Module 4 Design for Assembly

Transcription

1 Module 4 Design for Assembly

2 Lecture 2 Design for Welding-I

3 Instructional Objective By the end of this lecture, the student will learn: (a) how a weld joint should be designed to improve the joint performance, and (b) what are the possible welding defects and how the same can be avoided. Introduction Welding is the most prominent process for joining large components into complex assemblies or structures. A necessary condition for welding is that the two or more surfaces to be joined must be brought into intimate contact. When fusion takes place, the joint is achieved by melting of two or more workpiece materials in a localized region. In contrast, the solid-state joining processes rely on plastic deformation of the surface asperities along the contact surface representing the original weld interface or the impending weld joint. A variety of welding processes are available today that are outlined earlier in the Lecture 1 of Module 4. The success of a welding process not only depends on the selection of appropriate welding conditions or process parameters, but also on the joint configuration and welding position. The loads applied on a welded structure in the actual application are transferred from one member to another through the weld joints. Thus an examination of the types of welds and weld joint configurations is of interest of the present lecture. Types of Welds and Weld Joints There are eight basic types of welds (Figure 4.2.1), which are commonly used to prepare welded joints. Fillet weld is the most commonly used one and named so due to the approximately triangular cross-sectional shape of the weld profile. The fillet is regarded as being on the joint. Fillet welds are economical and requires no joint preparation and hence, widely used to join corner, T- and Lap joint configurations. Groove weld is another type of weld that is made between two members and regarded as being in the joint. Commonly, the grooves are usually made V-shaped due to ease of machining while U-shaped and J-shaped grooves are also used. Backing weld is usually made on the root side of a previously made weld to improve the quality of the weld joint by ensuring complete penetration. Usually, the root of the original weld is gouged, chipped and ground prior to the backing weld is made. Spot and projection welds are made typically at the interface of the members being joined. Seam welds are similar to spot welds while the actual weld geometry varies with the type of the welding process. Stud weld is used to weld a metallic stud onto a workpiece. Surfacing

4 welds refer to a group of welds that are used to build up a broken surface on a base metal. Figure shows five basic types of weld joints commonly used in welded fabrication. Joint Configuration Symbol Remarks FILLET Most popular of all welds (may be single or double) GROOVE Second most popular-may be single or double- has many variants BACK OR BACKING WELD Bead type back or backing welds of single groove welds PLUG OR SOPT WELD Used with prepared holes SOPT OR INJECTION WELD Used without prepared holes Use arc or resistance SEAM WELD Continuous-use are or resistance STUD WELD Special application welding process SURFACING WELD Surface built up by welding Figure Schematic outlines of eight basic types of welds [1]

5 Butt Joint Corner Joint Tee Joint Lap Joint Edge joint Figure Schematic outlines of five basic types of weld joints [1] Design Recommendations for Economical and Efficient Welded Fabrication 1. Welded assemblies should be made up of as few parts as possible. Reduction in the number of joints leads to less handling, processing time, equipment, service inspection, testing, less distortion etc. It is always suggested to reduce the number of weld joints by introducing formed parts. 2. Placement of weld joints should facilitate easy access of the welding nozzle. This is particularly important for consumable electrode fusion arc welding processes. 3. Whenever possible, design should be such that the joint is horizontal during welding with the electrode pointing downward. 4. Parts should fit-up properly. This is essential not only for welding speed but also for minimizing distortion of the finished weldment (Figure 4.2.3). Especially with butt joints, the edges of mating workpiece surfaces should be straight and uniform. 5. The fillet weld deposits should be kept to a minimum. Additional material in the convex portion of the fillet weld cross section contributed little to the strength of the joint. 6. Cast and forged parts should be designed so that the wall thickness of both these parts to be joined is equal at the joint interface to minimize weld joint distortion (Figure 4.2.4).

6 7. It is preferable to locate welds out of sight rather than in locations where special finishing operations are required to improve the appearance of the final assembled part. 8. The joint should be designed so that it requires minimal edge preparation. It is often advisable to use lap joints in welded assemblies to avoid the cost of edge preparation. 9. In some cases, curved edges parts can be used to provide the equivalent of a grooved edge for the weld joints (Figure 4.2.5). 10. If machining after welding is required, welds should be placed away from the material to be machined to avoid machining near to the weld joints (Figure 4.2.6a). 11. Back-up strip can be included as an integral part of the component to be welded to reduce the effort related to holding the back strip (Figure 4.2.6b). Figure Poor and good fit-up of weld joints [1]

7 Figure The wall thickness of parts to be joined should be equal at the joint [1] Figure Joints that have natural grooves and thus need little or no edge preparation [1] (a) (b) Figure (a) Keep the weld metal outside the portion of the weldment, (b) Integral backup strip [1]

8 Design Recommendations for Weld Strength Following are a few recommendations to improve the strength of the welded joint. 1. Square edged butt joint can be employed saving the edge preparation time if deeppenetration welding is used or the stock thickness is not great. Thicker stock or less penetrating methods may require grooved edges or groove welding. 2. For efficient and economical welding, minimize the stress that the joint must carry. Design the weld joint in such a fashion that it stays away from the stressed area or the part itself bears the load instead of the weld joints [Figure 4.2.7] 3. Groove welds should be designed to be in either compression or tension. Fillet welds should be in shear only. When using grooves for welds, follow the standard American Welding Society weld-groove dimensions. 4. Length of each fillet should be at least 4 times the fillet thickness when intermittent welds are used. If the joint is in compression, the spacing of the welds should not exceed 16 times the fillet thickness and for tension, the spacing may be as much as 32 times the fillet thickness. Not Recommended Recommended Figure Joint design for minimize stress concentration field [1]

