Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Shielded Metal Arc Welding (SMAW), or Manual Metal Arc (MMA) welding or Stick Welding Shielded metal arc welding is performed by striking an arc ( electronic current AC or DC) between a coated-metal electrode and the base metal. Once the arc has been established (the weld is laid), the molten metal from the tip of the electrode flows together with the molten metal from the edges of the base metal to form a sound joint. This process is known as fusion. The coating from the electrode forms a covering over the weld deposit, shielding it from contamination; therefore the process is called shielded metal arc welding.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Fact 1: The world's most popular welding process, Fact 2: The process is used primarily to weld iron and steels (including stainless steel) but aluminum, nickel and copper alloys can also be welded with this method. Fact 3: The main advantages of shielded metal arc welding are that high-quality welds are made rapidly at a low cost. Shielded Metal Arc Welding (SMAW), or Manual Metal Arc (MMA) welding or Stick Welding
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri SMAW system setup Stick welding equipment typically consists of a constant current welding power supply and an electrode, with an electrode holder, a work clamp, and welding cables (also known as welding leads) connecting the two.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Welding current When electrical current moves through a wire, heat is generated by the resistance of the wire to the flow of electricity. The greater the current flow, the greater the heat generated. The heat generated during the SMAW process comes from an arc that develops when electricity jumps across an air/gas gap between the end of an electrode and the base metal. The air/gas gap produces a high resistance to the flow of current. This resistance generates intense heat (6000 ◦ F to 10000 ◦ F or 3300 ◦ C to 5500 ◦ C).
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Welding current Welding current can be Direct Current (DC) or Alternating Current (AC). Direct current or Dc is an electrical current that flows in one direction only. There is no change in the direction of the current. www.pbs.org
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Welding current Alternate current or AC is an electrical current that has alternating positive and negative values. In the first (positive) half-cycle, the current flows in one direction; the current then reverses and for the second (negative) half-cycle flows in the opposite direction. www.pbs.org + -
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Transformer, Rectifier and Inverters A transformer is an electrical device that changes voltage from one level to another. It produces AC current. Transformer V1V1 V2V2 AC DC Rectifier AC A rectifier is an electrical device contained within a transformer welding machine that changes AC current into DC current. AC Inverter DC An inventer is an electrical device that changes DC current into AC current.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri AC and DC AC current: Allows a welder to easily maintain an arc during welding. Some features of AC current include low operating and maintenance costs, high overall electrical efficiency. AC welding machines typically operate on single-phase primary power. DC Current: DC output has usually polarity switches with both positive and negative terminals. DC current output for SMAW may use single-phase or 3-phase primary electrical power. The most stable DC welding is provided by welding machines that provide 3-phase transformers and full-wave rectifiers.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri AC/DC AC/DC welding machines are commonly used for SMAW. AC/DC current output is available on constant-current welding machines that operate using a single-phase or 3-phase primary power source. AC/DC output contains a rectifier (not available in AC ones). Rectified single-phase welding power is not as stable as rectified 3-phase DC welding power. A welding machine in which a steady supply of current is produced over a wide range of welding voltages caused by changes in arc length.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Welding Machine Output AC DC AC/DC
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri +- Polarity Polarity is the positive ( + ) or negative ( - ) state of an object. Polarity determines the direction of current flow in a DC circuit. Polarity is of no consequence in AC welding machines because current is constantly changing directions. Direct Current Electrode Positive (DCEP) Cables should be switched -+ Direct Current Electrode Negative (DCEN)
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Polarity Polarity determines the location of heat concentration in a welding circuit. With DCEN, more heat is located in the work piece. With DCEP, more heat is directed to the electrode. The type of welding to be performed and the electrode used determine the polarity. Electrodes are designed for use with a specific polarity.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Voltage Voltage: The force or pressure that causes current to flow in a circuit. (In a water system, a pump provides the pressure to make the water flow, whereas in an electrical circuit a power supply produces the force or voltage that pushes the current through the wires). Voltage is measured using a voltmeter. Voltage has the most effect on the height and width of the weld deposit.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Voltage Voltage drop is the voltage decrease across a component due to resistance to the flow of current. When there is too great a drop, the welding machine cannot supply enough current for welding. Open-circuit voltage is the voltage produced when the machine is ON and no welding is being done (50 V to 100V). Arc Voltage (Working voltage) is the voltage present after an arc is struck and maintained (18V to 36 V).
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Avoid electrical shock! Make certain that the electrode holder and all electrical connections and cables are properly insulated. Check to see that the welder is properly grounded. Do not dip the electrode holder in water to cool it because this practice may result in electrical shock. Never weld in damp locations because of the shock hazard. Never touch the electrical connection, bare wire, work, or a machine part which may cause electrical shock.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Safety Instructions Protect welding cables. Keep the cables from coming in contact with hot metal and sharp edges. When welding, avoid wrapping electrode cables around your body. Handle hot metal with pliers or tongs. Submerge hot metal completely in water to prevent steam burns. Do not allow electrode to stick. If the electrode sticks, cut off the switch, allow electrode to cool, and then break it loose with your gloved hand.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Safety Instructions Never look at a welding arc without wearing a welding helmet. Safety glasses should be worn under face shields, hoods, and helmets and at all times when working in the shop. Use both hands. To reduce fatigue, use both hands for welding.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Test yourself! Which one is Direct Current Electrode Positive (DCEP)? -+ +- This is DCEP, Because the electrode is connected to the positive terminal.
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Test yourself! Place the name of machines in the empty boxes. Choose from: Rectifier, Inverter, and Transformer. Transformer Rectifier Inverter V1V1 V2V2
Manufacturing Processes Lab 1 (MET 1321) Prof S. Nasseri Is this sentence true or false: Handle hot metal with pliers or tongs. Submerge the electrode completely in water to prevent steam burns. True False Test yourself! With DCEP, more heat is directed to the …. a)Electrode b)Work piece Hot metal