Unit-4 Unconventional Manufacturing Process

November 1, 2017 | Author: brijkishor2017 | Category: Welding, Electric Arc, Plasma (Physics), Scuba Diving, Industrial Processes
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Explosive welding, Cladding etc. Under water welding, Metalizing, Plasma are welding/cutting etc....


Introduction Welding and joining are essential for the manufacture of a range of engineering components, which may vary from very large structures such as ships and bridges, to very complex structures such as aircraft engines or miniature components for micro-electronic applications. Joining processes The basic joining processes may be subdivided into:    

mechanical joining; adhesive bonding; brazing and soldering; welding.

A large number of joining techniques are available and, in recent years, significant developments have taken place, particularly in the adhesive bonding and welding areas. Existing welding processes have been improved and new methods of joining have been introduced. The proliferation of techniques which have resulted makes process selection difficult and may limit their effective exploitation. The aim of this book is to provide an objective assessment of the most recent developments in welding process technology in an attempt to ensure that the most appropriate welding process is selected for a given application. This chapter will introduce some of the basic concepts which need to be considered and highlight some of the features of traditional welding methods. Classification of welding processes Several alternative definitions are used to describe a weld, for example: A union between two pieces of metal rendered plastic or liquid by heat or pressure or both. A filler metal with a melting temperature of the same order as that of the parent metal may or may not be used, or alternatively:

A localized coalescence of metals or non-metals produced either by heating the materials to the welding temperature, with or without the application of pressure, or by the application of pressure alone, with or without the use of a filler metal.

Many different processes have been developed, but for simplicity these may be classified in two groups; namely ‘fusion’ and ‘pressure’ welding as shown in Fig., which summarises some of the key processes. Conventional welding processes

A brief description of the most common processes, their applications and limitations is given below. The more advanced processes and their developments are dealt with in more detail in the remaining chapters. An international standard ISO 4063 identifies processes by a numeric code. The first digit of this code specifies the main process grouping whilst the second and third digit indicate sub-groups. The main groups and some

Some important welding processes Resistance welding The resistance welding processes are commonly classified as pressure welding processes although they involve fusion at the interface of the material being joined. Resistance spot , seam and projection welding rely on a similar mechanism. The material to be joined is clamped between two electrodes and a high current is applied (Fig.). Resistance heating at the contact surfaces causes local melting and fusion. High currents (typically10000 A) are applied for short durations and pressure is applied to the electrodes before the application of current and for a short time after the current has ceased to flow.

Resistance welding system.

Accurate control of current amplitude, pressure and weld cycle time are required to ensure that consistent weld quality is achieved, but some variation may occur due to changes in the contact resistance of the material, electrode wear, magnetic losses or shunting of the current through previously formed spots. These ‘unpredictable’ variations in process performance have led to the practice of increasing the number of welds from the design requirement to give some measure of protection against poor individual weld quality. To improve this situation significant developments have been made in resistance monitoring and control.

Features of the basic resistance welding process include:   

the process requires relatively simple equipment; it is easily and normally automated; once the welding parameters are established it should be possible to producerepeatable welds for relatively long production runs.

The major applications of the process have been in the joining of sheet steel in the automotive and white-goods manufacturing industries. Cold pressure welding If sufficient pressure is applied to the cleaned mating surfaces to cause substantial plastic deformation, the surface layers of the material are disrupted, metallic bonds form across the interface and a cold pressure weld is formed.

The main characteristics of cold pressure welding are:   

the simplicity and low cost of the equipment; the avoidance of thermal damage to the material; it is most suitable for low-strength (soft) materials.

Schematic illustration of cold pressure welding

The pressure and deformation may be applied by rolling, indentation, butt welding, drawing or shear welding techniques. In general, the more ductile materials are more easily welded.

This process has been used for electrical connections between smalldiameter copper and aluminium conductors using butt and indentation techniques. Roll bonding is used to produce bimetallic sheets such as Cu/Al for cooking utensils, Al/Zn for printing plates and precious-metal contact springs for electrical applications. Friction welding In friction welding, a high temperature is developed at the joint by the relative motion of the contact surfaces. When the surfaces are softened, a forging pressure is applied and the relative motion is stopped (Fig.). Material is extruded from the joint to form an upset.