9 Residual Stress and Distortion in Welding The inherent local non-uniform heating and cooling cycles associated with the joining processes, in particular with the fusion welding processes, results in complex stresses and strains in and around the weld joint. These finally lead to the development of residual stress and distortion in welded structure. Residual stresses are referred to as internal stresses that would exist in a body after the removal of all external loads. Distortion refers to the permanent (plastic) strain that would be exhibited in terms of dimensional change after the welding is over. While the residual stresses can reduce the service life of a structure or even cause catastrophic failures, distortion usually results in misalignment with consequent difficulties in assembly and poor appearance of the final structure. Residual Stress Three main reasons for the development of residual stresses in welded structure are 1. non-uniform heating and cooling of metal in and adjacent to the weld region, 2. volume shrinkage of metal in weld during solidification (freezing), and 3. microstructural change of metal, in particular solid state phase change in steel, on solidification leading to volumetric strain. Typical changes in temperature and thermally induced stresses during welding are shown schematically in the Figure The section A-A is far away from the welding heat source and thus, is not affected by the same. The welding heat source is along the section B-B. Thus, the longitudinal stresses will be close to zero in the region underneath the heat source since molten metal cannot withstand any stress. In the region slightly away from the melt pool in the transverse direction along section B-B, compressive stresses will be generated since thermal expansion of this region will be restrained by the surrounding colder material, which is at lower temperature. Along the section C-C, the weld metal and the adjacent base metal are cooled and would shrink, thus producing tensile stresses since the compressive stresses due to shrinkage in solidified melt pool will be opposed by the surrounding colder material. As the weld metal will cool further, greater tensile stresses will be generated in the weld center and compressive in the base metal as apparent along the section D-D.

10 Figure Schematic distributions of temperature and longitudinal residual stresses in fusion welding [2] Effects of Residual Stress and Stress Relieving Methods The effect of residual stress can be summarized as follows: The effect of weld induced residual stresses on the performance of a welded structure is significant only for the phenomena that occur at low applied stresses, such as brittle fracture, fatigue and stress corrosion cracking. As the level of applied stress increases, the effect of residual stress decreases. The effect of residual stress tends to decrease after repeated loading. The effect of residual stress on the performance of welded structures under applied stresses greater than the yield strength is negligible. Due to very high thermal gradient during welding process, it is not possible to avoid the generation of residual stress. However, a number of stress-relieving methods are available and applied after the welding. These stress-relieving methods include (a) vibratory stress relieving, (b) peening of weld area, (c) post-weld heat treatment etc. Residual stress can also

11 be minimized if proper measures are taken during welding, such as preheating to primarily reduce the temperature gradient and cooling rate, weld sequencing, preferring fillet weld over a butt weld, etc. Welding Distortion Welding distortion is caused by the non-uniform expansion and contraction of the weld metal and the adjacent base metal during the heating and cooling cycle of the welding process. The extent of welding distortion will depend on various factors such as: (a) geometry of the joint, (b) type of weld preparation, (c) width or volume of the web, (d) rate of heat input during welding process, (e) volume of weld deposition, (e) alignment of structural elements in the weldments, and (f) the sequence in which welds are made. Figure schematically shows six types of welding distortion which are common in fusion arc welding (a) transverse, (b) longitudinal, (c) angular, (d) rotational, (e) longitudinal bending, and (f) buckling. Figure Types of distortion in fusion welding [2] Guidelines for Minimizing Distortion (1) Good fit-up of parts is important for minimizing distortion. As shown in Figure 4.2.3, the maximum contact of all the mating surfaces minimizes the welding time and also the requirement of the volume filler material. The more gap to fill, the greater the possible weldment distortion. (2) Heavier sections are less prone to distortion. So if distortion prevention is important to the application, designers should consider the use of thicker, more rigid components.

12 (3) Long and thin sections are readily distorted and buckled unless a good rigid support is provided. Use of short-flanged butt joint can minimize distortion in this case. (4) Whenever possible, try to place welded joints in symmetric position. Distortion can be reduced by placing welds opposite to one another which balance the shrinkage force in the weld fillets. (5) If sections of unequal thickness are to be welded together, then distortion can be reduced by machining a groove in the thicker section such that the thickness of the both part at the welded section becomes equal. (Figure ). (6) When dimensioning welded assemblies, it is essential that consideration be given to the shrinkage inherent in each weld. Figure A short-flanged butt joint is often preferable for joining thin material [3] Figure Use of a machined groove to equalize wall thickness [3]

13 Exercise 1. Residual stresses can be measured by (a) stress relaxation method, (b) X-ray, (c) cracking Method, (d) all the above. 2. What is the major problem when joining thin sheet by lap joint? 3. It is preferable to design groove joint in such a way that it would be subjected to (a) tension only, (b) compression only, (c) tension or compression only, (d) tension and compression simultaneously. Ans. 1(d), 3(c) References: 1. Bralla, Design for Manufacturability Handbook, The McGraw-Hill Companies 2. R. W. Messler (Jr.), Principles Of Welding (Processes Physics, chemistry and Metallurgy), Wiley International, NY, G Dieter, Engineering Design - a materials and processing approach, McGraw Hill, NY, 2000.