Welding Concepts, about Welding, Solidworks Design.



	WELDING CONCEPTS Soldering employs lead- or tin-base alloys with melting points below 800 degrees F and is commonly referred to as soft soldering. Use of hard solders, silver solders and spelter solders which have silver, copper, or nickel bases and have melting points above 800degrees F is known as brazing. Soldering is used to provide a convenient joint that does not require any great mechanical strength. It is used in a great many instances in combination with mechanical staking, crimping or folding, the solder being used only to seal against leakage or to assure electrical contact. The accompanying table gives some of the proper-ties and uses of various solders that are generally available. Brazing is a metal joining process which uses a non-ferrous filler metal with a melting point below that of the base metals but above 800 degrees F. The filler metal wets the base metal when molten in a manner similar to that of a solder and its base metal. There is as light diffusion of the filler metal into the hot, solid base metal or a surface alloying of the base and filler metal. The molten metal flows between the close-fitting metals because of capillary forces. Welding of metals requires that they be heated to a molten state so that they fuse together. A filler wire or rod is held in the heated zone to add material that will replace metal consumed by the process and to produce a slightly raised area that can be dressed down to make a level surface if needed. Most welding operations today use an electric arc, though the autogenous method using a torch that burns a mixture of (usually) acetylene and oxy-gen gases to heat the components is still used for certain work. Lasers are also used as the heating medium for some welding operations. In arc welding, a low-voltage, high-current arc is struck between the end of an electrode in a holder and the work, generating intense heat that immediately melts tile surface. Gas Metal Arc Welding (GMAW) the two most cost-effective manual arc welding processes are GMAW and FCAW. These two welding processes are used with more than 50 per cent of the arc welding consumable electrodes purchased. Gas metal arc welding modes extend from short circuit welding, where the consumable electrode wire is melted into the molten pool in a rapid succession of short circuits during which the arc is extinguished, to pulsed and regulars pray transfer, where a stream of fine drops and vaporized weld metal is propelled across the continuous arc gap by electromagnetic forces in the arc. Flux-Cored Arc Welding FCAW welding offers unique benefits for specific applications, but flux-cored consumable electrodes cost more than the solid electrodes used in gas metal arc welding, so users need to be aware of FCAW benefits and disadvantages compared with those of GMAW welding. Generally, flux-cored electrodes designed for use without a shielding gas are intended for welding outdoors. Most indoor FCAW welding is done with gas-shielded FCAW welding electrode wire. Shielded Metal Arc Welding With the shielded metal arc welding (SMAW) process, commonly known as stick welding, it is most important to select an electrode that is suited to the application. For welding austenitic stainless or high-alloy steels, the electrode is first selected to match the mechanical and chemical requirements of the metal to be welded. Secondary requirements such as the welding position, penetration potential, deposition capabilities, and ease of slag removal are then considered. Many electrodes for SMAW welding of low- to medium-car-bon steels have unique characteristics making them the most suitable and cost-effective for a specific welding application. Gas Tungsten Arc Welding Often called TIG (for tungsten inert gas) welding, gas tungsten arc welding (GTAW)uses a non consumable tungsten electrode with a gas shield, and was, until the development of plasma arc welding (PAW), the most versatile of all common manual welding processes. Plasma arc welding is a modified GTAW process. In contrast to GTAW, plasma arc welding has less sensitivity to arc length variations, superior low-current arc stability, greater potential tungsten life, and the capability for single-pass, full-penetration welds on thick sections. Plasma Arc Welding (PAW) When an electric current passes between two electrodes through certain gases, the energy of the gas molecules is increased so that they accelerate and collide with each other more often. With increases in energy, the binding forces between the nuclei and the electrons are exceeded, and electrons are released from the nuclei. The gas now consists of neutral molecules, positively charged atoms, and negatively charged electrons. The plasma gas is said to be ionized, so that it is capable of conducting electric current. Plasma forms in all welding arcs but in plasma arc welding it is generated by a series of events that begins with inert gas passing through the welding torch nozzle. High-frequency current is then generated between the tungsten electrode (cathode) and the torch nozzle (anode), forming a low current pilot arc. The ionized path of this non transferred arc is then transferred from the tungsten electrode to the work, and preset plasma current is generated. Plasma Arc Surface Coating Plasma Arc Surfacing uses an arc struck between the electrode and the work piece, or transferred arc, to apply coatings of other metals or alloys to the work piece surface. This high-temperature process produces homogeneous welds in which the ionized plasma gas stream melts both the work surface and a stream of powdered alloy or filler wire fed into the arc. Dilution of the base metal can be held below 5 per cent if required. With