The process may be divided into several operating modes in terms of the means of supplying the energy: (1) Continuous drive: in which the relative motion is generated by direct coupling to the energy source. The drive maintains a constant speed during the heating phase. (2) Stored energy: in which the relative motion is supplied by a flywheel which is disconnected from the drive during the heating phase. Rotational motion is the most commonly used, mainly for round components where angular alignment of the two parts is not critical (Fig.). If it is required to achieve a fixed relationship between the mating parts, angular oscillation may be used and for non-circular components the linear and orbital techniques may be employed. Features of the process include:     

one-shot process for butt welding sections; suitable for dissimilar metals; short cycle time; most suited to circular sections; robust and costly equipment may be required.

The process is commonly applied to circular sections, particularly in steel, but it may also be applied to dissimilar metal joints such as aluminium to steel or even ceramic materials to metals. Early applications of the process

Friction welding. Stage 1: A fixed, B rotated and moved into contact with A. Stage 2: A fixed, B rotated under pressure, interface heating. Stage 3: A fixed, forge pressure applied. Stage 4: relative motion stopped, weld formed.

Friction welding variants: (a) normal rotational motion, (b) linear oscillation, (c) angular oscillation.

included the welding of automotive stub axles, but the process has also been applied to the fabrication of high-quality aero-engine parts, duplex stainless teel pipe for offshore applications [6] and nuclear components. Recent developments of the process include the joining of metal to ceramics, the use of the process for stud welding in normal ambient conditions and underwater, and the use of the process for surfacing. The linear technique has recently been successfully demonstrated on titanium alloy welds having a weld area of 250 mm2 using an oscillation frequency of 25 kHz, 110 N mm–2 axial force and an oscillation amplitude of ± 2 mm.

Examples of numbering system from ISO 4063 Explosive welding In explosive welding, the force required to deform the interface is generated by an explosive charge. In the most common application of the process, two flat plates are joined to form a bimetallic structure. An explosive charge is used to force the upper or ‘flier’ plate on to the baseplate in such a way that a wave of plastic material at the interface is extruded forward as the plates join (Fig.). For large workpieces, considerable force is involved and care is required to ensure the safe operation of the process. Features of the process include:

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it is a one-shot process; it offers a short welding time; it is suitable for joining large surface areas; it is suitable for dissimilar thickness and metals joining; careful preparation is required for large workpieces; safety is an issue.

The process may also be applied for welding heat exchanger tubes to tube plates or for plugging redundant or damaged tubes.

Explosive welding.

Applications       

Joining of pipes and tubes. Major areas of the use of this method are heat exchanger tube sheets and pressure vessels. Tube Plugging. Remote joining in hazardous environments. Joining of dissimilar metals - Aluminium to steel, Titanium alloys to Cr – Ni steel, Cu to stainless steel, Tungsten to Steel, etc. Attaching cooling fins. Other applications are in chemical process vessels, ship building industry, cryogenic industry, etc.

Advantages       

Can bond many dissimilar, normally unweldable metals. Minimum fixturing/jigs. Simplicity of the process. Extremely large surfaces can be bonded. Wide range of thicknesses can be explosively clad together. No effect on parent properties. Small quantity of explosive used.

