JOINING PROCESS WELDING - IN BRIEF

That chapter presents the fundamental approaches used in manufacturing namely casting, forming, welding and machining. Further, common methods of developing joint and selection of suitable methods have been described. Applications, advantages and limitations of welding as a fabrication technique have also been covered.

Keywords: Manufacturing process, selection of joint, welding vs. manufacturing processes, selection of welding process, advantages, application and limitation of welding processes

1.1 Introduction 

The manufacturing technology primarily involves sizing, shaping and imparting desired combination of the properties to the material so that the component or engineering system being produced to perform indented function for design life. A wide range of manufacturing processes have been developed in order to produce the engineering components ranging from simple to complex geometries using materials of different physical, chemical, mechanical and dimensional properties. 

There are four chief manufacturing processes i.e. casting, forming, machining and welding. Selection of suitable manufacturing process for a produce/component was dictated by complexity of geometry of the component, number of units to be produced, properties of the materials (physical, chemical, mechanical and dimensional properties) to be processed and economics. Based on the approach used for obtaining desired size and shape by different manufacturing processes; these can be termed as positive, negative and or zero processes

• Casting: zero process

• Forming: zero process

•  Machining: negative process

• Joining (welding): positive process

Casting and forming are categorized as zero processes as they involve only shifting of metal in controlled (using heat and pressure singly or in combination) way from one region to another to get the required size and shape of product. Machining was considered as a negative process because unwanted material from the stock was removed in the form of small chips during machining for the shaping and sizing of a product purpose. During  manufacturing, it was frequently required to join the simple shape components to get desired product. Since simple shape components are brought together by joining in order to obtain desired shape of end useable product therefore joining was categorized as a positive process. Schematic diagrams of few typical manufacturing processes are shown in Fig. 1.1.  

 

Fig. 1.1 Schematic diagram showing shaping approaches using different manufacturing processes a) forming, b) casting, c) machining and d) joining 

1.2 Selection of Joint   

The fabrication of engineering systems frequently needs joining of simple components and parts. Three types of joining methods namely mechanical joining (nuts & bolts, clamps, rivets), adhesive joining (epoxy resins, fevicol), welding (welding, brazing and soldering) are commonly used for manufacturing variety of engineering product/component. Each type of joint offers different load carrying capacity, reliability, compatibility in joining of similar or dissimilar materials besides their fitness for use in different environments and cost. It will be appropriate to consider following aspects while selecting type of joints for an application:

a) type of joint required for an application was temporary or permanent  

b) Whether similar or dissimilar materials are to be joined in order to take care of the compatibility aspect as metallurgical incompatibility can be disastrous for performance of the joints  

c) Physical, chemical metallurgical properties of materials to be joined   

d) requirements of the service from the joint under special conditions of temperature, corrosion, environment, and reliability

e) type and nature of loading conditions (static and dynamic loading under tension, shear, compression, bending etc.) 

f) economy or cost effectiveness was one most important factors influencing the selection of joint for manufacturing an engineering component

1.3 Welding and its comparison with other manufacturing processes

Welding was one of the most commonly used fabrication techniques for manufacturing engineering components for power, fertilizer, petro-chemical, automotive, food processing, and many other sectors. Welding generally uses localized heating during common fusion welding processes (shielded metal arc, submerged arc, gas metal arc welding etc.) for melting the faying surfaces and filler metal. However, localized and differential heating & cooling experienced by the metal during welding makes it significantly different from other manufacturing techniques:

•  Residual stresses are induced in welded components (development of tensile residual stresses adversely affects the tensile and fatigue properties of work piece) 

•  Simple shape components to be joined are partially melted

•  Temperature of the base metal during welding in and around the weld varies as function of time (weld thermal cycle)

•  Chemical, metallurgical and mechanical properties of the weld are generally anisotropic  Reliability of weld joint was poor.

• Little amount of metal was wasted in the form of spatter, run in and run off

• Process capabilities of the welding in terms of dimensional accuracy, precision and finish are poor.

•  Weld joints for critical applications generally need post weld treatment such as heat treatment or mechanical working to get desired properties or reline residual stress.

• Problem related with ductile to brittle transition behaviour of steel was more severe with weld joints under low temperature conditions.

