MM235 MANUFACTURING PROCESSES II WELDING MODULAR NOTES. Lecturer: Dr. Joseph Stokes Room S370 Tel: E-mai:

Transcription

1 MM235 MANUFACTURING PROCESSES II WELDING MODULAR NOTES Lecturer: Dr. Joseph Stokes Room S370 Tel: E-mai:

2 WELDING Welding: process in which two materials, usually metals, are permanently joint together coalescence under the influence of temperature, pressure or metallurgical conditions. Classification of welding processes (showing some of the processes): Fusion welding: Chemical Oxy- acetylene welding Electrical Arc welding Shielded metal arc welding (SMAW) Submerged arc welding (SAW) Metal inert gas welding (MIGW) Tungsten inert gas welding (TIGW) Resistance welding Spot welding Seam welding Solid state welding: Friction welding Common characteristics of all welding processes : Almost any size or shape can be welded. Properly welded constructions have high strength/weight ratio, (compared to other processes). A welded joint can be made as strong or even stronger than the base components. However, for ordinary butt joints, when no x-ray or ultrasonic inspection is available, design calculations should consider 80% of the strength of the base material. Typical applications are complex, large assemblies. Examples: tubes, storage tanks, machine frames, structures, platforms. Weldings are typical for repairing operations. Fusion welding: Melting together and coalescing materials by means of heat. The thermal energy required for melting the metal is usually supplied by chemical or electrical means. Fusion welding may be performed with or without adding filler metal to the weld area.

3 Fusion welding During fusion welding the high temperatures associated with the melted weld pool can cause rapid oxidation. The oxides can be entrapped within the weld metal => mechanical properties of the weld degrade => the weld zone should be protected from the surrounded atmosphere => protection is most often made with gaseous shield. Electric arc welding: All of the electric arc welding processes rely on the formation of an arc between an electrode and the base material to provide heat. An electric arc forms when there is an electric discharge between two metal objects that are not in contact with one another (in the case of arc welding the two objects are the electrode and the base metal workpiece). The arc ionises the gas between the electrode and the base metal, thus creating plasma, with temperatures exceeding 10,000 0 C (up to 30,000). The kinetic energy of the plasma is converted to heat which causes local melting of the metal. The electrode may or may not be consumed. Typical welds: A wide variety of weld geometries may be produced. Before welding the edges of the parts to be joined are prepared to form a repository for the weld metal (example: V- shape).

4 Multipass welds: Thicker sections may be welded by multiple passes. Multipass electric arc weld Shielded metal arc welding (SMAW) (stick welding): One of the oldest, simplest, and most versatile joining processes. About 50% of all industrial and maintenance weldings are made by SMAW. The electric arc is generated by touching the tip of a coated electrode against the workpiece and then withdrawing it quickly to a distance sufficient to maintain the arc. The operator moves the electrode slowly along the joint length, and to maintain constant arc length, moves the electrode holder closer to the weld pool as the electrode is consumed. The electrode is in the shape of thin, long sticks (=> stick welding). Manual process. The electrode is in rod form, covered with flux. They are consumed during welding. The electrode must be an alloy compatible with the base metal. The arc melts the base material and the tip of the electrode => the electrode acts as filler material => weld forms. The electrode coating deoxidises (decomposes into CO 2 cloud) and provides a shielding gas in the weld area => protects the weld from oxygen in the environment. The electrode coating also contains fluxes that remove impurities from the molten metal. The flux forms a slag layer on the surface of the molten metal => protects the cooling weld metal from atmospheric contamination. The slag is removed (by special hammer) once the joint area has cooled. The power supply can be a.c. or d.c. Current: normally A. Power: less than 10 kw. Low cost equipment. Slower process than the automated ones => best suited for low- volume production. Portable equipment. Can be used outdoor, in windy conditions. Can be applied in vertical, angular, or upside- down locations.