Limitations      

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The metals must have high enough impact resistance, and ductility. Noise and blast can require operator protection, vacuum chambers, buried in sand/water. The use of explosives in industrial areas will be restricted by the noise and ground vibrations caused by the explosion. The geometries welded must be simple – flat, cylindrical, conical. Introduction The fact that electric arc could operate was known for over a 100 years. The first ever underwater welding was carried out by British Admiralty – Dockyard for sealing leaking ship rivets below the water line. Underwater welding is an important tool for underwater fabrication works. In 1946, special waterproof electrodes were developed in Holland by ‘Van der Willingen’. In recent years the number of offshore structures including oil drilling rigs, pipelines, platforms are being installed significantly. Some of these structures will experience failures of its elements during normal usage and during unpredicted occurrences like storms, collisions. Any repair method will require the use of underwater welding Classification Underwater welding can be classified as 1) Wet Welding

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2) Dry Welding In wet welding the welding is performed underwater, directly exposed to the wet environment. In dry welding, a dry chamber is created near the area to be welded and the welder does the job by staying inside the chamber. Wet Welding Wet Welding indicates that welding is performed underwater, directly exposed to the wet environment. A special electrode is used and welding is carried out manually just as one does in open air welding. The increased freedom of movement makes wet welding the most effective, efficient and economical method. Welding power supply is located on the surface with connection to the diver/welder via cables and hoses. In wet welding MMA (manual metal arc welding) is used. Power Supply used : DC Polarity : -ve polarity When DC is used with +ve polarity, electrolysis will take place and cause rapid deterioration of any metallic components in the electrode holder. For wet welding AC is not used on account of electrical safety and difficulty in maintaining an arc underwater.

The power source should be a direct current machine rated at 300 or 400 amperes. Motor generator welding machines are most often used for underwater welding in the wet. The welding machine frame must be grounded to the ship. The welding circuit must include a positive type of switch, usually a knife switch operated on the surface and commanded by the welder-diver. The knife switch in the electrode circuit must be capable of breaking the full welding current and is used for safety reasons. The welding power should be connected to the electrode holder only during welding. Direct current with electrode negative (straight polarity) is used. Special welding electrode holders with extra insulation against the water are used. The underwater welding electrode holder utilizes a twist type head for gripping the electrode. It accommodates two sizes of electrodes. The electrode types used conform to AWS E6013 classification. The electrodes must be waterproofed. All connections must be thoroughly insulated so that the water cannot come in contact with the metal parts. If the insulation does leak, seawater will come in contact with the metal conductor and part of the current

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will leak away and will not be available at the arc. In addition, there will be rapid deterioration of the copper cable at the point of the leak. Principle of operation of Wet Welding The process of underwater wet welding takes in the following manner: The work to be welded is connected to one side of an electric circuit, and a metal electrode to the other side. These two parts of the circuit are brought together, and then separated slightly. The electric current jumps the gap and causes a sustained spark (arc), which melts the bare metal, forming a weld pool. At the same time, the tip of electrode melts, and metal droplets are projected into the weld pool. During this operation, the flux covering the electrode melts to provide a shielding gas, which is used to stabilize the arc column and shield the transfer metal. The arc burns in a cavity formed inside the flux covering, which is designed to burn slower than the metal barrel of the electrode. Developments in Under Water Welding Wet welding has been used as an underwater welding technique for a long time and is still being used. With recent acceleration in the construction of offshore structures underwater welding has assumed increased importance. This has led to the development of alternative welding methods like friction welding, explosive welding, and stud welding. Sufficient literature is not available of these processes. Scope for further developments Wet MMA is still being used for underwater repairs, but the quality of wet welds is poor and are prone to hydrogen cracking. Dry Hyperbaric welds are better in quality than wet welds. Present trend is towards automation. THOR – 1 (TIG Hyperbaric Orbital Robot) is developed where diver performs pipefitting, installs the trac and orbital head on the pipe and the rest process is automated. Developments of diverless Hyperbaric welding system is an even greater challenge calling for annexe developments like pipe preparation and aligning, automatic electrode and wire reel changing functions, using a robot arm installed. This is in testing stage in deep waters. Explosive and friction welding are also to be tested in deep waters. Hyperbaric Welding (dry welding) Hyperbaric welding is carried out in chamber sealed around the structure o be welded. The chamber is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure. The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place. This method produces high-quality weld joints that meet Xray and code requirements. The gas tungsten arc welding process is employed for this process. The area under the floor of the Habitat is open to water. Thus the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat. Risks Involved There is a risk to the welder/diver of electric shock. Precautions include achieving adequate electrical insulation of the welding equipment, shutting off the electricity supply immediately the arc is extinguished, and limiting the open-circuit voltage of MMA (SMA) welding sets. Secondly, hydrogen and oxygen are produced by the arc in wet welding. Precautions must be taken to avoid the build-up of pockets of gas, which are potentially explosive. The other main area of risk is to the life or health of the welder/diver from nitrogen introduced into the blood steam during exposure to air at increased pressure.