 

1.4            Selection of welding process

A wide range of welding processes are available to choose. These were developed over a long period of time. Each process differs in respect of their ability to apply heat for fusion, protection of the weld pool and soundmen of welds joint the so performance of the weld joint. However, selection of a particular process for producing a weld joint was dictated by the size and shape of the component to be manufactured, the metal system to be welded, availability of consumables and machines, precision required and economy. Whatever process was selected for developing weld joint it must be able to perform the intended function for designed life. Welding processes with their field of applications are given below: 

•  Resistance welding: Automobile 

• Thermite welding: Rail joints in railways 

• Tungsten inert gas welding: Aerospace and nuclear reactors 

• Submerged arc welding: Heavy engineering, ship building

•  Gas metal arc welding: Joining of metals (stainless steel, aluminium and magnesium) sensitive to atmospheric gases 

1.5            Advantages and Limitation of Welding as a Fabrication Technique

Welding was mainly used for the production of comparatively simple shape components. It was the process of joining the metallic components with or without application of heat, pressure and filler metal. Application of welding in fabrication offers many advantages, however; it suffers from few limitations also. Some of the advantage and limitations are given below.   Advantages of welding are enlisted below:

1. Permanent joint was produced, which becomes an integral part of work piece.

2. Joints can be stronger than the base metal if good quality filler metal was used.

3. Economical method of joining.

4. It was not restricted to the factory environment. 

Disadvantages of welding are enlisted also below:

1. Labour cost was high as only skilled welder can produce sound and quality weld joint.

2. It produces a permanent joint which in turn creates the problem in dissembling if of sub-component required.

3. Hazardous fumes and vapours are generated during welding. That demands  proper ventilation of welding area.

4. Weld joint itself was considered as a discontinuity owing to variation in its structure, composition and mechanical properties; therefore welding was not commonly recommended for critical application where there was a danger of life.  

1.6 Applications of welding  

General applications  

• The welding was widely used for fabrication of pressure vessels, bridges, building structures, aircraft and space crafts, railway coaches and general applications besides shipbuilding, automobile, electrical, electronic and defense industries, laying of pipe lines and railway tracks and nuclear installations.

• Specific components need welding for fabrication includes 

1. Transport tankers for transporting oil, water, milk etc.

2. Welding of tubes and pipes, chains, LPG cylinders and other items. 

3. Fabrication of Steel furniture, gates, doors and door frames, and body 

4. Manufacturing white goods such as refrigerators, washing machines,  microwave ovens and many other items of general applications  

The requirement of the welding for specific area of the industry was given in following section.  

Oil & Gas 

1.      Welding was used for joining of pipes, during laying of crude oil and gas pipelines, construction of tankers for their storage and transportation. Offshore structures, dockyards, loading and unloading cranes are also produced by welding.

Nuclear Industry 

2.      Spheres for nuclear reactor, pipe line bends, joining of pipes carrying heavy water require welding for safe and reliable operations.

Defense industry

3.      Tank body fabrication, joining of turret mounting to main body of tanks are typical examples of applications of welding in defense industry.

 Electronic industry

4.      Electronic industry uses welding to limited extent e.g. joining leads of special transistors but other joining processes such as brazing and soldering are widely used.

5.      Soldering was used for joining electronic components to printed circuit boards (PCBs).

6.      Robotic soldering was very common for joining of parts to printed circuit boards of computers, television, communication equipment and other control equipment etc.

Electrical Industry 

7.      Components of both hydro and steam power generation system, such as penstocks, water control gates, condensers, electrical transmission towers and distribution system equipment are fabricated by welding. Turbine blades and cooling fins are also joined by welding.

Surface transport 

8.      Railway: Railway uses welding extensively for fabrication of coaches and wagons, repair of wheel, laying of new railway tracks by mobile flash butt welding machines and repair of cracked/damaged tracks by thermite welding.

9.       Automobiles: Production of automobile components like chassis, body and its structure, fuel tanks and joining of door hinges require welding. 

Aerospace Industry 

10.  Aircraft and Spacecraft: Similar to ships, aircrafts were produced by riveting in early days but with the introduction of jet engines welding was widely used for aircraft structure and for joining of skin sheet to body.

 Space vehicles which have to encounter frictional heat as well as low temperatures require outer skin and other parts of special materials. These materials are welded with full success for achieving safety and reliability.

Ship Industry 

11.  Ships were produced earlier by riveting. Welding found its place in ship building around 1920 and presently all welded ships are widely used. Similarly submarines are also produced by welding.

 Construction industry 

12.  Arc welding was used for construction of steel building structures leading to considerable savings in steel and money. 

13.  In addition to building, huge structures such as steel towers also require welding for fabrication.

ERICSSON CYCLE - NOTES

Ericsson Cycle:

The Ericsson cycle consists of two isothermal and two constant pressure processes.

The processes are: 

Process 1-2: Reversible isothermal compression.

Process 2-3: Constant pressure heat addition.

Process 3-4: Reversible isothermal expansion.

Process 4-1: Constant pressure heat rejection. 

