Presentation on theme: "Chapter 5: Welding Process Shafizan Bt. Shariffuddin School of Manufacturing Engineering UniMAP 5.1 INTRODUCTION TO WELDING."— Presentation transcript:
Chapter 5: Welding Process Shafizan Bt. Shariffuddin School of Manufacturing Engineering UniMAP 5.1 INTRODUCTION TO WELDING
Introduction Welding is a fabrication process that joins materials, usually metals, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld.
Classification of Welding Processes The American Welding Society has defined and classified the various welding processes in the manner presented in Figure 2. In Fusion Welding processes, the work-pieces are melted together at their faying surfaces. They are the most commonly used processes. Arc, resistance, and oxy-fuel gas welding methods have melting points about the same as or just below those of the metals being joined.
In Solid State Welding, the work-pieces are joined by the application of heat and usually pressure, or by the application of pressure only. However, with these processes, the welding temperature is essential below the melting points of the materials being joined or if any liquid metal is present it is squeezed out of the joint. No filler material is added during welding.
Manual Joining Processes A manual joining process is completely performed by hand. The welder controls all of the manipulation, rate of travel, joint tracking and in some cases, the rate at which filler metal is added to the weld. The most commonly used manual joining processes are listed as follows :- –Shielded Metal Arc Welding (SMAW) –Gas Tungsten Arc Welding (GTAW) –Oxyacetylene Welding (OAW) –Torch Brazing (TB)
Chapter 5: Welding Process Shafizan Bt. Shariffuddin School of Manufacturing Engineering UniMAP 5.2 ARC WELDING PROCESS
Fundamental Of Arc Welding One of the most important processes in industry is the fusion of metals by an electric arc. This is commonly called arc welding or SMAW (Shielded Metal Arc Welding). Arc welding is widely used in the manufacturing and construction industries. Special applications of basic arc welding such as resurfacing steel parts, automatic welding processes, pipe welding and shielded arc welding are widely used.
Briefly, the process takes place in the following manner :- The work to be welded is connected to one side of an electric circuit, and a metal electrode is connected to the other side. These two parts of the circuit are brought together and the separated slightly. The electric current jumps the gap and causes a continuous spark called an arc. The high temperature of this arc melts the metal to be welded, forming a molten puddle. The electrode also melts and adds metal to the puddle. As the arc is moved, the metal solidifies. The metal fuses into one piece as it solidifies. The melting action is controlled by changing the amount of electric current which flows across the arc and by changing the size of the electrode.
Arc Welding Variables Factors to secure a weld to has proper penetration, :- –Correct electrode –Correct arc length –Correct current –Correct travel speed –Correct electrode angle
Correct electrode The choice of an electrode involves such items: –position of the weld, –properties of the base metal, –diameter of electrode, –type of joint and –current value.
Correct arc length If the arc is too long: –The metal melts off the electrode in large globules which wobble from side to side as the arc wavers. –This produces a wide, spattered and irregular bead without sufficient fusion between the original metal and the deposited metal. An arc that is too short: –Fails to generate enough heat to melt the base metal properly. –Furthermore, the electrode sticks frequently, producing a high, uneven bead with irregular ripples.
Figure 4 : This is how beads appear when : (top) the arc is to long, (middle) when the arc is too short, (bottom) when the arc is the correct length
From Figure 4: –The top sample illustrated a weld formed when the arc is too long. This is indicated by the large amount of spatter and the fact that the beads are coarse. –The middle sample shows a weld made with an arc that is too short. Notice the excessive height of the beads. Such beads are a sign of improper penetration. –The bottom sample is an example of a good weld. In this case, the beads have the proper height and width and the ripples are uniformly spaced.
Correct current If the current is too high: –the electrode melts too fast and –the molten pool is large and irregular. When the current is too low: –there is not enough heat to melt the base metal and –the molten pool will be too small. –poor fusion and the beads will pile up and be irregular in shape.
Correct travel speed Where the speed is too fast, –the molten pool does not last long enough and impurities are locked in the weld. –the bead is narrow and the ripples pointed. If the rate of travel is too slow, –the metal piles up excessively –the bead is high and wide with straight ripples as illustrated in Figure 5.