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Precautions include the provision of an emergency air or gas supply, stand-by divers, and decompression chambers to avoid nitrogen narcosis following rapid surfacing after saturation diving. For the structures being welded by wet underwater welding, inspection following welding may be more difficult than for welds deposited in air. Assuring the integrity of such underwater welds may be more difficult, and there is a risk that defects may remain undetected. Advantages of Dry Welding 1) Welder/Diver Safety – Welding is performed in a chamber, immune to ocean currents and marine animals. The warm, dry habitat is well illuminated and has its own environmental control system (ECS). 2) Good Quality Welds – This method has ability to produce welds of quality comparable to open air welds because water is no longer present to quench the weld and H2 level is much lower than wet welds. 3) Surface Monitoring – Joint preparation, pipe alignment, NDT inspection, etc. are monitored visually. 4) Non-Destructive Testing (NDT) – NDT is also facilitated by the dry habitat environment. Disadvantages of Dry Welding 1) The habitat welding requires large quantities of complex equipment and much support equipment on the surface. The chamber is extremely complex. 2) Cost of habitat welding is extremely high and increases with depth. Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required. The process is costly – a $ 80000 charge for a single weld job. One cannot use the same chamber for another job, if it is a different one. Advantages of Wet Welding Wet underwater MMA welding has now been widely used for many years in the repair of offshore platforms. The benefits of wet welding are: 1) The versatility and low cost of wet welding makes this method highly desirable. 2) Other benefits include the speed. With which the operation is carried out. 3) It is less costly compared to dry welding. 4) The welder can reach portions of offshore structures that could not be welded using other methods. 5) No enclosures are needed and no time is lost building. Readily available standard welding machine and equipments are used. The equipment needed for mobilization of a wet welded job is minimal. Disadvantages of Wet Welding Although wet welding is widely used for underwater fabrication works, it suffers from the following drawbacks: 1) There is rapid quenching of the weld metal by the surrounding water. Although quenching increases the tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness. 2) Hydrogen Embrittlement – Large amount of hydrogen is present in the weld region, resulting from the dissociation of the water vapour in the arc region. The H2 dissolves in

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the Heat Affected Zone (HAZ) and the weld metal, which causes Embrittlement, cracks and microscopic fissures. Cracks can grow and may result in catastrophic failure of the structure. 3) Another disadvantage is poor visibility. The welder some times is not able to weld properly. Introduction The plasma welding process was introduced to the welding industry in 1964 as a method of bringing better control to the arc welding process in lower current ranges. Today, plasma retains the original advantages it brought to industry by providing an advanced level of control and accuracy to produce high quality welds in miniature or precision applications and to provide long electrode life for high production requirements. The plasma process is equally suited to manual and automatic applications. It has been used in a variety of operations ranging from high volume welding of strip metal, to precision welding of surgical instruments, to automatic repair of jet engine blades, to the manual welding of kitchen equipment for the food and dairy industry. Plasma arc welding (PAW) Plasma arc welding (PAW) is a process of joining of metals, produced by heating with a constricted arc between an electrode and the work piece (transfer arc) or the electrode and the constricting nozzle (non transfer arc). Shielding is obtained from the hot ionized gas issuing from the orifice, which may be supplemented by an auxiliary source of shielding gas.

Transferred arcprocessproduces plasma jet of high energy density and may be used for high speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys. Non-transferred arcprocessproduces plasma of relatively low energy density. It is used for welding of various metals and for plasmaspraying(coating). Equipment: (1) Power source:- A constant current drooping characteristic power source supplying the dc welding current is required. It should have an open circuit voltage of 80 volts and have a duty cycle of 60 percent.

