Welding Fundamentals 1b1z4x

Module-II of Manufacturing Science-I

2. ING PROCESSES According to American Welding Society (AWS), all ing 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 t 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 ing takes place without fusion; consequently thee is no liquid (molten) phase in the t. 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 t to a temperature below the solidus temperature and applying pressure. • Forge welding Melting of the metal at the t (fusion welding). • Gas welding • Arc welding • Thermit welding • Flow welding Melting the metal at the t and applying pressure. • Pressure-gas welding • Stud welding • Flash welding • Resistance welding • Induction welding Applying pressure only to the metal t (done at room temperature). • Cold welding

Another criterion used for a classification of the welding process is based on the composition of the t. According to this scheme, all ing 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 ing. 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 t 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 ing processes.

Figure 2.2: Classification of welding processes

Types of ts: The relative positions of the two pieces being ed determine the type of t. The basic types of ts are classified as butt, lap, corner, tee or T and edge ts. The choice of the type of t depends on the weldment being made and sheet thickness. Each of the ts 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 ts

(a)

(b) Figure 2.4: Welding positions

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Module-II of Manufacturing Science-I The edges of the ts 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 ed is small, so that heat of welding penetrates the full depth of t. 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 t 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 t, it could be made as a V or U. The V t is easier to make but the amount of extra metal to be filled in the t increases greatly with an increase in thickness. From this a U t is preferable, since the amount of extra metal to be added to fill the t is greatly less beyond a certain plate thickness. But machining a U t is more difficult compared to a V t.

Figure 2.5: Butt t edge preparation methods The following are definitions of some of the welding that are commonly used. • Backing: it is the material provided at the root side of a weld to aid in the control of penetration. • Base metal: the metal to be ed or cut is termed as the base metal. • Bead or weld bead: Bead is the metal added during a single 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 t 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 t. • Puddle: The portion of the weld t that melted by the heat of welding is called puddle. • Root: It is the point at which the two pieces to be ed 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 t is called weld metal. It may be only base metal or a mixture of base metal and the filler metal. Weld : A single movement of the welding torch or electrode along the length of the t which results in a bead, is a weld .

Figure 2.6: Terminology to be used with various welded ts

Figure 2.7: Features of a good welding t 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 ing methods

Cost

Visual inspection

Ease of maintenance

Reliability

Tolerances

Large parts

Small parts

Design variability

Method

Strength

Characteristics

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2

3

1

2 1

3 3

1 1 1

2 3 1

3

2

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 ? 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 ts. 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 t 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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