Fabrication and Welding Technology for Mechanical Engineering - Assignment Example

Name : xxxxxx Tutor : xxxxxxx Title : welding Institution : xxxxxxx @2016 Task 4a: (LO4: 4.1) Arc welding Arc welding is a technique of joining two or more metal parts together by use of electric power to generate an electric arc between an electrode and the metal join to be welded. The electric supply used in most cases can either be a direct current electrical supply although alternating current supply is predominant in such operations (Squillace et al. 2004). There are various types of arc welding methods, some utilize the consumable electrodes while others use electrodes that are non-consumable. 1. Applications Arc welding is used in most engineering applications as a form of joining various metal parts together in order to achieve a desired engineering structure. Such applications range from constructions, to machinery to locomotives. Some of the few examples of applications of arc welding are covered below: Construction Arc welding has a great and common use in the construction industries. Such industries normally use large amounts of metal bars as a component or as an integral part of the construction material. Therefore, there is need to join these material to form construction components such as the trusses and columns and means in some cases. Arc welding is used to facilitate this joining. Automobiles Most automobile industries have parts joined by use of arc welding techniques. Such parts include the body works of vehicles such as Lorries, transit vehicles as well as farm machinery such as tractor trailers among other. These parts are mostly composed of metal parts that have been joined together by use of arc welding technique. Marine machinery Most ships that cruise our water ways have been constructed by purely joining huge pieces of metal parts together through welding. One of the most important parts of a ship is the hull. The hull of a ship requires experienced and massive amount of arc welding in order to meet its perfect and water tight requirements for safety of the ship. Arc welding is mostly employed to accomplish this task. 2. Arc welding operating principle Arc welding as a fusion technique as discussed earlier utilized high temperatures to accomplish its fusion task. The metal joints are subjected to high temperatures that results to both the electrode as well as the joint parts melting and intermixing for the case of consumable electrode arc welding methods. In other cases where the electrodes are not consumable, the metal joints are subjected to high temperatures (Wang et al. 2002). The high temperatures between the end of the electrode and the metal part is as a result of the electric arc formed between the metal joint and the end of the electrode. Thus, the electrical circuitry in order to achieve this arc is as shown below. Figure 1: Arc welding circuit Thus the electric arc is carefully guided along the metal joint as desired and this can either be done manual by the welding personnel or mechanically by use of programmed robots although in other cases, simple mechanical mechanisms are used to achieve this guiding. Some of the arc welding techniques include; Manual metal arc welding. Submerged arc welding. a) Manual metal arc welding Manual metal arc welding is also commonly referred to as shielded metal arc welding and thus abbreviated as SMAW. This method utilizes a consumable electrode as well as an alternating current that is used to strike an arc between the joint and the electrode. Thus for effective joining to occur, the electrode must be made from material that is compatible with the base material whose joining is under consideration. Such an electrode is normally covered with a material referred to as the flux. The flux forms gaseous vapours during welding and such vapours serves the task of shielding the weld. On cooling, the vapours forms a layer of slag that also serves as a protective layer to the weld from reaction with the air before cooling. Figure 2: Shielded metal arc welding The electrode during welding melts down and fills itself into the weld joint thus eliminating the need for a filler material during manual arc welding operation. Advantages Simple and do not require much operator training. It is comprises of cheap equipment. Disadvantages The welding process is slow. The process can only be used with working on ferrous materials. b) Submerged arc welding This type of arc welding is normally regarded as a high productivity welding process. The electric arc in this case is struck beneath a layer of flux. The flux is conventionally granular. The electrical circuitry is similar to that of the manual arc welding method however, this method produces higher quality weld due to the fact that the atmosphere is blocked from interacting with the weld by the flux. The flux in such a cases scraps off by itself and normally results in a clean weld. The method uses a wire feed as a filler and this normally results in an increased rate of deposition (Lancaster 1984). The figure below represents a simple figure of the submerged arc welding