Figure 5 : Examples of properly and improperly formed beads. A) Current, voltage and speed normal B) Current too low C) Current too high D) Voltage too low E) Voltage too high F) Speed too slow G) Speed too fast
Correct electrode angle The angle at which the electrode is held will greatly affect weld bead shape and is particularly important in fillet and deep groove welding. Electrode angle involves two positions – incline and side angles. Incline vary from 5° to 30° from the vertical, depending on welder preference and welding conditions. Side angle is the angle from horizontal, measured at right angles to the line of welding, which normally splits the angles of the weld joint. See Figure 6 for examples.
Basic Principles Of Sustaining A Welding Arc To sustain a stable arc, three elements will be necessary :- –arc gap (arc length) –arc voltage –load current (amperage)
arc length : length between the electrode and the work. A long arc unfortunately produces: –unstable welding arc, –reduces penetration, –increase spatter, –causes flat and wide beads –prevents the gas shield from protecting the molten puddle from atmospheric contamination. Arc too short –will not create enough heat to melt the base metal, –the electrode will have a tendency to stick, –penetration will be poor and –uneven beads with irregular ripples.
Arc voltage or load voltage, is the voltage measured across the arc during welding. This voltage is affected by the arc length and therefore must fall within a certain range for each welding condition. Load current represents the actual flow of electricity and is regulated by the current setting of the power supply.
Striking the arc There are two methods which can be used to start or strike the are :- –tapping –scratching motion In the tapping motion, the electrode is brought straight down and withdraws instantly, as shown in top of Figure 6. With the scratching method, the electrode is moved at an angle to the plate in a scratching motion as much as in striking a match shown in Figure 6.
Figure 6 : There are two methods of starting or striking a welding arc.
Chapter 5: Welding Process Shafizan Bt. Shariffuddin School of Manufacturing Engineering UniMAP 5.3 OXYACETYLENE WELDING PROCESS
Fundamentals Of Oxyacetylene Welding In oxyacetylene welding, one of the gas welding processes, the metal is heated by the hot flame of a gas-fed torch. The metal melts and fuses together to produce the weld. In many cases, additional metal from a welding rod is melted into the joint which becomes as strong as the base metal.
Advantages and Disadvantages can be applied to a wide variety of manufacturing and maintenance situations. The equipment is portable. The cost and maintenance of the welding equipment is low when compared to some other welding processes. The cost of welding gases, supplies and operator’s time, depends on the materials being joined and the size, shape and position in which the weld must be made. The rate of heating and cooling is relatively slow. In some cases, this is an advantage. In other cases where a rapid heating and cooling cycle is desirable, the oxyacetylene welding process is not suitable. A skilled welder can control the amount of heat supplied to the joint being welded. This always a distinct advantage. The oxygen and nitrogen in the air must be kept from combining with the metal to form harmful oxides and nitrides.
In general, the oxyacetylene process can be used to advantage in the following situations:- When the materials being joined are thin. When excessively high temperature or rapid heating and cooling of the work would produce unwanted or harmful changes in the metal. When extremely high temperatures would cause certain elements in the metals to escape into the atmosphere.
Equipment The basic oxyacetylene welding equipment is shown in Figure below. Figure 7 : Oxy-acetylene Equipment
WELDING TECHNIQUE When welding, an operator can concentrate the heat from the torch either in the weld bead, which is called the backhand technique; or he can concentrate the heat ahead of the weld bead or in the weld puddle, which is called the forehand technique (Figure 8). Figure 8 : Welding Technique : (a) Forehead, (b) Backhand
Forehand Welding Technique The forehand welding technique is usually used on relatively thin metals. The torch points in the same direction that the weld is being done so that the heat is not flowing into the metal as much as it could. The tip of the torch is held approximately at 45° angle, which makes some of the heat deflect away from the metal. Instead of the base metal absorbing all the heat, some of it is reflected off into the atmosphere. In this way, it is possible to weld very thin material. The weld bead appearance is characterized by an evenly flowing, ripple design.
Backhand Welding Technique The backhand welding technique is one used on heavier of thicker base metals. Basically, in this technique the torch is pointing in the direction opposite to that in which the weld is being done. In this technique, the heat is concentrates into the metal so that thicker materials can be welded successfully. Welds with penetrations of approximately 12 mm can be achieved in a single pass with the backhand technique. The bead is characterized by layers that form a much broader based ripple than that of the forehand technique.
