Реферат выпускная квалификационная работа 114 с., 15 рис., 23 табл., 50 источников, 1 прил
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Maintaining Equipment Thankfully, GMAW guns and consumables don’t require a lot of time consuming maintenance and upkeep. Nevertheless, failing to spend a small amount of time maintaining your equipment could result in spending a significant amount of time reworking bad welds.[50] 78 The majority of gun and consumables maintenance simply involves checking the visible components of the equipment for problems. This includes for looking loose fittings, damaged cables, clogged diffuser ports and the like. Below is a component-by-component guide to minimize downtime for reworking bad welds. Feeder Connection — The feeder connection, which carries the electrical current and gas from the wire feeder to the gun, should be tight fitting and free of excessive dirt and debris. The O-rings that ensure the shielding gas flows into the gun cable and nowhere else, should be in good working order, ie: not dry, cracked or otherwise damaged. If the feeder connection is loose and cannot be properly tightened, it will likely need to be replaced. The same goes for damaged O-rings. A dirty direct plug usually can be cleaned with an electrical contact cleaner. Cable — Cable maintenance involves little more than inspecting it on a daily basis to ensure there are no cuts, kinks or other damage that could interfere with weld quality and also cause a safety hazard. Avoid problems such as porosity, an erratic arc and damage to the copper cable stranding by keeping the cable from bending at too severe of an angle. Liner — Accessing the liner can be very time consuming, so you should limit routine maintenance activity to periods when the liner is easily reached, such as during wire changeovers or when the gun is disconnected from the feeder. You can clear out any built up debris, including metal filings from the welding wire, by using compressed air during these changeover times. Handle and Trigger —Daily visual inspection should be conducted to ensure there are no missing screws or other damage to the handle and that the trigger is not malfunctioning. These items should be replaced as necessary if they are found to be damaged. Neck — The neck connections, and the insulators that separate electrically live components from neutral components, should be checked on a regular basis as both a safety and weld quality measure. 79 Loose neck connections should be tightened or, if damaged, replaced. You should also check that the insulators are in place at either end of the neck and that they are undamaged. Proper GMAW gun maintenance and troubleshooting are essential to maintaining productivity and avoiding unnecessary downtime[50] Consumables — Consisting of the diffuser, nozzle and contact tip, the consumables require regular replacement simply by virtue of their role in the welding process and proximity to the arc. Extending the life of the consumables is relatively easy, however, and you can save a significant amount of downtime and equipment costs through some simple maintenance steps. Multiple times daily, use a welding pliers or reamer to clear out any spatter or other debris that could clog the nozzle and diffuser, being careful not to damage these parts in the process. 80 Also, you should check the O-rings on the diffuser, the connections between the diffuser, neck and contact tip, the nozzle insulator and the contact tip on a daily basis. Loose connections can usually be tightened, but you should replace these components if any other types of damage appear. Troubleshooting Of course, no amount of preventive maintenance will be able to stop every problem from occurring. So, when a problem does arise, it’s important to be able to identify and correct its cause. Often, the same problem, such as erratic wire feeding, can have more than one cause. In these cases, it’s usually a good idea to conduct the troubleshooting effort by working from the easiest component to check to the most difficult. For example, both the liner and the contact tip can be the source of erratic wire feeding. The liner takes approximately 20 times longer that the contact tip to check, so it makes sense to begin with the contact tip and only check the liner if necessary. Below are a few of the most common problems that occur as a result of gun and consumables malfunction. Wire does not feed — If your wire is not feeding at all, it is most likely being caused by a faulty feeder relay, control lead, adapter connection, liner or trigger switch. If the drive rolls are not turning when the gun trigger is pulled, it is either because an electrical continuity failure is occurring at the gun connection or the trigger is not functioning properly. Repair or replace any of these items discovered to be the cause of the problem. If the drive rolls turn, but the wire is not feeding, there may be inadequate drive roll pressure or a blockage in the contact tip or liner. As mentioned earlier, check the contact tip and drive rolls before proceeding to the liner. Consult the manufacturer of your wire feeder if the feeder relay turns out to be the cause of the problem. 