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Plasma Cutting

Course Outline for Welding and Joining Processes

  1. Introduction
    1. Significance of Welding and Joining (W&J)
      1. W&J has improved what we manufacture
        1. Increased size - buildings, bridges, pressure vessels, etc.
        2. Reduced cost - especially in high-volume production
        3. Reduced weight - high joint efficiency without flanges or overlap joints
        4. Improved reliability - high strength without large stress concentrations
        5. Increased product life - no crevices to corrode
      2. W&J is ubiquitous
        1. Virtually all manufactured products contain joints
        2. Quality of product is directly related to quality of joints
      3. Joining is a relatively large fraction of the product cost - W&J often comes near the end of the manufacturing cycle when scrap costs are high
      4. Many failures are caused by joining defects
        1. Even the best joints may have inferior strengths as compared to the base material
        2. Joints are usually placed at the most highly stressed locations
    2. Current limitations of W&J
      1. Often applied as an art
        1. Immediate industrial need for new technologies-science often left behind
        2. Often performed manually
        3. Complex process
      2. Few engineers/designers educated trained in W&J
      3. Inability to join materials limits their usefulness
    3. W&J is multidisciplinary
      1. Combines nearly every field of engineering and most fields of science
        1. Engineering - Civil, Structural, Mechanical, Materials, Electrical, Chemical, Ocean, Nuclear, Environmental, Safety, etc.
        2. Science - Physics, Chemistry , Mathematics, Computer, Robotics, etc.
      2. Requires broad engineer, willing to address complex and practical problems
  2. Fastening
  3. Fundamentals of Bonding
    1. Condensed phases have a net attraction between atoms and molecules
      1. Lennard-Jones Energy Potential
        1. Magnitude varies with bond type
          1. Ionic = 1-3 eV
          2. Covalent = 1-3 eV
          3. Metallic = I-3eV
          4. Hydrogen = 0.3 eV
          5. Van derWaals = 0.1-0.2 eV
          6. Dispersion = 0.02-0.05 eV
        2. Thermal energy can break bonds - eV=kT
        3. Derivative of energy is force
        4. Curvature is Young's Modulus
      2. Force of attraction
        1. Distance of interaction is small ~ 3-5 x atom size
        2. Magnitude is very large: 1 eV ~ 106 psi
        3. Derivative is Young's Modulus
    2. Factors inhibiting bonding
      1. Contamination
        1. Oxygen, carbon dioxide, water vapor, oil, etreadily absorbed on surfaces
        2. Monolayer time is 10E-8 atm-seconds
        3. Example: Graphite as lubricant on earth and abrasive in space (in oxygen-free atmosphere)
        4. Higher surface energies mean greater tendency to absorb contaminants
          1. Metals 0.5-3.0 J/M^2
          2. Covalent 0.2-1.0 J/M^2
          3. Ionic 0.2-0.5 J/M^2
          4. Water 0.072 J/M^2
          5. Teflon 0.020 J/M^2
          6. Gas - zero
      2. Surface Roughness
        1. On an atomic scale, virtually all surfaces are rough
        2. Roughness factor 2x to lOx apparent area
        3. True area of contact is small fraction of apparent area
        4. Under normal (perpendicular) loading, maximurn true area of contact is only 1/3 of apparent area of contact
      3. Interfacial shear is required to get strong cold welds
      4. Example: Ahrned and Svitak, Solid State Technology, November, 1975
        1. Bonding is difficult in central dead zone
        2. Magnitude of normal stress above that necessary to create contact is unimportant