5 Suitable for inaccessible, distant welds. Minimum normal thickness of sheet metal is 1.5 mm, best for 3-20 mm. No maximum sheet thickness (multiple passes can be used). Safety equipment is compulsory. Materials to be welded: mainly low carbon steel (< 0.3% carbon), some alloy steels. Application: Pressure vessels, pipeworks, pipeline joints, general construction, shipbuilding, repair and maintenance of industrial machinery. Shielded metal arc welding Submerged metal arc welding (SAW): Granular solid flux that submerges the end of the electrode. Supplied to the joint area (ahead of the electrode) immediately prior to welding, by gravity flow through a nozzle. The electrode is continuously fed wire (coil), submerged below a flux layer. Diameter: mm. It is fed automatically through a welding gun. A portion of the flux is melted which acts chemically to lower the oxygen, nitrogen and sulphur content of the weld pool. The unmelted flux protects the weld from the surrounding environment. Automatic movement of the weld gun and dispensing of the flux. Power supply can be a.c. or d.c. Current: A, normally connected to standard single or three- phase power lines. High heat input rate => suitable for heavier sections. 40 mm depth can be made in single pass. Low cooling rate => less brittle welds. The process gives deep penetration => minimum sheet thickness is 4.5 mm. Only for horizontal welds. Less portable than stick welding. Higher equipment cost, faster process => suitable for large- quantity work, especially for long seams. The flux completely covers the weld area => prevents sparks, ultraviolet radiation, fumes => safer for the operator.

6 Unused flux is recovered (by vacuum), treated, reused. Very high weld quality. Materials to be welded: mainly low carbon steel (< 0.3% carbon), some alloy steels, some stainless steel pales and sheets. With some preheat and postheat precautions, medium- carbon and alloy steels, some cast irons, copper alloys, nickel alloys can be welded. Not suitable for high- carbon steels, tool steels, aluminium, magnesium, titanium, lead, zinc. Application: Often used for long seams (pipes, tanks). Bridges, pressure vessels, shipbuilding, heavy structural applications. Thick sections. Submerged metal arc welding Metal inert gas arc welding (MIGW): Bare (uncoated) wire electrode (coil), continuously fed. The electrode is usually the same alloy as the base material. The molten pool is protected by argon (for ferrous alloys), argon/helium mixture (for aluminium and copper alloys), or carbon- dioxide (CO 2 is not an inert gas, but ionises at welding temperature and acts like inert gases) (for steel) shield gas => no atmospheric contamination. Unrestricted electrode length => can be used for long joints. Faster deposition speed, more stable arc than with SMAW => less operator skill is required. Can be used with thin sheets (down to 0.5 mm). Not suitable for thickness > 12 mm. Not suitable for outdoor welding. Not suitable for inaccessible welds. High quality welds (no slag).

7 Due to the absence of the slag the welds cool faster => not suitable for air- hardening steels. Can be easily automated. Although equipment cost is higher than with some other methods, but labour cost is low (no need of slack removal and electrode change). Can be used for high- volume production, too, and the most economical process for low- quantity weldments. Materials to be welded: carbon steels, low- alloy steels, stainless steels, aluminium, magnesium, titanium, copper, nickel. Application: Thin sections, sheets, lighter parts. Inert gas arc welding Tungsten inert gas welding (TIGW): Similar to MIGW, but the electrode is not consumed => does not provide filler metal. The electrode is made of tungsten (or tungsten alloy). The inert gas is mainly argon (or helium). For lighter sections can be used without filler metal => fuse is provided only by local melting of the base metal. If filler metal is needed, auxiliary rod is used (normally added manually by the operator). Can be used for thin sheets ( mm), not suitable for thicknesses > 6 mm. Power supply: current: 200 A d.c., or 500 A a.c., power: 8-20 kw. Advantageous when no filling metal is required. Suitable for all positions. Slower than other processes. Welds are of very high quality and surface finish. Lower equipment cost than with MIGW, but slower. Materials to be welded: if used without filler material, more suitable than MIGW for stainless steel, aluminium, non- ferrous metals (magnesium, titanium). Dissimilar metals can also be welded. Application: Critical applications, thin sections, sheets, lighter parts.