arc temperatures between 25,000 and 50,000°F (14,000 and 28,000°C), deposition occurs rapidly, and a rate of 15 lb/h (6.8 kg/h) of powdered alloy is not unusual. Deposition from wire can be performed at rates up to 28 lb/h (12.7 kg/h), much higher than with oxygen/fuel or gas metal/arc methods. Electron-Beam (EB) Welding Heat for melting of metals in electron-beam welding is obtained by generating electrons, concentrating them into a beam, and accelerating them to between 30 and 70 per cent of the speed of light, using voltages between 25 and 200 kV. The apparatus used is called an electron beam gun, and it is provided with electrical coils to focus and deflect the beam as needed for the welding operation. Energy input depends on the number of electrons impinging on the work in unit time, their velocity, the degree of concentration of the beam, and the traveling speed of the work piece being welded. Some 6.3 × 1015 electrons/s is generated in a 1-mA current stream. With beam diameters of 0.01 to 0.03 in. (0.25 to 0.76mm), beam power can reach 100 kW and power density can be as high as 107 W/in2 (1.55× 104 W/mm2), higher than most arc welding levels. Pipe Welding Pipe Welding Welding of (usually steel) pipe is commonly performed manually, with the pipe joint stationary, or held in a fixture whereby rotation can be used to keep the weld location in a fixed, down hand, position. Alternatively, pipe may need to be welded on site, without rotation, and the welder then has to exert considerable skill to produce a satisfactory, pressure-tight joint. Before welding stationary pipe, a welder must be proficient in welding in the four basic positions: 1G flat, 2G horizontal, 3G vertical and 4G overhead Nondestructive Testing Nondestructive testing (NDT) is aimed at examination of a component or assembly, usually for surface or internal cracks or other non homogeneities, to determine the structure, or to measure thickness, by some means that will not impair its use for the intended purpose. Traditional methods include use of radiography, ultrasonic vibration, dye penetrates, magnetic particles, acoustic emission, leakage, and eddy cur-rents. These methods are simple to use but some thought needs to be given to their application and to interpretation of the results. Space limitations preclude a full discussion of NDT here, but the nature of the welding process makes these methods particularly useful, so some information on use of NDT Welding Terms and Acronyms OAW – Oxyacetylene Welding SMAW – Shielded Metal Arc Welding aka Stick Welding TMAW – Tungsten Metal Arc Welding aka TIG (Tungsten Inert Gas) or Heliarc® GMAW TERMS GMAW – Gas Metal Arc Welding, aka MIG (Metal Inert Gas) or Wire Feed Welding FCAW – Flux Cored Arc Welding AWS – American Welding Society cfh – Cubic Feet per Hour (when setting the flowmeter for GMAW and TMAW, you need a minimum of 20cfh) psi – Pounds per Square Inch IPM – Inches Per Minute (for setting the wire speed on a GMAW welder) Arc Force – The pressure of the welding process Arc Gap – The distance between the tip of the electrode and the metal being welded Penetration – The depth of the weld into the base metal Porosity – Impurities trapped within the weld, usually caused by not enough shielding gas or dirty base metal. Stickout – How far the wire “sticks-out” out past the contact tip. Usually no more than 3/8 an inch for GMAW, and 1-1/2 inch for FCAW. Travel Speed – The rate in which you weld. Travel Angle – 5 to 15 degrees Gun Angle – Perpendicular (90°) for a flat weld joint, or half the angle (45° for T-Fillet, etc.) Shielding Gasses – Commonly used for GMAW is 75% Argon mixed with 25% C0₂, or 100% CO₂. For TMAW 100% Ar is used. For Dual Shield FCAW, either a 75/25 mix or 100% CO₂ is used. Dual Shield – A welding process that uses Flux Cored wire as well as shielding gas. Forehand – Pushing your welding puddle, giving a shallower penetrating weld. Backhand – Pulling your welding puddle, giving a deeper penetrating weld. Inert – A group of gasses that are non-reactive to other elements. These include Helium and Argon (Ar is really a Noble gas). Duty Cycle – The percentage of welding time that can be done in relation to the percentage of time the welder must rest. A 60% duty cycle welder can weld for 6 minutes, but then it must rest for 4 minutes. Be able to identify GMAW Welding Parts, Equipment, and Accessories Contact Tip Nozzle Diffuser Insulator Liner Gun Flowmeter Cylinder Drive Wheels Knurled Drive Wheels Hub Tension Nut Wire Spool Wire Spool Retainer Anti Spatter Spray Nozzle Dip Drive Wheel Tension Knob Guide to MIG Welding I have taken the liberty of writing a few pointers for anyone who is interested and please do appreciate that every welder has a slightly different style and will use slightly different settings etc, but don't get disorientated by differing advice, as long as you keep the basics right 'eventually' you will achieve a result: If possible get someone who is an experienced MIG welder to come and set preliminary settings on your machine - it is quite surprising how much of your welding will be done without varying too far from the basic settings. At the same time get this welder to give you an hour’s tuition and set up some basic practice welds - this will give you a huge head start. PRACTICE ON OFFCUTS FIRST! As a rule of thumb if a weld runs looks good it generally is mechanically good. Clean the parent metal to be welded first (sorry if this is juvenile but some people think that you can weld any crap and it will stick). If you are on your own and don't have anyone to help you set it up: Without the gas on squeeze the trigger and make sure the wire feeds smoothly out of the gun. If not clean the inner cable (wash it and lubricate it