CLASSIFICATION OF WELDING PROCESSESS - 1

Welding was a process of joining metallic components with or without application of heat, with or without pressure and with or without filler metal. A range of welding processes have been developed so far using single or a combination above factors namely heat, pressure and filler. Welding processes can be classified on the basis of following techological criteria:

·         Welding with or without filler material

·         Source of energy for welding

·         Arc and non-arc welding

·         Fusion and pressure welding   

2.1 Classification of welding processes on the basis of technical factors 

2.1.1 Welding with or without filler material

A weld joint can be developed just by melting of edges (faying surfaces) of plates or sheets to be welded especially when thickness was lesser than 5 mm thickness. A weld joint developed by melting the fating surfaces and subsequently solidification only (without using any filler metal) was called “autogenous weld”. Thus, the composition of the autogenous weld metal corresponds to the base metal only. However, autogenous weld can be crack sensitive when solidification temperature range of the base metal to be welded was significantly high (750o - 100oC). Following are typical welding processes in which filler metal was generally not used to produce a weld joint.  

·         Laser beam welding

·         Electron beam welding

·         Resistance welding, 

·         Friction stir welding 

However, for welding of thick plates/sheets using any of the following processes filler metal can be used as per needs according to thickness of plates. Application of autogenous fusion weld in case of thick plates may result in concave weld or under fill like discontinuity in weld joint. The composition of the filler metal can be similar to that of base metal or different one accordingly weld joints are categorized as homogeneous or heterogeneous weld, respecting.  In case of autogenous and homogeneous welds, solidification occurs directly by growth mechanism without nucleation stage. That type of solidification was called epitaxial solidification. The autogenous and homogeneous welds are considered to be of lesser prone to the development of weld discontinuities than heterogeneous weld because of a uniformity in composition and (b) if solidification largely occurs at a constant temperature. Metal systems having wider solidification temperature range show issues related with solidification cracking and partial melting tendency. The solidification in heterogeneous welds takes place in conventional manner in two stages i.e. nucleation and growth. Following are few fusion welding processes where filler may or may not be used for developing weld joints:      

·         Plasma arc welding 

·         Gas tungsten arc welding 

·         Gas welding

Some of the welding processes are inherently designed to produce a weld joint by applying heat for melting base metal and filler metal both. These processes are mostly used for welding of thick plates (usually > 5mm) with comparatively higher  deposition rate.    

·         Metal inert gas welding: (with filler)

·         Submerged arc welding: (with filler)

·         Flux cored arc welding: (with filler) 

·         Electro gas/slag welding: (with filler)

Comments on classification of welding processes based on with/without filler The gas welding process was the only fusion welding process earlier using which joining could be achieved with or without filler material. The gas welding

performed without filler material was termed as autogenous welding. However, with the development of tungsten inert gas welding, electron beam, laser beam and many other welding processes, such classification created confusion as many processes were falling in both the categories.  

2.1.2 Source of energy for welding 

Almost all weld joints are produced by applying energy in one or other form to develop atomic/metallic bond between metals being joined and the same was achieved either by melting the faying surfaces using heat or applying pressure either at room temperature or high temperature (0.5o to 0.9o Tm). Based on the type of energy being used for creating metallic bonds between the components to be welded, welding processes can be grouped as under: 

·         Chemical energy: Gas welding, explosive welding, thermite welding

·          Mechanical energy: Friction welding, ultrasonic welding

·          Electrical energy: Arc welding, resistance welding

·          Radiation energy: Laser beam welding, electron beam welding 

Comments on classification of welding processes based on source of energy  Energy in various forms such as chemical, electrical, light, sound, mechanical energies etc. are used for developing weld joints. However, except chemical energy all other forms of energies are generated from electrical energy for welding. Hence, categorization of the welding processes based on the source of energy criterion also does not justify classification properly.  

2.1.3 Arc or Non-arc welding 

Metallic bond between the plates to be welded can be developed either by using heat for complete melting of the faying surfaces then allowing it to solidify or by apply pressure on the components to be joined for mechanical interlocking. All those welding processes in which heat for melting the faying surfaces was provided after establishing an arc either between the base plate and an electrode or

between electrode & nozzle are grouped under arc welding processes. Another set of welding processes in which metallic bond was produced using pressure or heat generated from sources other than arc namely chemical reactions or frictional effect etc., are grouped as non-arc based welding processes. Welding processes corresponding to each group are given below.

Arc based welding processes

1.        Shielded Metal Arc Welding: Arc between base metal and covered electrode

2.       Gas Tungsten Arc Welding: Arc between base metal and tungsten electrode

3.       Plasma Arc Welding: Arc between base metal and tungsten electrode 

4.       Gas Metal Arc Welding: Arc between base metal and consumable electrode

5.       Flux Cored Arc Welding: Arc between base metal and consumable electrode    

6.       Submerged Arc Welding: Arc between base metal and consumable electrode 

Non-arc based welding processes

·         Resistance welding processes: uses electric resistance heating

·         Gas welding: uses heat from exothermic chemical reactions 

·         Thermit welding: uses heat from exothermic chemical reactions 

·         Ultrasonic welding: uses both pressure and frictional heat

·         Diffusion welding: uses electric resistance/induction heating to facilitate diffusion 

·         Explosive welding: involves pressure  

Comments on classification of welding processes based on arc or non arc based process Arc and non-arc welding processes classification leads to grouping of all the arc welding processes in one class and all other processes in non-arc welding processes. However, welding processes such as electro slag welding (ESW) and

flash butt welding were found difficult to classify in either of the two classes as  ESW process starts with arcing and subsequently on melting of sufficient amount flux, the arc extinguishes and heat for melting of base metal was generated by electrical resistance heating by flow of current through molten flux/metal. In flash butt welding, tiny arcs i.e. sparks are established during initial stage of the welding followed by pressing of components against each other. Therefore, such classification was also found not perfect. 

2.1.4 Pressure or Fusion welding 

Welding processes in which heat was primarily applied for melting of the faying surfaces are called fusion welding processes while other processes in which pressure was primarily applied (with little or no application of heat for softening of metal up to plastic state) for developing metallic bonds are termed as solid state welding processes.       

Pressure welding

·         Resistance welding processes (spot, seam, projection, flash butt, arc stud welding)

·         Ultrasonic welding

·          Diffusion welding

·          Explosive welding

 Fusion welding process

·         Gas Welding

·         Shielded Metal Arc Welding

·         o Gas Metal Arc Welding

·          Gas Tungsten Arc Welding

·          Submerged Arc Welding

·          Electro Slag/Electro Gas Welding 

Comments on classification of welding processes based on Fusion and pressure welding 

Fusion welding and pressure welding was most widely used classification as it covers all processes in both the categories irrespective of heat source and welding with or without filler material. In fusion welding, all those processes are included in which molten metal solidifies freely while in pressure welding, molten metal if any was retained in confined space (as in case of resistance spot welding or arc stud welding) and solidifies under pressure or semisolid metal cools under pressure. That type of classification poses no problems and therefore it was considered as the best criterion.

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.