Welding Fundamentals

Module-II of Manufacturing Science-I

2. JOINING PROCESSES According to American Welding Society (AWS), all joining processes can be classified into three categories. They are a) welding, brazing & soldering, b) adhesive bonding and c) mechanical fastening. Welding processes are in turn divided into three basic categories: a) fusion welding, b) solid-state welding, and c) brazing and soldering. Some types of welding processes can be classified into both the fusion and solid-state categories.

Welding was using in ancient (3,000 years ago) times by pouring molten metal into a cast located on another metal part. Sand casting is still used today.

Figure 2.1: Ancient welding process Fusion welding is defined as the melting together and coalescing of materials by means of heat (usually provided by chemical or electrical means); filler metals may or may not be used. This process constitutes a major category of welding; it comprises consumable or non consumable electrode arc welding and high energy beam welding processes .The welded joint undergoes important metallurgical and physical changes which in turn have a major effect on the properties and performance of the welded component or structure. In solid-state welding joining takes place without fusion; consequently thee is no liquid (molten) phase in the joint. The basic categories are diffusion bonding, ultrasonic, cold, friction, resistance, and explosive welding. Diffusion bonding, combined with super plastic forming, has become an important manufacturing process for complex shapes. Brazing and soldering use filler metals and involve lower temperatures than welding; the heat required is supplied externally.

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Another classification of welding processes is given below: Heating the metal joint to a temperature below the solidus temperature and applying pressure. • Forge welding Melting of the metal at the joint (fusion welding). • Gas welding • Arc welding • Thermit welding • Flow welding Melting the metal at the joint and applying pressure. • Pressure-gas welding • Stud welding • Flash welding • Resistance welding • Induction welding Applying pressure only to the metal joint (done at room temperature). • Cold welding

Another criterion used for a classification of the welding process is based on the composition of the joint. According to this scheme, all joining processes can be grouped into three different categories, namely,

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Module-II of Manufacturing Science-I (i) autogeneous (ii) homogeneous, and (iii) heterogeneous. In autogeneous processes no filler material is added during the joining. All types of solid phase welding and resistance welding are examples of this category. In homogeneous welding processes, the filler material used to provide the joint is the same as the parent material. Arc, gas, and thermit welding belong to this category. In third category of welding, a filler material different from the parent material is used. Soldering and brazing are two such joining processes.

Figure 2.2: Classification of welding processes

Types of Joints: The relative positions of the two pieces being joined determine the type of joint. The basic types of joints are classified as butt, lap, corner, tee or T and edge joints. The choice of the type of joint depends on the weldment being made and sheet thickness. Each of the joints can have any of the four different welding positions: flat, horizontal, vertical and overhead.

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Module-II of Manufacturing Science-I

Figure 2.3: Types of welded joints

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(b) Figure 2.4: Welding positions

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Module-II of Manufacturing Science-I The edges of the joints in figure 2.3 are shown to be straight. This type of edge preparation is used when the thickness of the two pieces to be joined is small, so that heat of welding penetrates the full depth of joint. However, when the thickness increases it becomes necessary to prepare the edge in such a way that heat would be able to penetrate the entire depth. To facilitate this, the joint is widened as shown in figure given below. For thick plates, the welding needs to be done from both sides. To provide the necessary access into the joint, it could be made as a V or U. The V joint is easier to make but the amount of extra metal to be filled in the joint increases greatly with an increase in thickness. From this account a U joint is preferable, since the amount of extra metal to be added to fill the joint is greatly less beyond a certain plate thickness. But machining a U joint is more difficult compared to a V joint.

Figure 2.5: Butt joint edge preparation methods The following are definitions of some of the welding terms that are commonly used. • Backing: it is the material support provided at the root side of a weld to aid in the control of penetration. • Base metal: the metal to be joined or cut is termed as the base metal. • Bead or weld bead: Bead is the metal added during a single pass of welding. The bead appears as a separate material from the base metal. • Crater: In arc welding, a crater is the depression in the weld metal pool at the point where the arc strikes the base metal plate. • Deposition rate: The rate at which the weld metal is deposited per unit time is the deposition rate and is normally expressed as Kg/h. • Fillet weld: the metal fused into the corner of a joint made of two pieces placed at approximately 900 to each other is termed as fillet weld. • Penetration: It is the depth up to which the weld metal combines with the base metal as measured from the top surface of the joint. • Puddle: The portion of the weld joint that melted by the heat of welding is called puddle. • Root: It is the point at which the two pieces to be joined by welding is nearest. • Tack weld: A small weld, generally used to temporarily hold the two pieces together during actual welding, is the tack weld. • Toe of weld: It is the junction between the weld face and the base metal.

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Module-II of Manufacturing Science-I • • • •

Torch: In gas welding, the torch mixes the fuel and oxygen and controls its delivery to get the desired flame. Weld face: It is the exposed surface of the weld. Weld metal: The metal that solidified in the joint is called weld metal. It may be only base metal or a mixture of base metal and the filler metal. Weld pass: A single movement of the welding torch or electrode along the length of the joint which results in a bead, is a weld pass.

Figure 2.6: Terminology to be used with various welded joints

Figure 2.7: Features of a good welding joint and imperfections in a weld

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Module-II of Manufacturing Science-I

Arc 1 2 3 1 3 1 2 welding Resistance 1 2 1 1 3 3 3 Welding Brazing 1 1 1 1 3 1 3 Bolts and 1 2 3 1 2 1 1 nuts Riveting 1 2 3 1 1 1 3 Fasteners 2 3 3 1 2 2 2 Seaming, 2 2 1 3 3 1 3 crimping Adhesive 3 1 1 2 3 2 3 bonding Note: 1, very good; 2, good; 3, poor. Table-I: Comparison of various joining methods

Cost

Visual inspection

Ease of maintenance

Reliability

Tolerances

Large parts

Small parts

Design variability

Method

Strength

Characteristics

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3

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2 1

3 3

1 1 1

2 3 1

3

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Reference: • Welding and Welding Technology by R.L.Little, MacGraw-Hill, NewYork, page 1-27 • The Science and Practice of Welding by A.C.Davis, Cambridge University Press, page 42 • Principles of Manufacturing Materials and Processes by J.S.Campbell, MacGraw-Hill, NewYork, page 375-376 • Manufacturing Engineering and Technology by Kalpakjian and Schmid, Pearson Education, page 775-779 • Manufacturing Technology by P.N.Rao, TMH, page361-366 Review questions: 1. Discuss the need for edge preparation in welding. 2. What is meant by penetration? Explain its relevance to welding. 3. How do you classify different weld positions? 4. What do you mean by root pass? Why it is important in welding? 5. Explain with sketch the features of a good weld. 6. For thicker material welding what type edge preparation you will select? 7. Explain autogeneous welding processes. 8. Differentiate between fillet weld and butt weld. 9. How people in ancient time were using welding process? 10. Describe various weld joints. 11. Why deposition rate is important in welding operation? 12. Differentiate between weld bead and base metal. 13. Which processes are called heterogeneous welding process? 14. Can you make metal joint by applying only pressure between two plates? Explain. 15. Is there any welding process where pressure is applied during fusion of base metals? 16. Describe solid state welding. 17. Why filler materials are required in welding? 18. Give a brief classification of the various fabrication processes. 19. Why welding is extensively used? 20. Explain the term weld face.

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