with kerosene) and drive rollers and reset the tension on the drive rollers. With the tension on you should still be able to hand turn the cable spool backwards - pulling the weld wire back through the gun - but there should be firm resistance - if your 12 year old daughter can't do it it's too tight. Check / clean the contact tip. If the contact tip is too small the weld wire will bind in the tip. If the tip is too big there will be an intermittent arc and the wire will wander in the weld pool. Set the wire feed and voltage to about 50%. If you have a larger machine than a hobby welder and it has more than two welding controls - some have four or more - (other than the wire speed and voltage) turn them all to the off position. Set the nozzle (gas shroud) level or just past the end of the contact tip by approx' 1 to 2mm (you can push the contact tip out past the end of the nozzle to get into tight corners, but as a rule they should be flush or just under flush). Feed 10 to 12mm of wire out of the contact tip (this should remain constant before, during & after welding i.e. 12mm is the gap to be maintained from the tip of the gun to the weld piece) when you have triggered off and ceased welding look at the end of the gun you should have 12mm of wire sticking out. This is called 'The Stick Out'! (Sounds like a perfect lead into a thousand one liners, but we won't go there). Set the gas flow to about 14 to 18 liters / min. Too little gas = atmospheric contamination too much gas = turbulence = a real waste of money and a shitty weld! Set up two flat plates (2 to 3mm thick) in a horizontal (flat) position with a small gap (approx' 0.5 to 1.0mm) between them and tack each end of the gap. Hold the (hand piece) gun vertically above the join to be welded with about 12mm from the contact tip to the plates (use the stick out to touch the plates before squeezing the trigger and this will set the gap to 10 to 12mm) and then lean the gun back to about 65 to 70 deg. Squeeze the trigger and start the weld run, remembering that you 'push' the gun in the direction (it is pointing) of the weld run not 'drag' it away from the weld. This is the opposite of ARC (stick) welding. If the wire 'stubs' (you will hear this as well as feel and see it) into the plate, increase the voltage one increment. If it still stubs turn up the wire speed one increment. And so on and so forth. Whilst doing this check your stick out (distance contact tip is away from the material) and the gun angle. This is very like driving a car - after a while (and loads of practice) you stop thinking about your hands and feet on the controls and start watching the road. With the welding you will begin to hold the gun at the correct angle and gap and watch the weld pool and listen to the sound of the weld arc. As close as I can describe; the sound of the weld arc is a cross between ripping silk and sizzling bacon. If you burn through the plate increase the stick out (pull the gun further away from the material) and increase the speed of your travel along the join to be welded. If it still burns through try backing the settings off one increment at a time. A point to note here is that the wire speed is also your amperage control so this is the major control over the amount of power or heat that you are putting into the material; the voltage control is basically a fine adjustment. So don't alter the wire speed settings too much at one go otherwise you will simply waste time chasing settings up and down the range of adjustment and you will never lay down any weld. If you get lots of spatter and large pin holes in the weld then the gas flow is incorrect or nonexistent. If your weld run is high and round and sits up on top of the material either: your weld current / voltage is too low. And / or, your travel speed is too fast. If your weld run is wide or undercut or both: your wire feed / voltage is too high. If your weld run is wide but is 0.5 to 1.5mm proud of the material to be joined and has a lot of spatter your wire speed / voltage is too high. A good MIG weld (as a rule of thumb for thin plate) is 1.5 to 2x as wide as the material is thick (i.e. 5 to 6mm on three millimeter plate - this does not apply to thicker plate sections and 'veed’ or beveled edge prepared weld joins) and will be 0.5 to 1.5mm proud of the material surface. The weld bead will have a clean smooth finish with the characteristic semi circular pattern lightly along its surface. Turn the plate over and there should be 0.5mm of penetration along the back of the weld join with a clear even zone of discoloration along either side of the join. After every run - check inside the nozzle for spatter and clean as necessary / clip off wire stick out to 12mm. Do not try to complete a run longer than 100mm generally or 150mm occasionally (4 to 6" for the Yanks!) without stopping and altering your posture. If your posture is not comfortable don't start welding - you will just produce a shitty weld if you do and then you have got to grind the bugger out. I would not advise beginners to use Co2. It burns hot, gives more penetration and produces lots of spatter. Great if you are welding oil tankers or earth moving machinery, but not much good for light gauge work. Use a general purpose 'MIG Mix' i.e. argon with about 10% oxygen and Co2 thrown in for good measure. There will be different brand names depending on your supplier. Disclaimer: This is not a welding lesson. It is a compilation of my observations of some of the difficulties that I and others I have observed have experienced. The only opinion expressed here that is mine is; that welding is not a simple and easy skill to learn and that if you have any doubts whatsoever about your ability get your parts welded professionally!