Welding Technology for Mechanical Engineering - Assignment Example

FABRICATION AND WELDING TECHNOLOGY FOR MECHANICAL ENGINEERING By Name Institution Instructor Course Class Date Task 4a: Metal Active Gas (MAG) welding process Equipment MAG setup includes a gas cylinder, roll of electrode wire, wire feed control unit, welding gun/torch, and a welding power supply. All these components are assembled as shown in figure 1, which shows a schematic diagram of a MAG process. Figure 1: Schematic diagram of a MAG process a. Gas cylinder: it provides the shielding gas, which shields or protects the arc and weld pool from atmospheric gases. In other words, shielding gas protects weld metal during transfer to prevent reaction that would interfere with weld quality. The shielding gas must not react with the molten steel, and it could be helium, argon, carbon dioxide, or a mixture of oxygen, carbon dioxide, and argon (Jeffus & Bower 2010). The choice of shielding gas to use depends on its compatibility with the metal being welded, type, and thickness of the joint, welding properties, and physical properties of the metal being welded (Jeffus 2017). b. Electrode roll: the wire is used as the electrode and the filler. c. Wire feed unit: this component controls the rate of feed of the electrode into the welding torch. d. Welding torch: all the inputs end up at the welding torch, which actualises the welding process. The electrode comes out at the middle of the welding torch surrounded by shielding gas that comes out through a concentric diffuser as shown in the figure below. The arrangement ensures that the shielding gas surrounds the molten weld and the arc. The welding torch as controls the wire feed unit (Jeffus 2017). Figure 2: Diagram showing arc development, metal transfer and gas shielding at the welding torch (Jeffus & Bower 2010) e. Power supply unit: this component provides the necessary power (direct current) to facilitate the welding process. Initially, MIG was designed to weld aluminum with argon being used as the shielding gas. However, developments have expanded the range of metals to include mild steel, aluminum, aluminum alloys, bronze, copper, magnesium alloys, and stainless steel. As earlier mentioned, the choice of shielding gas depends on the metal to be welded. Therefore, it is important to change the shielding gas when welding different types of metals to prevent reaction with the molten metal. Such a reaction would compromise weld quality in terms of appearance and strength. In addition, physical properties of the metal being welded determine gas selection. Moreover, weld thickness determines shielding gas to use because gas selection determines metal transfer method (Jeffus 2017). Consequently, selecting the best gas for the metal to use affects weld performance and quality. MIG is most preferred welding method for industrial applications because it offers a number of benefits compared to other methods such as arc welding. Firstly, it is possible to automate (full or semi-automation) the process. Secondly, it is relatively easy to carry out with little training because it does not depend so much on welder skills and experience as is the case with arc welding. Further, it is applicable to a wide variety of industrial metals especially heat resistant stainless steels. Moreover, increase in current does not negatively affect the process and weld quality. Besides, it lends itself suitable for thin sheet welding where other methods would not produce good quality work (Smith 2014). Other benefits include good rate of deposition, high productivity, and deep fusion penetration compared to other welding techniques. Task 4b: Welding Power Sources Available in Modern Welding Workshops A welding power source provides the desired current to facilitate welding. They range from simple designs to highly complicated systems with computer programs for welding control. The most common power supplies include transformer, generator, and inverter designs. a) Transformers The most common power supply type is the transformer. Arc welding uses much lower voltages than utility values. The transformer reduces line voltage, which ranges between 240 and 480V to generate lower welding voltages of between 60 and 80V (Phillips 2016). Since they reduce high AC utility voltage to lower welding voltages, these transformers are step-down types. As shown in figure 3, they achieve this reduction by having more primary windings (on the input side) and fewer secondary windings (on the output side). Figure 3: A step down transformer with higher number of coils on the primary winding than on the secondary winding. It steps down voltage in accordance with the ratio of coils between the two windings b) Rectifier A rectifier is used in welding power source to convert alternating current (AC) to direct current (DC) to facilitate arc welding. There are two types of rectifiers, half wave and full wave. In half wave rectification, only half of the AC wavelength is converted to DC but in full wave rectification, the entire AC is converted to DC. One design that facilitates full wave rectification is the full wave bridge rectifier as shown in figure 4. Figure 4: full wave bridge rectifier c) Inverter Inverter welding machines are small compared with others of the same ampere range. Therefore, they are portable and energy efficient compared to other power supply types. The inverter increases mains frequency from 60Hz to thousands of cycles a second using solid-state electronics parts. Consequently, it is possible to use small-sized transformers. Further use of electronics makes it possible to lower the high frequency in the output power. Therefore, it is possible to alter welding power to suit typically any application such that a single inverter power source is applicable to different welding techniques including MIC, tungsten arc welding, arc welding, and plasma arc welding (Jeffus 2012). d) Generators In areas not connected to the grid, a diesel generator may be used to supply the required DC for arc welding. This arrangement is also used to facilitate portability. A diesel-powered engine is used to generate DC whereby it rotates an armature loop between two magnetic poles (split magnets) also known as stator as shown in figure 5 (Phillips 2016). As a result, a voltage is induced around the magnetic loop leading to generation of DC. Figure 5: Schematic diagram of a DC generator showing only one armature set Task 4c: Welded Joints The four main types of welded joints are butt, lap, T, and corner joint. 