process. Figure 3: Submerged arc welding Task 4b: (LO4: 4.2) Power supply The arc welding process depends mainly on the power supply and thus, the welding [power supply is that device that will provide the required electrical current to facilitate welding process. Arc welding processes requires electrical current as high as 80 amperes and therefore, a power supply must be identified before commencing and welding activity. Some if the common welding power supply in use include: Transformers. Inverter power sources. Generators. a) Transformers Transformer welding power supplies normally steps up the normal power from the utility mains or house hold mains into a higher current and lower voltage for purposes of welding. Such transformers are used hand in hand with rectifiers whose main task is to convert the AC current from the transformer to DC. The use of rectifiers with the transformers normally incurs more cost on the equipment (Wang et al. 2002). The transformer welding equipment has a mechanism of allowing the user or the welder to select the amount of current he/she want to work with by manipulation the distance between the primary and secondary coil within the transformer. Thus the user can adjust the amount of current based on the material he/she is working on or based on the thickness of the material involved. The picture below shows a typical arc welding transformer. Picture 1: A typical arc welding transformer power supply. b) The generator The generators may be used to produce arc welding power supply by converting mechanical energy into electrical energy. Such generator may be driven by diesel, petrol or gasoline based internal combustion engine and may produce either the AC or the DC current without the need of rectification. Most of these generators may be used with alternators for purposes of converting the current supply from AC to DC or vice-versa (Gunuraj & Murugan 1999). Picture 2: A typical arc welding generator. c) Inverter power sources An inverter is a switched mode power supply developed from high power semiconductor technologies and are capable of facilitating high power electrical requirements such as arc welding. The inverter power sources are commonly referred to as inverter welding units. Typically inverters utilize the utility mains power supply by converting such AC current to DC current and subsequently inverting the DC power into the desired reduced voltage by use of an integral step down transformer. The inverters have been designed to sustain a switching frequency of above 10 kHz depending on the requirements. Such inverters have other features such as over load protection mechanism that helps to protect the equipment from damage in case of overloads. Picture 3: A typical welding inverter. Task 4c: (LO4: 4.3) Describe typical joint configurations used in welding. (You may wish to use annotated images to help give a clear, straightforward description which includes all of the main points of the joint configurations). As discussed earlier, arc welding or generally welding process is used to join two or more metal parts to form a desired joint. There are therefore, various types of joints that can be done through welding and the choice of the joint to be used depends on the structure of the end product desired (Cao et al. 2006). Typically, the common joint used in welding include: Square but joints. V-joints. J-joints. U-joints There are several other welding joints that can be used and their use is initiated by the desired outcome. a) Square butt joints Square butt weld joint involves joining of two metal pieces that are flat and parallel to each other. This type of welding is mostly used for purposes of work piece elongation or similar operations. It is the simplest configuration with regards to preparation, economic benefits as well as strength. Figure 4: Square butt joint configuration. b) V-joints In V-joint welds, both sides of the weld joints are beveled to join two materials in a V-shaped configuration. A double V-joint is normally used when welding involves thicker materials. The double V-joints will help to facilitate for warping forces subjected to the welds and is thus used to increase its strength. Figure 5: A typical V-joint. c) U-joints In single –U butt welds, the welds have both edges of the weld surfaces having a J-shape but form a uniform U-shaped when both pieces are brought together. U joints are probably the most expensive weld joints to prepare and are commonly used in cases where the v-joints have an undesirably extreme angle Figure 6: Single-U joint butt weld d) J-joints J-joint welding is when one piece of metal piece in the shape of a J and the other is a square piece and are joined together by use of filler material. J-groove is normally formed artificially by processes such as grinding. J-grooves are normally very expensive to form thus making the j-joint welds very expensive as compared to other welding joints such as V-joints (Buste et al. 2000) Figure 7: Single J- butt weld. Task 4d: (LO4: 4.4,) Discuss common weld defects, your response must include the following: Types of common weld defects