        3. Mutual extension of two clean surfaces is sufficient for bond formation
        4. Small interfacial shear is very beneficial
        5. High interfacial shears are not beneficial as they break existing bonds
      5. Houldcroft grouping of welding processes
  4. Cold and Pressure Welding
    1. Hard oxide - soft metal is beneficial for cold welding - see Tylecote
    2. Mohmed and Washburn, Weld 1975 (9) p. 302-s showed that must crack the oxide and metal flow through gaps in oxide
      1. Probabilistic analysis of contact area of two surfaces
    3. Friction weld - high interfacial shear - extrudes contamination out of joint - generates heat
    4. Ultrasonic weld microscopic interfacial shear - oxides displaced, not removed
    5. Forge and Explosive Welding
    6. Microjoining
  5. Adhesive Bonding
    1. Different from other bonding methods in that surface contamination films remain
    2. Bonding achieved by wetting of solid by liquid
      1. Type I - Pressure differential of wetted liquid and ambient - Thermodynamic analysis
      2. Type II - Mechanical interlocking - liquid must wet the solid in order to enter the pores
      3. Young's equation
        1. Contact angle less than 30 degrees for wetting (preferably less than 10 degrees)
        2. Roughness factor enhances wetting
        3. Hystersis of contact angle
        4. Only applies to static situations
        5. Example: Water (ice) on Teflon
    3. Kinetics of wetting - Stefan Equation
      1. Effect of joint thickness
      2. Change in viscosity
        1. Solvent removal
        2. Polymerization
        3. Cooling and solidification
    4. Effect of roughness
      1. Limits thinness in Type I bonding
      2. Promotes Type II
      3. Example: Anodizing and phosphating metals
      4. Can produce plane of weakness if wetting is marginal
    5. Composition of Adhesives
      1. Cost - wide range
      2. Proteins - animal
      3. Starches - vegetable
      4. Chemicals
    6. Performance of Adhesives
      1. Good strength when large surface/volume ratio (sheets, fibers, powders)
      2. Overlap joints - criterion for failure in base metal or joint
      3. Corrosion
      4. Distributed stresses - improved fatigue behavior
  6. Diffusion Bonding
    (Pressure Welding with Heat)
    1. Pressure Temperature and Surface Finish are Primary Variables
    2. Addition of Heat Permits:
      1. Deformation of asperities to improve area of contact
      2. Diffusion and/or evaporation of contaminants
    3. Requirements:
      1. A void intermetallic formation
        1. Slows diffusion
        2. Often brittle
        3. Third material for interlayer
      2. Match thermal expansion - may need interlayer
      3. Compatible joining (softening) temperatures
    4. Transient Liquid Phase
      1. Speeds diffusion - reduces time
      2. Creates full area of contact
      3. Only low pressure (for fixturing) required
    5. Difficulties
      1. Metals lose cold work and prior TMT
      2. Thermal expansion mismatch creates stresses and weakens joint
      3. Refractory oxides may not be removed (Al, Mg) if oxygen solubility in base metal is low
    6. Stages
      1. Stage 1 - asperity contact - higher pressure improves contact area
      2. Stage 2 - interfacial grain boundary - elimination of pores by vacancy annihilation at grain boundary - pressure not required
      3. Stage 3 - grain growth leaves pores enclosed in grain - process effectively stopped
      4. Higher temperatures speed the process but cause Stage 3 to be reached sooner
    7. Activated Diffusion Bonding - addition of an easily bonded coating changes a difficult-to-bond swface to an easily bonded surface (e.Ag or Ni)
  7. Soldering
    (below 800° F or 425° C)
