Transcript Chapter 11
Chapter 11 Gas Metal Arc Welding © 2012 Delmar, Cengage Learning Objectives • Demonstrate how to properly set up a GMA welding installation • Demonstrate how to thread the electrode wire on a GMAW machine • Demonstrate how to set the shielding gas flow rate on a GMAW machine • Use various settings on a GMA welding machine, and compare the effects on a weld • Show the effect of changing the electrode extension on a weld © 2012 Delmar, Cengage Learning Objectives (cont'd.) • Describe the effects of changing the welding gun angle on the weld bead • Tell what must be considered when selecting the right shielding gas for a particular application • Evaluate weld beads made with various shielding gas mixtures • Tell why hot-rolled steel should be cleaned to bright metal before welding © 2012 Delmar, Cengage Learning Objectives (cont'd.) • Demonstrate how to properly make GMA welds in butt joints, lap joints, and tee joints in all positions that can pass the specified standard © 2012 Delmar, Cengage Learning Introduction • Performing satisfactory GMA welds relies on many factors – – – – – Setup Voltage Amperage Electrode extension Welding angle • Excellent welding conditions – Increase productivity © 2012 Delmar, Cengage Learning Setup • Basic GMAW installation consists of: – – – – – – – Welding gun and gun switch circuit Electrode conduit-welding contractor control Electrode feed unit and electrode supply Power supply and shielding gas supply Shielding gas flowmeter regulator Shielding gas hoses Power and work cables © 2012 Delmar, Cengage Learning Gas Density and Flow Rates • Density – Determines effectiveness of a gas for arc shielding • Correct flow rate – Set by checking welding guides FIGURE 11-15 A wind screen can help prevent the shielding gas from being blown away. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Wire-Feed Speed • Changes in wire-feed speed – Automatically change amperage • Possible to set amperage by using a chart and measuring length of wire fed per minute • Wire-feed speed – Generally recommended by electrode manufacturer • Selected in inches per minute (ipm) © 2012 Delmar, Cengage Learning Arc-Voltage and Amperage Characteristics • Arc-voltage and amperage characteristics of GMA welding are different from other processes – Voltage is set on welder – Amperage is set by changing wire-feed speed • Voltage and amperage settings – Specified according to a WPS or other codes and standards © 2012 Delmar, Cengage Learning Electrode Extension • CP power supply – Welding current changes as distance between contact tube and work changes – Change is enough to affect the weld – Longer extension: greater resistance FIGURE 11-18 Heat buildup due to the extremely high current for the small conductor (electrode). © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Welding Gun Angle • Angle between GMA welding gun and work – As it relates to the direction of travel • Backhand welding: deep penetration • Forehand welding: shallow penetration • Perpendicular welding: balance • Changing electrode extension and angle at the same time – Can result in a quality weld being made with lessthan-ideal conditions © 2012 Delmar, Cengage Learning FIGURE 11-19 Backhand welding or dragging angle. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning FIGURE 11-21 Perpendicular gun angle. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Effect of Shielding Gas on Welding • Protects molten metal from oxidation and contamination – Factors in selecting the right gas • • • • • • • Arc influence Metal transfer characteristics Weld penetration Width of fusion zone Surface shape patterns Welding speed Undercut tendency © 2012 Delmar, Cengage Learning Effect of Shielding Gas on Welding (cont'd.) • Pure argon and helium – Produce flaws on ferrous metals • Add controlled quantities of reactive gases – Good arc action – Metal transfer • Oxygen or carbon dioxide – Often added to argon © 2012 Delmar, Cengage Learning Effect of Shielding Gas on Welding (cont'd.) • Adding oxygen or CO2 to an inert gas – Causes shielding gas to become oxidizing • May cause porosity in some ferrous metals • Presence of oxygen causes loss of alloying elements • Pure carbon dioxide – Widely used as a shielding gas for GMA welding of steel © 2012 Delmar, Cengage Learning Table 11-7 Shielding Gas Mixtures. © 2012 Delmar, Cengage Learning Practices • Grouped according to those requiring similar techniques and setups – Major skill required is the ability to set up the equipment and weldment • Variations requiring changes in technique and setup – Material thickness – Position – Type of joint © 2012 Delmar, Cengage Learning Practices (cont'd.) • Correctly set-up station – Can be operated with minimum skill – Often, the main factor to success is ability to make correct machine setup • Practices – Some give the welder general operating conditions – Differences in the type of machine affect settings © 2012 Delmar, Cengage Learning Metal Preparation • Hot-rolled steel – Has an oxide layer – Mill scale is a thin layer of dark gray or black iron oxide • Not removed for noncode welding • GMA welding wire – Difficult to add enough deoxidizers • Porosity caused by mill scale is often confined to interior of the weld • Mill scale is not visible on surface: goes unnoticed © 2012 Delmar, Cengage Learning Metal Preparation (cont'd.) • Welding surfaces must be cleaned if: – Practices will be destructively tested – Work is of a critical nature • Clean weld groove and surrounding surfaces – Within one inch – To bright metal – Methods: grinding, filling, sanding, or blasting © 2012 Delmar, Cengage Learning Flat Position, 1G and 1F Positions FIGURE 11-25 Stringer beads in the flat position. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Vertical Up 3G and 3F Positions FIGURE 11-31 Vertical up position. Larry Jeffus © 2012 Delmar, Cengage Learning Vertical Down 3G and 3F Positions • Major advantages – Speed – Shallow penetration – Good bead appearance • Often used on thin sheet metals or in the root pass in grooved joints – Combination of controlled penetration and higher weld speeds © 2012 Delmar, Cengage Learning Horizontal 2G and 2F Positions FIGURE 11-37 The actual size of the molten weld pool remains small along the weld. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Overhead 4G and 4F Positions • Several advantages to short-circuiting arc metal transfer in the overhead position – Small molten weld pool size • Allows surface tension to hold it in place • Improved bead contour – Direct metal transfer • Efficient metal transfer • Less spatter and loss of filler metal © 2012 Delmar, Cengage Learning FIGURE 11-38 Overhead weld. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Globular Metal Transfer, 1G Position FIGURE 11-44 Weld bead made with GMAW globular metal transfer mode. Larry Jeffus © 2012 Delmar, Cengage Learning Axial Spray FIGURE 11-51 Weld bead made with GMAW axial spray metal transfer. Larry Jeffus © 2012 Delmar, Cengage Learning Summary • Slight changes in welding gun angle and electrode extension – Make significant differences • Small adjustments in welding technique – Required to compensate for slight changes that occur along a welding joint • Variations in conditions – Significantly affect welding setup for GMA process • Have someone assist you in making changes to machine's settings as you weld © 2012 Delmar, Cengage Learning