1) Butt joint This weld joint joins two pieces placed parallel to each other such as metal pipes, flat pieces of metal, shafts, and metal rods. The weld is applied within the outline of the final component and it fuses the entire cross section. Most applicable weld processes for this type of weld include gas, laser, arc, electron beam, friction, flash, and arc welding (Bhandari 2010). 2) Lap joint This type of joint is most applicable to two pieces of metal with different thickness. One metal is placed on top of the other with a desirable overlap provided. Alternatively, one piece of metal may be sandwiched between two others. Then, the welder may apply a weld on one or both sides depending on design and requirements. This type of weld joint is most applicable to metal plates and sheets. Applicable types of weld include resistance spot, seam, arc spot, and arc welding (Bhandari 2010). 3) T joint The edge of one metal (pipe, shaft, bar, or plate) rests on the surface of another metal at an intersection angle of 90 degrees. A weld beam is applied around the point of intersection using gas, laser, arc, electron beam, friction, flash, or arc welding (Bhandari 2010). 4) Corner joint It connects two pieces of metal in such a way that they form an L section at the joint. The two pieces, mostly (but not exclusively) pipes must be cut into 45 degrees before welding. The weld can be achieved through gas, laser, arc, electron beam, friction, flash, and arc welding (Bhandari 2010). Selecting type of joint for a full penetration weld using 20 mm mild steel plate For this job, the best weld joint would be a double sided full penetration butt weld shown in the figure below. The work would begin by machining both edges to be to be joined at an included angle of 60 degrees to a depth of 8 mm on each side of each metal. A root face of 1 mm would be left at the middle of the thickness. During welding, the two pieces of metal would then be separated at a gap (root gap) of 1.5 mm to facilitate root penetration. Filling runs would then run side to side (from one metal to the other) so that molten metal from each piece will mix with filler from the electrode to form a tight weld. The capping run would then be made along the edge of each piece and afterwards in a weave motion from side to side. This type of joint would be used in repairing a broken beam such as those used in the construction of bridges. These beams are usually made of thick metals. Task 4d: Welding Defects and Corrective Measures MIG welding defects and corrective measures 1. Porosity: it is readily visible on weld face whereby weld contains air spaces. Causes include: Weld contamination by atmospheric gases especially when rate of supply of shielding gas is so fast such that some or all of it is lost, supply rate is too slow such that there is insufficient shielding, or rate of supply is too high that there is turbulence leading to drawing of atmospheric air (Bhandari 2010) Shielding gas is entrapped during solidification of the molten metal (Bhandari 2010) Shielding gas is contaminated (Jeffus 2012) Corrective measures include: Making sure shielding gas is dry and pure (Bhandari 2010) Matching shielding gas with the metal being welded (Jeffus 2012) Controlling rate of shielding gas release (Bhandari 2010) 2. Whiskers: electrode wire sticks to the weld. It is caused by pushing the electrode wide beyond edge of the weld pool. It is corrected by reducing wire feed rate. 3. Excessive penetration: this defect is characterised by too much melt-through. Too much heat on the weld area, improper preparation of the joint especially having a too wide root or too small root face cause the defect. Remedies include proper joint preparation and design depending on material thickness, reducing electrode feed rate or increasing travel speed to reduce current and hence heat on the weld area, and increasing distance of electrode extension. 4. Incomplete penetration: causes include insufficient heat and poor weld techniques/design. Remedies include increasing current by increasing electrode feed rate and reducing electrode extension. Proper preparation of the work piece and using correct weld design also corrects the problem (Jeffus 2012). TIG welding defects and corrective actions a. Porosity: characteristics include cavities in the weld metal. Causes include: Absorption of oxygen, hydrogen, and oxygen during welding Fumes generated from metal coatings on metal being welded. Such coatings include paint, oil, and metal coatings (Bhandari 2010). Corrective measures include: Cleaning the work piece before welding Ensure that electrode is dry Check for and seal any air leak (Jeffus 2012) b. Incomplete penetration: its main cause is poor weld design and metal preparation. Therefore, corrective measures include preparing the work piece properly and using the correct weld design (Jeffus 2012). MMA welding defects and corrective measures i. Incomplete penetration is very common and is caused by poor preparation of the work piece and improper weld design. Corrective measure include preparing the metal to the correct root angle depending on thickness, adequate root gap and root face. Selection of the correct weld design also helps. Insufficient head also causes under penetration and this can be remedied by adjusting power source to increase voltage depending on metal thickness (Bhandari 2010). ii. Over penetration may be caused by excessive heat. It can be corrected by adjusting voltage to reduce heat depending on metal thickness and type (Jeffus 2012). Task 5a: Designing a Structural Weld Joint When designing a welded joint, one should consider a number of factors. The first issue of consideration is joint application (what the joint will be used for). One should consider forces to be applied on the joint and whether the joint will interfere with intended application. The amount and type of load to be applied will determine joint preparation and design to ensure that the welded joint withstands forces. The location of the joint determines the weld type and preparation. For example, if the part to be welded is supposed to move within a cylinder, the weld must achieve a smooth surface finish. It is crucial to perform a simple cost-benefit analysis