(2) Welding torch:- The welding torch for plasma arc welding is similar in appearance to a gas tungsten arc torch but it is more complex. (a) All plasma torches are water cooled, even the lowest-current range torch. This is because the arc is contained inside a chamber in the torch where it generates considerable heat.During thenon transferred period, the arc will be struck between the nozzle or tip with the orifice and the tungsten electrode.

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(b) The torch utilizes the 2 percent thoriated tungsten electrode similar to that used for gas tungsten welding. (3) Control console:- A control console is required for plasma arc welding. The plasma arc torches are designed to connect to the control console rather than the power source. The console includes a power source for the pilot arc, delay timing systems for transferring from the pilot arc to the transferred arc, and water and gas valves and separate flow meters for the plasma gas and the shielding gas. The console is usually connected to the power source. The high-frequency generator is used to initiate the pilot arc. Principles of Operation The plasma arc welding process is normally compared to the gas tungsten arc process. But in the TIG-process, the arc is burning free and unchanneled, whereas in the plasmaarc system, the arc is necked by an additional water-cooled plasma-nozzle. Aplasma gas – almostalways 100 % argon –flows between thetungsten electrode andthe plasma nozzle. The welding process involves heating a gas called plasma to an extremely high temperature and then ionizing it such that it becomes electrically conductive. The plasma is used to transfer an electric arc called pilot arc to a work piece which burns between thetungsten electrode and the plasma nozzle. By forcing the plasma gas and arc through a constricted orificethe metal, which is to be welded is melted by the extreme heat of the arc. The weld pool is protected by the shielding gas, flowing between the outershielding gas nozzle and the plasma nozzle. As shielding gas pure argon-rich gas-mixtures with hydrogen or helium are used.

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The high temperature of the plasma or constricted arc and the high velocity plasma jet provide an increased heat transfer rate over gas tungsten arc weldingwhen using thesame current. This results in faster welding speeds and deeper weld penetration. This method of operation is used for welding extremely thin material and for welding multi pass groove and welds and fillet welds. Uses & Applications Plasma arc welding machine is used for several purposes and in various fields. The common application areas of the machine are: 1. Single runs autogenous and multi-run circumferential pipe welding. 2. In tube mill applications. 3. Welding cryogenic, aerospace and high temperature corrosion resistant alloys. 4. Nuclear submarine pipe system (non-nuclear sections, sub assemblies). 5. Welding steel rocket motor cases. 6. Welding of stainless steel tubes (thickness 2.6 to 6.3 mm). 7. Welding of carbon steel, stainless steel, nickel, copper, brass, monel, inconel, aluminium, titanium, etc. 8. Welding titanium plates up to 8 mm thickness. 9. Welding nickel and high nickel alloys. 10. or melting, high melting point metals. 11. Plasma torch can be applied to spraying, welding and cutting of difficult to cut metals and alloys.

Plasma Arc Machining (PAM) Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to melt and displace material in its path called PAM, this is a method of cutting metal with a plasma-arc, or tungsten inert-gas-arc, torch. The torch produces a high velocity jet of high-temperature ionized gas called plasma that cuts by melting and removing material from the work piece. Temperatures in the plasma zone range from 20,000° to 50,000° F (11,000° to 28,000° C).

It is used as an alternative to oxyfuel-gas cutting, employing an electric arc at very high temperatures to melt and vaporize the metal. Equipment: A plasma arc cutting torch has four components:    

The electrode carries the negative charge from the power supply. The swirl ring spins the plasma gas to create a swirling flow pattern. The nozzle constricts the gas flow and increases the arc energy density. The shield channels the flow of shielding gas and protects the nozzle from metal spatter.

Principle of operation PAM is a thermal cutting process that uses a constricted jet of high-temperature plasma gas to melt and separate metal. The plasma arc is formed between a negatively charged electrode inside the torch and a positively charged work piece. Heat from the transferred arc rapidly melts the metal, and the high-velocity gas jet expels the molten material from the cut. Applications The materials cut by PAM are generally those that are difficult to cut by any other means, such as stainless steels and aluminum alloys. It has an accuracy of about 0.008".