Both of these techniques can be used with or without filler rod. Welding done without filler rod is called pudding. When pudding in the flat position, the torch is usually held somewhere between the angles of 35° and 45° (Figure 9). Even penetration can be determined by observing the amount that the metal sags in the bead’s path. The amount of sag should be just enough to be noticeable. Pudding is generally used with metals that is approximately 10 gage to 3 mm thick. Metals heavier than this are generally welded with filler material.
Figure 9 : Welding (a) Without filler rod (puddling) (b) With filler rod
JOINT DESIGN Joint design is greatly influenced by :- The cost of repairing the joint, The accessibility of the weld, Its adaptability for the product being designed or welded, The type of loading the weld is required to withstand. The five basic joints used in welding are :- butt T Lap edge corner
Surfacing Weld A type of weld composed of one or more stringer or weave beads deposited on an unbroken surface to obtain desired properties or dimensions. Fillet Weld A fillet weld is approximately a triangle in cross-section, joining two surfaces at right angles to each other in a lap, T or corner joint. Groove Weld A groove weld is a weld made in the groove between two members to be joined. The weld is adaptable for joints classified as square butt, single-V, double-V, single-U, single-J and double-J. Plug and Slot-Weld These welds are used to join two overlapping pieces of metal by welding through circular holes or slots. Such welds are often used instead of rivets.
Chapter 5: Welding Process Shafizan Bt. Shariffuddin School of Manufacturing Engineering UniMAP 5.4 FLAME CUTTING
Fundamentals Of Flame Cutting Advantages of this cutting method are :- A relatively smooth cut is produced. Very thick steel can be cut. The equipment is portable. Underwater cutting is possible with some adaptations. The equipment lends itself to automatic processes in manufacturing
THE FLAME CUTTING PROCESS In flame cutting, a jet of the purest possible oxygen (purity not less than 99.5%) blown on the starting point of the cut, which has been preheated to the ignition points to the material by means of a heating flame. The oxygen burns the material at this point, releasing a considerable amount of heat which in turn heats the zone below it to igniting temperature. The rapid and continuous process of preheating and burning produces a cut in even the thickest work-piece. The movement of the torch produces a cutting kerf, the combustion products being blown clear by the kinetic energy of the oxygen jet which strike the work-piece vertically in relation to the cutting direction.
EQUIPMENT Cutting is performed with a manual torch, Figure 14, and with different machine torches as in Figure 15 and 16. Tips for these torches are interchangeable so that can be adapted to cut a variety of metal thickness. The torches and tips are constructed so that they can preheat the work to the kindling temperature. The manual torch also includes a level for starting and stopping the stream of high- pressure cutting oxygen as required.
Figure 14: Flame-cutting torch with one oxygen needle valve (left) and one acetylene needle valve (right) Figure 15 : Machine cutting torch.Figure 16 : Shape cutting machine.
Chapter 5: Welding Process Shafizan Bt. Shariffuddin School of Manufacturing Engineering UniMAP 5.5 MIG/MAG WELDING PROCESS
MIG/MAG PROCESS MIG/MAG welding (often called metal inert-gas or metal active gas is done by using a consumable wire electrode to maintain the arc and to provide filler metal. The wire electrode is fed through the torch or gun at a present controlled speed. At the same time, an inert gas active gas is fed through the gun into the weld zone to prevent contamination from the surrounding atmosphere.
Advantage of MIG/MAG Welding Arc visible to operator High welding speed No slag to remove Sound welds Weld in all positions
Types of MIG Welding Spray-arc welding, Figure 17, is a high-current- range method which produces a rapid deposition of weld metal. It is effective in welding heavy-gage metals, producing deep weld penetration. Short-arc welding, Figure 18, is a reduced-heat method with a pin arc for use on all common metals. It was developed for welding thin-gage metals to eliminate distortion, burn-through and spatter. This technique can be used in the welding of heavy thicknesses.
MIG CO2 (carbon-dioxide) welding is a variation of the MIG proves. Carbon dioxide is used as the shielding gas for the welding of carbon and low-alloy steel from 16 gage (1.5 mm) to 6 mm or heavier. It produces deeper penetration than argon or argon mixtures with slightly more spatter.
Flux-cored welding (FCAW) is an intense- heat, high-deposition-rate process using flux-cored wire on carbon steel. Electrically, cored-wire welding is similar to spray-arc welding. Flux-cored wires are available in diameters as small as 1 mm. The process can be used on material as thin as 3 mm and welded in all positions.