8 Tungsten inert gas welding Oxy-Acetylene welding: Most of the welds that can be done by arc welding, can also be done by oxy-acetylene welding. However, the success of the welds largely depends on the skills of the operator. Mixing a combustion (fuel) gas (acetylene, C 2 H 2 ) and oxygen in a nozzle which is ignited to generate heat. The flame produced is directed onto the joint area to provide rapid melting of the base metal. Initially, the mixed gas is ignited manually as it exits the welding tip (the flow rates have to be adjusted to maintain a stable flame). Acetylene and oxygen are stored in high pressure cylinders (1.7 MPa) => the process is portable, no electrical supply is needed. Nozzle (torch): provides independent adjustment for acetylene and oxygen flow rates. Mixes the gases. Manual process. Filler metal is provided by filler rods (1.5-9 mm). The rods can be bare or coated with flux. Low cost equipment, slower process than the automated ones => best suited for lowvolume production. The heat is far less than with arc weldings => slow heating rate => advantageous for thin sheets (less than 6 mm). Filler metal is provided by separate rods. Low cost equipment, but slow process. suitable for low- quantity work. Materials to be welded: most metals (ferrous and non-ferrous) can be gas welded. Application: structural sheet- metal fabrication, automotive bodies, repair work.

9 Oxy- acetylene welding equipment Resistance welding: Makes use of a material's resistance to an electrical current to provide heat for fusion (as a metal impedes the flow of electric current, heat is generated): H I 2 Rt H - heat generated, J (watt- seconds) I - current, A R - resistance, t - time of current flow, s. Due to energy losses through radiation and conduction the actual heat is less. The total resistance is composed of: Resistance of electrodes Electrode- workpiece contact resistance Resistance of the individual parts to be welded Workpiece- workpiece contact resistance. The metal makes a much better path for the current than the arc in welding => the current must be much higher. The highest resistance is at the interface between the two workpieces => the most heat is generated there. The parts to be joined are held together under force and electrodes apply a short electrical pulse (60 ms to 2 s) at low voltage (< 10 V) but high current (several thousands A, might be up to 100,000 A). The electrical pulse causes melting at the contact between the two parts. After the pulse, the parts remain clamped while the metal parts solidify together and cool.

10 The electrodes are not consumed. No shielding gases or fluxes are required. Two major types of resistance welding: spot welding and seam welding. For these processes the electrodes also provide the clamping force. Materials to be welded: carbon steels (best with carbon content %), low alloy steels, stainless steels. Aluminium is more difficult to weld (narrow plastic temperature range, high electrical conductivity => much higher current is required), copper and its alloys: difficult to weld (high electrical conductivity). Dissimilar metals can be welded if they can combine to form an alloy and if their melting temperatures are not too far apart. Spot welding: The tips of two cylindrical electrodes contact the lap joint of two sheet metals. The electrodes and workpieces are stationary relative to one another. The phases of the process: Apply pressure to clamp the sheets together (in order to obtain good bond in the weld nugget). Current impulse => heat is generated => workpieces melt locally. Current off, hold pressure till joint solidifies. Pressure released. The strength of the bond depends on the cleanliness of the mating surfaces => oil, paint, stain should be removed. Accurate control of timing of the current and the pressure are essential. Simple process, does not require high qualification of the operator. Mostly single electrode pairs are used, but press- type machines with a lot of electrodes may be developed for simultaneous welding. The process can be automated (robot welding). Rapid operation spots per minute. Normal sheet thickness: mm, but can be used mm (for steel), up to 8 mm for aluminium. Low tooling costs, fast setup => can be used for low production rate, too. Application: Sheet- metal fabrication, automotive bodies (an automobile may have 10,000 spot welds), stainless steel cookware.

11 Spot welding Seam welding: Circular electrodes produce a continuous weld along the workpieces. The electrically conducting rollers produce overlapping spot welds (continuous spot weld) => the joint is liquid tight and gas (pressure) tight. Application: cans for household products, gasoline tanks, containers, ductworks. Seam welding Friction welding (friction bonding): The energy required for joining is produced by frictional heating of a rotational part as it is pressed against a stationary part. Rapid heating is made by high rotational speed. Once the mating (rubbing) surfaces have been heated to a elasticised state, rotation is stopped, but the axial force joining the surfaces is increased to complete the weld. The mating surfaces are not exposed to atmosphere => clean surface for bonding (the base materials do not melt!). Quick process (may be less than 10 s).