81 Contact tip burn back — Contact tip burnback, when the wire fuses with the contact tip, occurs occasionally as a normal part of welding. If you are noticing an increase in burnback frequency, it could be a result of using the wrong contact tip recess, holding the gun too close to the workpiece or a faulty work lead. If you have not changed your welding parameters, shielding gas and base metal, then it’s unlikely the contact tip recess is the cause of the problem. Additionally, if those variables are the same and you are confident you are not welding any closer to the material than normal, it may be time to consider the work lead as the cause of the burnback. Repair or replace a faulty work lead as necessary. A final cause of increased burnback, erratic wire feeding, is discussed below. Erratic wire feeding — If the wire is not feeding from the gun at a consistent rate, it is most likely being caused by the liner, drive rolls or contact tip. Begin troubleshooting an erratically feeding wire by ensuring the contact tip is the correct size for the wire being used, and that it is not damaged from excessive wear by the wire or from heat exposure from the arc. If the contact tip is worn out from excessive wear, it could be a result of the drive rolls causing small deformities in the wire. After replacing the contact tip, be sure to check for burrs or other abnormalities along the length of the wire and adjust or replace the drive rolls as necessary. Drive rolls that are improperly tensioned, either too tight or too loose, can also lead to erratic wire feeding. 82 As seen, GMAW gun consumables are exposed to a lot of abuse during the normal course of welding. Regularly maintaining these products can extend their life and increase a company's productivity[50] Erratic arc — Interruptions in electrical conductivity are often the primary cause of an erratic arc. These are commonly caused by the wire maintaining only intermittent contact with a worn out contact tip instead of the constant contact required for a consistent arc. Simply replace the worn out contact tip with a correctly sized new one if this proves to be the case. Other possible causes of an erratic arc, all of which relate to inconsistent electrical conductivity, are a neck that is too straight, a worn or kinked liner, debris built up inside the liner, an improperly trimmed liner and a faulty work lead connection. Porosity — Holes in the weld bead, called porosity, are almost always caused by problems with the shielding gas coverage. This can be caused by excessive wind blowing the shielding gas away, worn out or damaged diffusers, insulators, o-rings and fittings, a ruptured gas hose, too much or too little gas flow or a faulty solenoid. 83 If porosity occurs without any changes to your work environment and equipment set-up, troubleshoot the problem by checking all of the above mentioned components and replacing as necessary. Good GMAW welds are not a product of luck, and poor welds can usually be attributed to operator technique, equipment malfunction or incorrect electrical parameters. Following these maintenance and troubleshooting tips won’t ensure excellent GMAW welds, but it will guarantee that your gun and consumables are not the cause of any problems that arise. Conclusion MIG advantages 1. High productivity, because you don't have to stop to change rods or chip and brush the weld frequently. 2. Easy to learn and makes great-looking welds. 3. Almost no cleanup. 4. Can weld on stainless, mild steel, and aluminum. 5. Can weld in all positions. MIG disadvantages 1. Requires a cumbersome bottle of shielding gas. 2. Costs money for consumables, such as tips and nozzles. 3. Isn't worth a dang on paint, rust, or dirty surfaces. 4. Not good for thick steel, because it doesn't get the proper penetration. The applications of shielding gases are limited primarily by the cost of the gas, the cost of the equipment, and by the location of the welding. Some shielding gases, like argon, are expensive, limiting its use. The equipment used for the delivery of the gas is also an added cost, and as a result, processes like shielded metal arc welding, which require less expensive equipment, might be preferred in certain situations. Finally, because atmospheric movements can cause the dispersion of the shielding gas around the weld, welding processes that require shielding gases are often only 84 done indoors, where the environment is stable and atmospheric gases can be effectively prevented from entering the weld area.[49] The desirable rate of gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode being utilized. Welding flat surfaces requires higher flow than welding grooved materials, since the gas is dispersed more quickly. Faster welding speeds, in general, mean that more gas needs to be supplied to provide adequate coverage. Additionally, higher current requires greater flow, and generally, more helium is required to provide adequate coverage than argon. Perhaps most importantly, the four primary variations of GMAW have differing shielding gas flow requirements—for the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min (20 ft3/h) is generally suitable, while for globular transfer, around 15 L/min (30 ft3/h) is preferred. The spray transfer variation normally requires more because of its higher heat input and thus larger weld pool; along the lines of 20–25 L/min (40–50 ft3/h). [49] |