Causes of common weld defects Acceptance criteria, what is acceptable/unacceptable Welding defects Welding process just like any other engineering design process is subject to defects especially if the welding operations is nit carried out in a desired manner. Such defects may be attributed to incorrect welding practices including but not limited to wrong welding positions, improper timing or improper setting of the welding equipment including electrical circuitry. Some of the common defects as well as their causes and acceptability standards are discussed below. a) Undercut Undercutting is a welding defect that occurs when the weld results in reduction of the reduction of the thickness of the base material under consideration. Thus, this leads to a significant reduction in the mechanical strength of the work piece under consideration due to compromise subjected to its molecular structure (Lancaster 1984). Figure 8: An undercut defect in welding. The figure above shows the typical undercut defect in welding as a result of improper welding practices. Causes The major causes of undercut as welding defect includes; Too much welding current from the power supply. Use of inappropriate stick electrode during the welding process. Excessively long arc length. Improper cleaning of base material before welding. Overheating of the base material by the arc before actual welding begins. The defect may also be caused by too fast travel speed during welding. Remedy The following can be used to avoid such defects in welding. Regulate the welding current to the desired level before commencing actual welding. Choose the appropriate electrode based on the material under consideration before actual welding. Compatibility of the electrode with the base material is important for high quality weld. Properly maintain the arc length in a continuous manner. This can be achieved by experience and practise in welding. Use proper and appropriate electrode angles and pause after a specified amount of weld beads will help the welder to determine the quality of weld and regulate as desired. Always ensure that the base metal surface is clean before commencing the welding activity. The electrode sizes should be smaller so as to avoid too much arc on the metal surface. Acceptable limits Undercut has an allowable limits and this varies with the various standards and codes that exists and changes depending on the locality. However, the standard allowable limit of the plate should be less than 1/32” or 5% of the thickness of the material. b) Cracks The cracks as a welding defect varies in magnitude and ranges from micro to macro. There are different categories of cracks depending on their direction of occurrence with regards to the base material. They are normally caused by stresses as a result of shrinkage especially when the weld solidifies. Causes Shrinkage as a result of solidification. Improper matching of the welding rods with the materials i.e. improper choice of the welding rods. To fast cooling rate of the welds. Remedy The current used should not be excessive and should be regulated to the desirable level. Avid too fast cooling of the weld at all times. Choose a proper weld rod before commencing the welding process. Acceptable limits Cracks in welds are totally not allowed at all cost. Thus for any welding process, the user must ensure that the weld is free from cracks. c) Porosity Porosity is a welding defect that manifests when gases are trapped within the solidifying weld surfaces. These gases normally originate from the flux although sometimes, it may result from absorbed moisture present in the coating (Teng et al. 2003). Causes Too high speed of travel during welding. Too much current from the power supply. Moisture in the electrodes coating. Remedy Regulate and maintain proper welding speed during welding. Regulate the current from the power supply to the desired level depending on the use. Molten should be puddle so as to enhance escaping of the gas. Acceptable limits The porosity should not cover more than 10% of the welded area as it will compromise quality and weld strength beyond a desired limit. d) Concavity Concavity as a weld defect is a shallow groove that normally occurs at the root of a butt weld. The defect normally compromises the shape of the weld as well as its mechanical strength. Causes Too large root face. Excessive current. Too heavy hot pass while welding. Remedy Regulate the amount of current depending on the use before commencing welding activity. Use an appropriate welding speed. Use the right and appropriate root face while carrying out welding activity. Acceptable limit The concavity of the root surface shall not lead to reduction of the total joint thickness to less than the nominal thickness of the thinner work piece. Task 5a (LO5: 5.1) Factors influencing design of welded joints Several factors affect the design of welding joints to be used for a particular task. Some of the common factors under consideration are discussed below. a) Joint configuration The joint configuration is a major factor that determines the type of weld joint to be used in a