Materials & Processes in Manufacturing

Transcription

1 2003 Bill Young Materials & Processes in Manufacturing ME 151 Chapter 37 Arc Processes Chapter 38 Resistance Welding Chapter 39 Brazing and Soldering 1 Introduction Arc welding processes produce fusion of work pieces using an as the heat source Arc welding is the most common of the welding processes It offers a wide range of flexibility and a variety of processes With manual processes weld quality depends on the skill of the operator Types of Arc Welding Processes: Shielded Metal Arc Welding (SMAW) Gas Tungsten Arc Welding (GTAW) Gas Metal Arc Welding (GMAW) Flux Cored Arc Welding (FCAW) Submerged Arc Welding (SAW) Plasma Arc Welding (PAW) Stud Welding (SW) 2003 Bill Young 2 Welding Process Classification Figure 35-1 Page Bill Young 3

2 2003 Bill Young 4 Arc Welding Circuitry All arc welding processes utilize the same basic electrical circuit The application dictates polarity: Figure 37-1 Page 941 Straight Polarity (Electrode -) - Faster electrode burn-off & less penetration Reverse Polarity (Electrode +) - Deeper penetration Basic Types of Arc Welding There are 2 basic types or groups of arc welding processes: Electrode Electrode is melted in the arc Carried across the gap to the workpiece Provides the filler metal for the joint Modes of Metal Transfer Electrode Electrode not consumed by the arc Separate wire/rod provides filler metal for the joint Figure 37-2 Page Bill Young 5 Arc Welding Process Parameters Both the type of process and specific application require selection of the following variables: Welding Voltage Welding Current Arc Polarity (Straight Polarity, Reverse Polarity, or Alternating) Arc Length Welding Speed Arc Atmosphere Electrode or Filler Metal Flux The end result will also be highly dependent on operator skill 2003 Bill Young 6

3 2003 Bill Young 7 Most common of the arc processes Provides versatility and low cost Also known as welding Shielded Metal Arc Welding (SMAW) Electrode is a metal wire with a chemical coating which: Provides protective atmosphere Acts as a flux Provides a protective slag Reduces spatter Adds alloying elements Affects arc penetration Influences weld bead shape Adds additional filler metal Figure 37-5 Page 944 Standard marking system & color codes identify electrodes Used to weld carbon steels, alloy steels, stainless steels, & cast iron Flux - Cored Arc Welding (FCAW) Overcomes some of the shielded metal arc limitations by having the flux inside the electrode Continuous electrode vs. a stick electrode Higher heat inputs increase penetration depth Best used for welding steels Figure 37-6 Page Bill Young 8 Gas Metal Arc Welding (GMAW) Uses consumable wire electrode & inert gas to shield arc & weld pool Also known as welding Automatic wire feed to maintain the arc & provide filler metal Process characteristics: Fast & Economical No flux required No special cleanup or slag removal Readily automated Figure 37-7 Page Bill Young 9

4 2003 Bill Young 10 Shielding gas is not required Submerged Arc Welding (SAW) Blanket of flux provides thermal insulation for cooling Slow cooling rates helps produce soft ductile welds Best suited for flat butt or fillet welds in low carbon steels Process characteristics: High welding speeds High welding currents High deposition rates Deep penetration Clean welds Figure 37-8 Page 949 Gas Tungsten Arc Welding (GTAW) Uses tungsten electrode & inert gas to shield arc & weld pool Also known as welding Arc is stable & easy to maintain (Nonconsumable Electrode) If needed the filler metal is supplied as a separate wire / rod Process characteristics: Clean welds No special cleanup or slag removal Scarcely visible welds All metals and alloys can be welded Figure Page Bill Young 11 Resistance Welding Group of processes that use electrical resistance heating to form the weld joint Coalescence temperatures are achieved by passing a large electrical current through the workpieces while held between the electrodes The maximum resistance is at the workpiece interface which is where the maximum amount of heat will be generated Total resistance between the electrodes consist of: Resistance of the workpieces themselves Contact resistance between electrodes and workpieces Resistance between the surfaces to be joined Figure 38-1 Page 962 Figure 38-2 Page Bill Young 12

