe in any metal.

Advantages

1. The main advantages of brazing process are the joining of dissimilar metals (which cannot be welded) and thin sections.

2. The process is faster.

3. Brazed joints need not be finished.

4. Joints are tougher.

Limitations

1. It has lesser strength compared to welding.

2. Joint preparation cost is more.

3. It can be used only for thin sheet metal sections.

 

4. Applications

This process is mostly used for joining pipes (Fig. 19(b)] and other fittings, carbide tips on steel tool holders, repairing radiators and heat exchangers. Brass, bronze, copper, steel and stainless steel can be brazed with each other. Care must be taken when brass or copper parts are brazed, in order that they shall not be fused. Aluminum is brazed by aluminum alloys that melt just before the parent metal.

5. Comparison of Soldering and Brazing