particular part. Depending on the nature of the structure, the configuration of the various parts vary. For parts that require square shaped outcome, square butt welding will be desirable. Therefore, the type of weld joint desirable for a particular task is highly dependent on the joint configuration (Squillace et al. 2004). b) Welding position Welding position plays a very big role in the choice of welding joint desirable for a particular task. In cases where the location or the position of the weld is uncomfortable for the use or is loosely clamped, intermittent welding may be desired in order to avoid cases of distortion. c) Work piece manipulation The appearance, nature and cleanliness of the weld material is an important factor that in the long run affects the quality of the welding process. Therefore, the material must be ensured that it is clean and fit for welding processes. d) Materials Welding process is affected by the materials under consideration. The welding joints varies with the materials under consideration and is especially important to select the appropriate welding joint for particular tasks. e) In-service behaviour requirements. The nature of behaviour of the material while in use or service will be important in choosing the correct weld joint to be used. It is therefore important to understand the functionality and behaviour of every part of the final structure in order to choose the right weld joints for every part. f) Joint access The access to the joint under consideration with regards to the position of the welder will be an important factor in determining the nature of weld to be used in that particular position. For easily accessible parts, complex joint weld as desirable can be used where as in inaccessible or parts with limited access, simple joints may be used so access to avoid compromising on quality of the joint as well as the strength. Task 5b: (LO5 :) When a structure has been welded using an electric arc welding process, mechanical changes take place to the area in and around the weld. These changes will affect the operational performance of the welded joint. Describe the effects of residual stresses on welded structures. Effects of residual stresses on welded structures Welding processes subjects the meal piece under consideration to high temperatures. Therefore after cooling, the metal will undergo internal stresses as a result of these high temperatures as well as heating and cooling (Wang et al. 2002). Such heating and cooling normally results from residual stresses as well as the reaction stresses from the welding. During welding, heat flows outwards from the area of weld and thus causes the joint to expand and thus this thermal expansion and contraction can leave behind permanent stress. Residual tensile stress normally exist within the base material as well as the weld metal whereas residual compressive stresses exist further away from the weld metal. Figure 9: Stress distribution within a weld metal. The figure below shows quantitave distribution of stress within the various regions of a weld surface. Figure 10: Quantitave distribution of stresses within a weld surface. Effects of residual stress The residual stresses on the weld surface reduces the fracture strength. It also leads to reduced buckling strength. The compressive residual stress that occurs further away from the weld region results in increased fatigue strength. In some case, residual stress results in cracking even in cases where there are no loads applied. Remedy In order to reduce the effects of the residual stresses, the following remedies can be employed or put in place. Select the appropriate welding process, procedures and use appropriate equipment for any welding activity. Always select the best method of stress relive and distortion elimination. Select the best material to minimize the effect of residual stress. Task 5c (LO5: 5.3,) Weld distortion Weld distortion occurs as a result of expansion and contraction as a result of the high temperatures involved in welding. Welding on a single face in known to cause more distortion as compared to welding on both faces of the welding material or the metal base. The physical and mechanical properties changes during welding and encompassed with the high temperatures involved results in distortion (Cao et al. 2006). Pre-welding distortion control a) Avoid over welding Over welding normally results in shrinkage and therefore, the bigger the weld, the more the shrinkage. Therefore, the correct sizing of the weld will go a long way in minimizing the effects of distortion as well as optimizing material use. b) Intermittent welding Since continuous welding may result in over welding, intermittent welding may be used to minimize this effects. This will help reduce the amount of heat subjected to the weld surface at any time9. c) Use few weld passes Using fewer but big passes will minimize the amount of heat as a result of welding subjected to the welding surface under consideration. Thus using fewer big passes will help in reducing the risk of distortion. d) Balancing welds along neutral axis As discussed