5 2003 Bill Young 13 Resistance Spot Welding (RSW) Resistance Seam Welding (RSEW) Projection Welding (RPW) Resistance Spot Welding (RSW) Simplest and most widely used form of resistance welding Resistance Welding Processes Fast and economical means of joining overlapped materials Dominant method for joining metal The welded region of a spot welded joint is called a A good spot weld will be as strong or stronger than the parent metal Virtually all commercial metals can be joined with steel being the most common Figure 38-4 Page 964 Figure 38-5 Page 964 Resistance Spot Welding (RSW) Resistance Seam Welding (RSEW) Projection Welding (RPW) Resistance Seam Welding (RSEW) Resistance Welding Processes 2 types of processes that produce continuous welds: Using rotating disks as electrodes sheet metal is joined to produce gas or liquid tight vessels Resistance butt welding is used mainly in the production of pipes and tubes Figure 38-9 Page 967 Figure Page Bill Young 14 Resistance Spot Welding (RSW) Resistance Seam Welding (RSEW) Projection Welding (RPW) Projection Welding (RPW) Projection welding overcomes 2 limitations of spot welding High maintenance of electrodes Spot welding only produces one weld at a time Larger area of electrodes results in less maintenance Resistance Welding Processes Heating is concentrated at projections resulting in multiple weld points and a stronger joint Projections can be incorporated in part manufacturing No indentation mark left on exterior surface of part Figure Page Bill Young 15

6 2003 Bill Young 16 Advantages and Limitations of Resistance Welding Advantages Fast Can often be fully automated Conserve material (No shielding gases, filler metal or flux required) Minimal distortion of parts Doesn t require skilled operators Dissimilar metals can be joined High degree of repeatability Disadvantages High initial cost for equipment Generally limited to thinner materials (< ¼ ) Requires skilled maintenance personnel for equipment Some materials may require special surface preperation Brazing and Soldering Classified as a low temperature joining method suitable for: Applications where the heat of welding is objectionable Materials that possess poor weldability Where welding is too expensive The joint involves thin or dissimilar materials Basic process steps include: Cleaning the joint surfaces Assembling or fixturing the components A low melting point nonferrous metal is melted and drawn into the joint by capillary action 2003 Bill Young 17 Brazing Brazing is the permanent joining of similar or dissimilar metals or ceramics using a filler metal with a melting temperature 840 o F Brazing differs from welding in that: The chemistry of the brazing alloy is different from the base metal The strength of the brazing alloy is lower than the base metal The melting point of the brazing alloy is lower than the base metal Bonding requires capillary action to distribute the filler metal Due to these differences, brazing has distinct advantages: A wide range of metallic and nonmetallic materials can be brazed Assembling or fixturing the components The process is quicker and economical due to lower heating requirements Lower process temperatures reduce problems with warping and distortion Assembly tolerances can be closer than for most welding processes Highly adaptable to automation A strong permanent joint is formed Figure 39-5 Page Bill Young 18

7 2003 Bill Young 19 Soldering Soldering is a joining process where the filler metal with a melting temperature 840 o F Applications include: Connecting thin metals Connecting electronic components Joining metals while avoiding high temperatures Filling surface flaws and defects Six important steps for a quality solder joint: Proper joint design Correct solder selection for the job Selection of the proper type of flux Cleaning the surfaces to be joined Application of flux, solder, and sufficient heat to fill the joint by capillary action Removal of flux residue Chapter 37 - Arc Processes Basic knowledge of the arc welding process Basic understanding of Shielded Metal Arc Welding, Flux Cored Arc Welding, Gas Metal Arc Welding, Submerged Arc Welding, and Gas Tungsten Arc Welding Review Questions: Chapter 38 Resistance Welding Basic concept behind resistance welding Basic understanding of resistance welding processes Advantages and limitations of resistance welding Review Questions: Chapter 39 Brazing and Soldering What distinguishes brazing from soldering Applications where brazing and soldering might be used Differences between brazing and welding Advantages of brazing Steps required for a quality solder joint Review Questions: 2003 Bill Young 20