The heat addition and rejection take place at constant pressure as well as isothermal processes. Since the process 2-3 and 3-4 are parallel to each other on the T-s diagram, the net effect is that the heat need to be added only at constant temperature T3=T4 and rejected at the constant temperature T1=T2. The cycle is shown on p-v and T-s diagrams in Fig.4.3. The advantage of the Ericsson cycle over the Carnot and Stirling cycles is its smaller pressure ratio for a given ratio of maximum to minimum specific volume with higher mean effective pressure. 

Fig.4.3. Ericsson cycle on p-v and T-s diagrams   

The thermal efficiency of Ericsson cycle is given by, (derivation is same as that of Stirling cycle),  

 The Ericsson cycle does not find practical application in piston engines but is

approached by a gas turbine employing a large number of stages with heat exchangers,

insulators and reheaters. 

CARNOT CYCLE AND STIRLING CYCLE

Carnot Cycle :  

A Carnot gas cycle operating in a given temperature range is shown in the T-s diagram  in Fig. 4.1(a). One way to carry out the processes of this cycle is through the use of  state, steady-flow devices as shown in Fig. 4.1(b). The isentropic expansion process 2-3 and the isentropic compression process 4-1 can be simulated quite well by a well-designed turbine and compressor respectively, but the isothermal expansion process 1-2 and the isothermal compression process 3-4 are most difficult to achieve. Because of these difficulties, a steady-flow Carnot gas cycle is not practical. 

The Carnot gas cycle could also be achieved in a cylinder-piston apparatus (a reciprocating engine) as shown in Fig. 4.2(b). The Carnot cycle on the p-v diagram is as shown in Fig. 4.2(a), in which processes 1-2 and 3-4 are isothermal while processes 2-3 and 4-1 are isentropic. We know that the Carnot cycle efficiency is given by the expression. 

Fig.4.1. Steady flow Carnot engine      

                                             Fig.4.2. Reciprocating Carnot engine        

                                  Fig.4.3. Carnot cycle on p-v and T-s diagrams

Fig.4.4. Working of Carnot engine 

Since the working fluid is an ideal gas with constant specific heats, we have, for the

isentropic process, 

Now, T1 = T2 and T4 = T3, therefore

Carnot cycle efficiency may be written as,

From the above equation, it can be observed that the Carnot cycle efficiency increases as ‘r’ increases. This implies that the high thermal efficiency of a Carnot cycle is obtained at the expense of large piston displacement. Also, for isentropic processes we have,

Since, T1 = T2 and T4 = T3, we have

Therefore, Carnot cycle efficiency may be written as, 

From the above equation, it can be observed that, the Carnot cycle efficiency can be

increased by increasing the pressure ratio. This means that Carnot cycle should be

operated at high peak pressure to obtain large efficiency. 

Stirling Cycle (Regenerative Cycle)

The Carnot cycle has a low mean effective pressure because of its very low work output. Hence, one of the modified forms of the cycle to produce higher mean effective pressure whilst theoretically achieving full Carnot cycle efficiency is the Stirling cycle. It consists of two isothermal and two constant volume processes. The heat rejection and addition take place at constant temperature. The p-v and T-s diagrams for the Stirling cycle are shown in Fig.4.2. 

Fig.4.2. Stirling cycle processes on p-v and T-s diagrams

Stirling Cycle Processes:

(a) The air is compressed isothermally from state 1 to 2 (TL to TH).

(b) The air at state-2 is passed into the regenerator from the top at a temperature T1. The air passing through the regenerator matrix gets heated from TL to TH.

(c) The air at state-3 expands isothermally in the cylinder until it reaches state-4.

(d) The air coming out of the engine at temperature TH (condition 4) enters into regenerator from the bottom and gets cooled while passing through the regenerator matrix at constant volume and it comes out at a temperature TL, at condition 1 and the cycle is repeated. (

e) It can be shown that the heat absorbed by the air from the regenerator matrix during the process 2-3 is equal to the heat given by the air to the regenerator matrix during the process 4-1, then the exchange of heat with external source will be only during the isothermal processes. 

Now we can write, Net work done = W = Qs - QR

Heat supplied = QS = heat supplied during the isothermal process 3-4. 

Heat rejected = QR = Heat rejected during the isothermal compression process, 1-2.  

Now,

and 

Thus the efficiency of Stirling cycle is equal to that of Carnot cycle efficiency when both are working with the same temperature limits. It is not possible to obtain 100% efficient regenerator and hence there will be always 10 to 20 % loss of heat in the regenerator, which decreases the cycle efficiency. Considering regenerator efficiency, the efficiency of the cycle can be written as, 

Where, R η is the regenerator efficiency.