when designing welded joints. Cost of welded joints includes work piece(s) preparation costs and the cost of carrying out the weld. The life span of the welded joint needs also be factored in so that one can decide to use a welded joint, replace the broken component (if the welding is aimed at repairing a component), use another type of joint (such as nut and bolt), or use a different weld joint design. Finally, the designer should consider if a full or partial penetration is required, which then determines joint preparation techniques required. Full penetration is required when the piece is thick or when there are significant loads to be applied on the weld (Messler, Jr. 2004). Task 5b During welding, the weld and the heat affected zone (HAZ) develops extremely high temperature. If cooled rapidly, the weld and HAZ form crystalline grain structures making them hard and brittle. These changes will mostly be prominent in cast iron, tool steel, and high carbon steel although other metals will still become more brittle. Consequences, chances of cracking will increase especially when the weld is subject to significant loading due to internal stresses that developed when the metal was cooled rapidly from high temperature. Materials can cool very quickly due to conduction and convection. Conduction occurs when materials gain or lose heat through collisions that occurs between neighboring molecules (Bridigum 2008). In this case, the weld and HAZ loses heat by heating up energising molecules on other parts of the material. Convection is a process through which a material gains or losses heat through mass movement of a fluid such as water or air (Bridigum 2008). In the case of welding, the weld and HAZ heats up adjacent cold air, which rises and another current of cold air falls. The cycle continues until the weld and HAZ becomes cold. Cooling through convention is highly prominent and fast when welding in cold weathers. To prevent rapid cooling of the weld and HAZ, which would make the joint weak, it is important to preheat the metal to between 300 and 400 degrees F before welding (Bridigum 2008). This process prevents rapid cooling through conduction and convection. This process is known as pre-weld treatment of the metal. Post-weld treatment involves treating the metal immediately after welding to prevent structural change. The most important post-weld treatment is slow cooling of the weld and HAZ, which promotes ductility and strength owing to even distribution of carbon. Strategies to ensure slow cooling include burying the welded part in dry sand or kitty litter or wrapping it with fire blanket. Another post-weld treatment is annealing, which removes stresses in the metal. For steels, annealing involves heating the metal to about 1300 degrees F, holding the metal at this temperature for some time, and then allowing slow cooling (Bridigum 2008). Task 5c: Techniques Used to Control and Rectify Welding Distortion It is easier and cost effective to control welding distortion during the welding process than performing post-welding rectification. When the weld metal and HAZ expand and then contract rapidly, distortion occurs (Lincoln Electric 2016). One technique for controlling welding distortion is through sequence welding. Every weld made should be in such a way that it counters shrinking forces made by a previous weld (Lincoln Electric 2016). For example, one can alternate welds on each side of the T-joint intermittently as shown in the figure below. Afterwards, the welder can fill the remaining gaps. The second method is through presetting of sections to be welded so that when welding is done, it corrects the presetting as shown in the figure below (Lincoln Electric 2016). As shown in the figure, welding is supposed to correct the preset through shrinkage thereby pulling the two pieces together to the required shape. However, the correct amount of preset requires a number of trials. However, it may not be possible to eliminate distortion completely during welding, which means one has to remove it after welding. A welder can use a big hammer or hydraulic jacking to force the parts back to the desired shape and form (Lincoln Electric 2016). However, cracks may form but this can be corrected during the final pass. Task 5d: Quality Control Procedures Used in Welding Fabrication Quality control procedure is a process used to ensure that the product meets all client requirements or set quality criteria. In welding fabrication, quality control ensures that the welded joint meets set performance standards. The procedure includes observing the welding task to ensure that the welder performs it as expected and then performing tests to make sure the weld joint performs as per expectations. For example, a torsion test may be necessary to ensure that a welded shaft carries the intended twisting load. The quality of the weld is fundamentally dependent on the welder. Certified coded welders are highly competent technicians capable of performing excellent weld jobs or carrying out quality assessment. To become a certified coded welder one must undergo welding fabrication training after which he/she undertakes a welding test. Passing this test enables one to be a certified welder. Bibliography Bhandari, BV 2010, Design of machine elements (3rd ed.). New Delhi, India: Tata McGraw Hill Education Private Limited. Jeffus, L 2012, Welding: Principles and applications (7th ed.). Boston, MA: Cengage Learning. Jeffus, L 2017, Welding: Principles and applications (8th ed.). Boston, MA: Cengage Learning. Jeffus, L & Bower, L 2010, Welding: Skills, processes and practices for entry-level welders. Clifton Park, NY: Delmar Cengage Learning. Lincoln Electric 2016, Weld distortion. [Online] Retrieved from [Accessed June 15, 2016]. Messler, RW, Jr. 2004, Principles of welding: Processes, physics, chemistry, and metallurgy. Wiley-VCH Verlag GmbH & Co. KGaA. Phillips, DH 2016, Welding engineering: An introduction. West Sussex, UK: John Wiley & Sons, Ltd. Smith, F 2014, Metal active gas shielded welding (MAGS/MIG). [Online] retrieved from https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwjqmY664qbNAhUH1xQKHSkeCOMQFgg3MAM&url=http%3A%2F%2Flocal.ecollege.ie%2FContent%2FAPPRENTICE%2Fliu%2Fpipefitting%2Fpdf%2FM2_U4_Metal%2520Active%2520Gas%2520Shielded%2520Welding.pdf&usg=AFQjCNHLxNmSt2HOX5xP6esDyTk6R6y1Bg&sig2=wecEz0QXvVVm6gBZX9uNSw Read More

