MT-253 MACHINE DESIGN

1. MT-253 MACHINE DESIGN Lecture #11 Syed EhtishamGillani Lecturer Department of Mechanical Engineering Technology University of Technology Nowshera

2. Welding • The process of permanently joining two or more metal parts by fusion of edges of the metals with or without application of pressure and filler material is called welding. • If pressure is applied, then it is called Forge welding. • If no pressure is applied, then it is called Fusion welding. Industrial Applications Aerospace engineering, Train and rail carriages, Automotive industry, Domestic hardware, Radiators and containers, Medical instruments and supplies, Nuclear equipment.

3. Types of Welding Processes

4. Types of Welding Fusion Welding • Fusion welding is a welding in which the parts to be jointed are held in position while the molten metal is supplied to joint. • The molten metal may come from the parts themselves (i.e. parent metal) or filler metal which normally have the composition of the parent metal. • The fusion welding can be classified into three types based on the method of heat generation. 1) Thermit Welding 2) Electric Arc Welding 3) Gas Welding

5. Types of Welding 1) Thermit Welding • The combination of Aluminium+ Iron oxide is called Thermit. • In this type of welding, Thermitis heated, melted and then poured into a mouldmade around the joint and fuses with the parts to be welded. Applications: • Fabrication of rails, locomotive frames, etc.

6. Types of Welding • Gas Welding • In this type of welding, flame of an oxy-acetylene or hydrogen gas from a welding torch is applied upon the surfaces of the prepared joint. • The intense heat coming from the flame heats up the local surfaces to fusion point and using the welding rod supplies metal for the weld. • Heating rate is slow and hence it is used for thinner metals. • Applications: Welding of thin metals, joining metals in automotive and aircraft industries, joining metals in sheet metal fabricating plants.

7. Types of Welding • Electric Arc Welding • In electric arc welding, the work is prepared in the same manner as for gas welding. • In this case the filler metal is supplied by metal welding electrode. • The operator, with his eyes and face protected, strikes an arc by touching the base metal with the electrode. • The base metal in the path of the arc stream is melted, forming a pool of molten metal. • A small depression is formed in the base metal and the molten metal is deposited around the edge of this depression, which is called the arc crater. • The slag is brushed off after the joint has cooled.

8. Types of Welding Forge WeldingIn forge welding, the parts to be jointed are first heated to a proper temperature in a furnace orforge and then hammered. This method of welding is rarely used now-a-days.

9. Advantages & Disadvantages of Welded Joints Advantages:-1. Comparatively lighter in weight than riveted joints.2) Greater strength compared to riveted joints.3) Addition and alterations can be done easily.4) Lesser time consuming.5) Tension members are not weakened in welded joints compared to riveted joints.

10. Advantages & Disadvantages of Welded Joints Disadvantages:-1) Requires skilled workers.2) Possibility of additional stress development due to uneven heating and cooling. Or in other words, the members may get distorted.3) Testing is difficult.4) As there is no provision for expansion or contraction of joints, cracks may developand propagate.

11. Types of Welded Joints The two important types of welded joints are:-1. Butt weld joint 2. Lap weld joint (Fillet joint) 1) Butt weld joint • The butt joint is obtained by placing the plates edge to edge as shown in Figure • In butt welds, the plate edges do not require bevelling if the thickness of plate is less than 5 mm. • On the other hand, if the plate thickness is 5 mm to 12.5 mm, the edges should be bevelled to V or U-groove on both sides

12. Types of Welded Joints • Butt weld joint The butt joints may be of following types: • Square butt joint • Single V-butt joint • Single U-butt joint • Double V-butt joint • Double U-butt joint.

13. Types of Welded Joints • If the edges of the two plates are touching each other and are joined by welding, then the joint is called butt weld joint. • Let l= length of weldt= Depth of weld (in case of full penetration)F=Tensile forceσt= Allowable tensile stress The maximum force weld can withstand: F= σt x A F= σt x l x t

14. Types of Welded Joints • The size of the butt weld depends upon the thickness of the throat. • In case of complete penetration butt weld, thickness of throat will be taken as the thickness of the plate. This is applicable to Double U and Double V butt welds. • The throat thickness in case of incomplete penetration butt weld depends upon the standardized actual data. However, if the actual data is not available, the thickness of in complete penetration but weld will be taken as 5/8 of thickness of plate. This is applicable to single V butt weld.

15. Types of Welded Joints Problem: Two plates of length 15 cm are welded by a single V butt joint. If the safe stress in the weld is 135MPa and the permissible load carried by the plates is 253 KN, Find the thickness of the plates?

16. Types of Welded Joints • Lap weld joint or Fillet joint The lap joint or the fillet joint is obtained by overlapping the plates and then welding the edges of the plates. The cross-section of the fillet is approximately triangular. The fillet joints may be1. Single transverse fillet 2. Double transverse fillet 3. Parallel fillet joints.

17. Types of Welded Joints • Lap weld joint or Fillet joint

18. Types of Welded Joints Problem: A plate 100 mm wide and 10 mm thick is to be welded to another plate by means of double transverse fillets. The plates are subjected to a static load of 80 kN. Find the length of weld if the permissible tensile stress in the weld does not exceed 55 MPa.