earlier, welding on one side of the plate will result in much distortion as compared to welding on both sides. Therefore, it is important to weld on both sides so as to offset the effect of distortion during welding. Post-welding rectification a) Peening Peening is a process used to relief stresses on the weld bead and this technique must be used with a lot of care as it may lead to further distortion rather than reducing it. Peening may also lead to exposure of the crack thus compromising the quality and the strength of the weld. Peening is not normally recommend in a final part as it may cover cracks on the metal surface and lead to compromised quality of the weld (Nishikawa et al. 2007). b) Thermal stress relieving Thermal stress relieving is a common method employed mostly in removal of shrinkage forces. This involves heating of the weld in a controlled manner to a desired temperature then carry out controlled cooling. This will help in relieving the stresses within the weld and thus prevent distortion11. Task 5d (LO5: 5.4,) Evaluate the quality control procedures used in welding and fabrication which are in place to ensure the ongoing process of quality control. From the testing of the welding operative, to the tracking and logging respective elements, via quality records and site plans, with element locations. Investigate and describe the process of how a potential welding operative becomes a certified coded welder, including the qualification process and procedure. Quality control procedures used in welding The visual inspection procedures for all welds should not lapse for a period of more than half an hour per session. A written description as well as method of documentation for identification and tracking of all welds must be in place. Any required repairs as well as re-inspection of non-conforming parts must be done. The contractor should also have recording systems identifying each weld together with the personnel who carried out the welding part under consideration. Copies of quality control documents should include certificates of compliance, daily production logs, daily reports, and weld repair tracking among others should be updated. Documentation of the filler metals, flux as well as combination of shielding gas certification should be used in accordance to manufacturer’s recommendations. Original and authorized copies of codebooks for each welding activity shall be used for any welding operation and activity. Use standard procedures in performing non-critical repairs as well as logging and tracking of rejected welds to welders (Gunuraj & Murugan 1999). Qualifications and certifications In order to qualify as a certified welder the following requirements are mandatory. A copy of AISC certification for cases where such is provided. Information and recommendations on qualification recorded of the welders. Certifications includes the following procedures. Collection of copies of qualifications from the welders e.g. AISC Inspection of qualification records of the welders. Testing of the welders on some welding techniques as well as assessment and grading. Generation of a master list of qualified welders. Certification and licencing of the welders. Weld planning Welding operation must be well planeed with reagrds to the following proceedures. Types of materials for the welding task with regards to righ choice of welding method, selction of appropriate welding rods and amount of current should be determined. The amount of materials required for the task should also be planned prior to welding. The number of workers as well as their identity must be determined and right workers for the job selected based on their experience and specialization. The type of wel joints for the various parts must be determined prior to welding. The requirements of the welding operations must be met at all cost by putting into consideration the available time and resources. References 1. Gunaraj, V. and Murugan, N., 1999. Application of response surface methodology for predicting weld bead quality in submerged arc welding of pipes. Journal of Materials Processing Technology, 88(1), pp.266-275. 2. Nunes Jr, A.C., Bayless Jr, E.O., Jones III, C.S., Munafo, P.M., Biddle, A.P. and Wilson, W.A., 1984. Variable polarity plasma arc welding on the space shuttle external tank. 3. Kim, Y.S. and Eagar, T.W., 1993. Analysis of metal transfer in gas metal arc welding. WELDING JOURNAL-NEW YORK-, 72, pp.269-s. 4. Nishikawa, H., Serizawa, H. and Murakawa, H., 2007. Actual application of FEM to analysis of large scale mechanical problems in welding. Science and Technology of Welding and Joining, 12(2), pp.147-152. 5. Cao, X., Jahazi, M., Immarigeon, J.P. and Wallace, W., 2006. A review of laser welding techniques for magnesium alloys. Journal of Materials Processing Technology, 171(2), pp.188-204. 6. Wang, G. and Liao, T.W., 2002. Automatic identification of different types of welding defects in radiographic images. Ndt & E International, 35(8), pp.519-528. 7. Shakeri, H.R., Buste, A., Worswick, M.J., Clarke, J.A., Feng, F., Jain, M. and Finn, M., 2002. Study of damage initiation and fracture in aluminum tailor welded blanks made via different welding techniques. Journal of Light Metals, 2(2), pp.95-110. 