Moreover, weld thickness determines shielding gas to use because gas selection determines metal transfer method (Jeffus 2017). Consequently, selecting the best gas for the metal to use affects weld performance and quality. MIG is most preferred welding method for industrial applications because it offers a number of benefits compared to other methods such as arc welding. Firstly, it is possible to automate (full or semi-automation) the process. Secondly, it is relatively easy to carry out with little training because it does not depend so much on welder skills and experience as is the case with arc welding.

Further, it is applicable to a wide variety of industrial metals especially heat resistant stainless steels. Moreover, increase in current does not negatively affect the process and weld quality. Besides, it lends itself suitable for thin sheet welding where other methods would not produce good quality work (Smith 2014). Other benefits include good rate of deposition, high productivity, and deep fusion penetration compared to other welding techniques. Task 4b: Welding Power Sources Available in Modern Welding Workshops A welding power source provides the desired current to facilitate welding.

They range from simple designs to highly complicated systems with computer programs for welding control. The most common power supplies include transformer, generator, and inverter designs. a) Transformers The most common power supply type is the transformer. Arc welding uses much lower voltages than utility values. The transformer reduces line voltage, which ranges between 240 and 480V to generate lower welding voltages of between 60 and 80V (Phillips 2016). Since they reduce high AC utility voltage to lower welding voltages, these transformers are step-down types.

As shown in figure 3, they achieve this reduction by having more primary windings (on the input side) and fewer secondary windings (on the output side). Figure 3: A step down transformer with higher number of coils on the primary winding than on the secondary winding. It steps down voltage in accordance with the ratio of coils between the two windings b) Rectifier A rectifier is used in welding power source to convert alternating current (AC) to direct current (DC) to facilitate arc welding.

There are two types of rectifiers, half wave and full wave. In half wave rectification, only half of the AC wavelength is converted to DC but in full wave rectification, the entire AC is converted to DC. One design that facilitates full wave rectification is the full wave bridge rectifier as shown in figure 4. Figure 4: full wave bridge rectifier c) Inverter Inverter welding machines are small compared with others of the same ampere range. Therefore, they are portable and energy efficient compared to other power supply types.

The inverter increases mains frequency from 60Hz to thousands of cycles a second using solid-state electronics parts. Consequently, it is possible to use small-sized transformers. Further use of electronics makes it possible to lower the high frequency in the output power. Therefore, it is possible to alter welding power to suit typically any application such that a single inverter power source is applicable to different welding techniques including MIC, tungsten arc welding, arc welding, and plasma arc welding (Jeffus 2012). d) Generators In areas not connected to the grid, a diesel generator may be used to supply the required DC for arc welding.

This arrangement is also used to facilitate portability. A diesel-powered engine is used to generate DC whereby it rotates an armature loop between two magnetic poles (split magnets) also known as stator as shown in figure 5 (Phillips 2016). As a result, a voltage is induced around the magnetic loop leading to generation of DC. Figure 5: Schematic diagram of a DC generator showing only one armature set Task 4c: Welded Joints The four main types of welded joints are butt, lap, T, and corner joint. 1) Butt joint This weld joint joins two pieces placed parallel to each other such as metal pipes, flat pieces of metal, shafts, and metal rods.

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