12 Surface speed may be up to 1000 m/min. The bonding is a complete metallurgical- joint continuity without brittle phases or voids. Dissimilar metals can be bonded (like copper and aluminium). The required energy is much less than with other welding processes. The parts must be able to withstand the high torque. One of the parts normally should be cylindrical or easily mounted in the chuck or holding fixture. The heat and the pressure develop a flash by plastic deformation => it can be removed by machining Applications: joining rods and tubes, pinions, welding a cylindrical part to a bigger one. Friction welding

13 Welding Tutorial Questions Questions (These short questions (a i) would be typically 5 to 8 marks each, question m overleaf would be 16 marks) (a) Describe the Oxyacetylene Gas Welding process, showing the basic equipment with the aid of sketches, and list two of its industrial applications. (b) Describe the Shielded Metal Arc Welding (SMAW) process. Explain the major characteristics of the process. Give three applications of the process. (c) Explain the basic principal of Submerged metal Arc Welding (SAW), listing two applications of the process (d) Briefly describe the following two methods of Welding; Fusion Welding and Electric Arc Welding (e) Describe the Metal Inert Gas-arc Welding (MIGW) process with the aid of sketches, and list some its industrial applications. (f) Describe the Resistance Welding process and explain in detail the spot welding process and its industrial applications. (g) Explain the basic principle of Friction Welding, listing two applications of the process (h) Name each of the welding processes used to join the following products; a. Can of Beans = Seam Welding b. Pinions = Friction Welding c. Wing section of a car = Spot Welding d. Thick sections of bridges = Submerged Metal Arc Welding (i) Describe the Spot Welding process, showing the basic equipment with the aid of sketches, and list two of its industrial applications. (j) Briefly describe the Seam Welding process, showing the basic equipment with the aid of a sketch. Give three applications of the process. (k) Explain the basic principal of Friction Welding, listing three applications of the process (l) Name the following pieces of Welding Equipment and give one application of each. + + (i) (ii) (iii) + -

14 (m) Draw table 2 below into your examination script and place the letter (in the correct? position) of the following welding equipment under their associated welding process; (a) Seam welding, (b) Shielded Metal Arc Welding (SMAW), (c) Oxy- Acetylene Welding, (d) Metal Inert Gas Welding (MIGW), (e) Friction Welding, (f) Spot Welding, (g) Submerged Arc Welding (SAW), and (h) Tungsten Inert Gas Welding (TIGW). FUSION WELDING TABLE 2: WELDING PROCESSES & EQUIPMENT Chemical Welding I.? Electrical Welding Arc welding II.? III.? IV.? V.? Resistance welding VI.? VII.? SOLID STATE WELDING VIII.? REFERENCES 1. Serope Kalpakjian, Manufacturing Processes for Engineering Materils 3-rd edition, Addison- Wesley, John A. Schey, Introduction to Manufacturing Processes, McGraw- Hill, E. Paul DeGarmo et all., Materials and Processing in Manufacturing, Prentice Hall, New Jersey, J. Beddoes et.all., Principles of Metal Manufacturing Processes, Arnold, London, R. L. Timings, Manufacturing technology; Volume 1, Longman Scientific & Technical, R. L. Timings, Manufacturing technology; Volume 2, Longman Scientific & Technical, B. H. Amstead at.all., Manufacturing processes, John Wiley & Sons, New York, R. A. Lindberg, Processes and Materials of Manufacture, Allyn & Bacon, P. F. Ostwald et.all., Manufacturing Processes and Systems, John Wiley and Sons, W. A. J. Chapman, Workshop Technology Part 3, Edward Arnold, J. G. Bralla, Handbook of Product Design for Manufacturing, McGraw Hill, 1986.