8. Squillace, A., De Fenzo, A., Giorleo, G. and Bellucci, F., 2004. A comparison between FSW and TIG welding techniques: modifications of microstructure and pitting corrosion resistance in AA 2024-T3 butt joints.Journal of Materials Processing Technology, 152(1), pp.97-105. 9. Teng, T.L., Chang, P.H. and Tseng, W.C., 2003. Effect of welding sequences on residual stresses. Computers & Structures, 81(5), pp.273-286. 10. Lancaster, J.F., 1984. The physics of welding. Physics in technology, 15(2), p.73. 11. Matsuoka, S.I. and Imai, H., 2009. Direct welding of different metals used ultrasonic vibration. Journal of materials processing technology, 209(2), pp.954-960. 12. Buste, A., Lalbin, X., Worswick, M.J., Clarke, J.A., Altshuller, B., Finn, M. and Jain, M., 2000. Prediction of strain distribution in aluminum tailor welded blanks for different welding techniques. Canadian metallurgical quarterly, 39(4), pp.493-502. Read More

In other cases where the electrodes are not consumable, the metal joints are subjected to high temperatures (Wang et al. 2002). The high temperatures between the end of the electrode and the metal part is as a result of the electric arc formed between the metal joint and the end of the electrode. Thus, the electrical circuitry in order to achieve this arc is as shown below. Figure 1: Arc welding circuit Thus the electric arc is carefully guided along the metal joint as desired and this can either be done manual by the welding personnel or mechanically by use of programmed robots although in other cases, simple mechanical mechanisms are used to achieve this guiding.

Some of the arc welding techniques include; Manual metal arc welding. Submerged arc welding. a) Manual metal arc welding Manual metal arc welding is also commonly referred to as shielded metal arc welding and thus abbreviated as SMAW. This method utilizes a consumable electrode as well as an alternating current that is used to strike an arc between the joint and the electrode. Thus for effective joining to occur, the electrode must be made from material that is compatible with the base material whose joining is under consideration.

Such an electrode is normally covered with a material referred to as the flux. The flux forms gaseous vapours during welding and such vapours serves the task of shielding the weld. On cooling, the vapours forms a layer of slag that also serves as a protective layer to the weld from reaction with the air before cooling. Figure 2: Shielded metal arc welding The electrode during welding melts down and fills itself into the weld joint thus eliminating the need for a filler material during manual arc welding operation.

Advantages Simple and do not require much operator training. It is comprises of cheap equipment. Disadvantages The welding process is slow. The process can only be used with working on ferrous materials. b) Submerged arc welding This type of arc welding is normally regarded as a high productivity welding process. The electric arc in this case is struck beneath a layer of flux. The flux is conventionally granular. The electrical circuitry is similar to that of the manual arc welding method however, this method produces higher quality weld due to the fact that the atmosphere is blocked from interacting with the weld by the flux.

The flux in such a cases scraps off by itself and normally results in a clean weld. The method uses a wire feed as a filler and this normally results in an increased rate of deposition (Lancaster 1984). The figure below represents a simple figure of the submerged arc welding process. Figure 3: Submerged arc welding Task 4b: (LO4: 4.2) Power supply The arc welding process depends mainly on the power supply and thus, the welding [power supply is that device that will provide the required electrical current to facilitate welding process.

Arc welding processes requires electrical current as high as 80 amperes and therefore, a power supply must be identified before commencing and welding activity. Some if the common welding power supply in use include: Transformers. Inverter power sources. Generators. a) Transformers Transformer welding power supplies normally steps up the normal power from the utility mains or house hold mains into a higher current and lower voltage for purposes of welding. Such transformers are used hand in hand with rectifiers whose main task is to convert the AC current from the transformer to DC.

The use of rectifiers with the transformers normally incurs more cost on the equipment (Wang et al. 2002). The transformer welding equipment has a mechanism of allowing the user or the welder to select the amount of current he/she want to work with by manipulation the distance between the primary and secondary coil within the transformer. Thus the user can adjust the amount of current based on the material he/she is working on or based on the thickness of the material involved. The picture below shows a typical arc welding transformer.

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