Plasma Arc Cutting Plasma arc cutting employs an extremely high-temperature, high-velocity, constricted arc between an electrode contained within the torch and the piece to be cut. The arc is concentrated by a nozzle onto a small area of the workpiece. The metal is continuously melted by the intense heat of the arc and then removed by the jetlike gas stream issuing from the torch nozzle. Because plasma arc cutting does not depend on a chemical reaction between the gas and the work metal, because the process relies on heat generated from an arc between the torch electrode and the workpiece, and because it generates very high temperatures (28,000 °C, or 50,000 °F, compared to 3000 °C, or 5500 °F, for oxyfuel), the transferred arc cutting mode can be used on almost any material that conducts electricity, including those that are resistant to oxyfuel gas cutting. Using the nontransferred arc method, nonmetallic objects such as rubber, plastic, styrofoam, and wood can be cut with a good quality surface to within 0.50 to 0.75 mm (0.020 to 0.030 in.) tolerances. The past decade has seen a great increase in use of plasma arc cutting, because of its high cutting speed (Fig.). The process increases the productivity of cutting machines over oxyfuel gas cutting without increasing space or machinery requirements.

Operating Principles and Parameters The basic plasma arc cutting torch is similar in design to that of a plasma arc welding torch. For welding, a plasma gas jet of low velocity is used to melt base and filler metals together in the joint (see the article "Plasma Arc Welding" in Welding, Brazing, and Soldering, Volume 6 of the ASM Handbook). For the cutting of metals, increased gas flows create a high-velocity plasma gas jet that is used to melt the metal and blow it away to form a kerf. The basic design and terminology for a plasma arc cutting torch are shown in Fig.

Components of a plasma arc cutting torch. All plasma arc torches constrict the arc by passing it through an orifice as it travels away from the electrode and toward the workpiece. As the orifice gas passes through the arc, it is heated rapidly to high temperature, expands, and accelerates as it passes through the constricting orifice. The intensity and velocity of the arc plasma gas are determined by such variables as the type of orifice gas and its entrance pressure, constricting orifice shape and diameter, and the plasma energy density on the work. The basic plasma arc cutting circuitry is shown in Fig.. The process operates on direct current, straight polarity (dcsp), electrode negative, with a constricted transferred arc. In the transferred arc mode, an arc is struck between the electrode in the torch and the workpiece. The arc is initiated by a pilot arc between the electrode and the constricting nozzle. The nozzle is connected to ground (positive) through a current-limiting resistor and a pilot arc relay contact. The pilot arc is initiated by a high-frequency generator connected to the electrode and nozzle. The welding power supply then maintains this low current arc inside the torch. Ionized orifice gas from the pilot arc is blown through the constricting nozzle orifice. This forms a low-resistance path to ignite the main arc between the electrode and the workpiece. When the main arc ignites, the pilot arc relay may be opened automatically to avoid unnecessary heating of the constricting nozzle.

Plasma arc cutting was originally developed for severing nonferrous metals using inert gases. Modifications of the process and equipment to allow the use of oxygen or compressed air in the orifice gas permitted the cutting of carbon and alloy steel with improved cutting speeds and a cut quality similar to that obtained with oxyfuel cutting. Because the plasma constricting nozzle is exposed to the high plasma flame temperatures (estimated at 10,000 to 14,000 °C, or 18,000 to 25,000 °F), the nozzle is sometimes made of water-cooled copper. In addition, the torch should be designed to produce a boundary layer of gas between the plasma and the nozzle. Several process variations are used to improve the plasma arc cutting quality for particular applications. They are generally applicable to materials in the 3 to 38 mm ( 1/8 to1(1/2) in.) thickness range, depending on the current rating of the plasma machine. Auxiliary shielding in the form of gas or water is used to improve cutting quality.

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