    1. Uses flux to remove contamination and protect from atmosphere
    2. Spread of solder by surface tension forces - liquid metals have high surface tension and low interfacial energies
    3. Requirements of Flux
      1. 1 .Chemical activity
      2. Spreading activity
      3. Thermal stability
      4. Non-corrosive or easily removed
      5. Example: Oxalic acid, Glucose and Abeitic Acid
    4. Types of Flux
      1. Chemical Flux - forms compound with contamination
      2. Reduction Flux - reduces oxide to metallic state
      3. "Reaction" Flux - penetrates oxide layer and floats it away
    5. Wettability Tests
      1. Dip
      2. Rotary dip
      3. Globule
      4. Surface tension balance
    6. Bond Number
    7. Chemical Requirements of Solder
    8. Corrosion Mechanisms
      1. Chlorides
      2. Green patina on Cu.
  8. Brazing
    (above 800° F or 425° C)
    1. Higher Temperatures
      1. More flexibility in choice of fluxes -any high temperature material can be brazed
      2. Room temperature strength of filler metal is stronger than solders
      3. Some al1oy elements are volatile
      4. Intermetallic compounds may form
      5. Thermal mismatch stresses may be severe
      6. Base metal erosion may occur
    2. Contact strengthening - thinner joints are stronger
  9. Fusion Heat Sources
    1. Heat Intensity
      1. Low Intensity Limit -100 W/cm^2 heat cannot be carried away by conduction - vaporization
      2. High Intensity Limit- 10^6 W/cm^2 heat cannot be carried away by conduction -vaporization
      3. Trends with increasing heat intensity
        1. Increasing heat efficiency
        2. Decreasing HAZ size
        3. Increasing travel speed
        4. Increasing need to automate
        5. Increasing equipment cost
        6. Increasing penetration (pool assigned ratio)
        7. Increasing production volume requirements
    2. Flames
      1. Temperature depends on:
        1. Enthalpy of reaction
        2. Fuel/oxygen ratio
        3. Inerts
      2. Combustion Intensity-Boundary Layer
      3. Types of flame
        1. Diffuse
        2. Pre-mixed
        3. Jet-burner
      4. MAPP vs. Acetylene
      5. Oxygen cutting
        1. Low-melting oxide
        2. Effect of N2 and CO2
    3. Arcs - Electrically augmented flame
      1. Effect of pressure
      2. Effect of current
      3. Partitioning of heat
        1. Electron flow
        2. Convection
        3. Conduction
        4. Radiation
      4. Effect of gas composition
      5. Heating of anode
      6. Cooling of cathode
      7. 7. Constricted arcs -Plasma arcs
      8. 8. Characteristic lengths
        1. gamma r - recombination
        2. gamma E - Energy exchange
        3. gamma e - elastic collisions
        4. gamma D - Debye screening length
      9. Arc ignition
        1. Paschen breakdown
        2. Field emission
        3. Touch start
        4. High frequency
      10. Extinction of arc
      11. Plasma jets
        1. Convection constricts arcs greater than 30 amperes!
        2. Arc pressure
          1. Arc stiffness
          2. Droplet transfer
        3. Metal vapor
          1. Changes plasma conductivity
          2. Gets upper limit on weld pool temperature
          3. Produces welding fume
          4. Influences arc stability
      12. Electromagnetic forces - arc blow
    4. Vaporization and High Energy Beams
      1. Vaporization within 10^-5 sec
      2. Liquid movement 10^-3 to 10^-3 sec
      3. D/W
        1. Parallel sides
        2. Alignment of joint
      4. Adiabatic melting - HAZ growth on cooling
      5. Comparison of electron beam and laser
        1. Heating particleslphotons - energy
        2. Heat location
        3. Sample conductivity
        4. Atmosphere
        5. Depth of penetration
          1. Orientation
          2. Ambient pressure
        6. Effect of vapor
        7. Heat efficiency
        8. Beam distortion
        9. Radiation
        10. Beam interaction with itself
        11. Beam shape
        12. Heating of sample
      6. Heavy Section Electron Beam
        1. Poor long term beam stability
        2. Inadequate repair of defects
        3. Spiking penetration
        4. High equipment cost
        5. Low utilization factor
        6. Extremely clean steel required
        7. Seam tracking
        8. Poor steel HAZ toughness
        9. No good NDT method
        10. End crater defects
        11. Large vacuum or local vacuum
        12. Narrow range of process variables for thick plate
    5. Heat flow
      1. Stationary point heat source
      2. Stationary line heat source
      3. Stationary planar heat source
      4. Traveling point heat source
    6. Convection in weld pool
      1. Forces
        1. Buoyancy
        2. Electromagnetic
        3. Marangoni (surface tension)
        4. Plasma drag
      2. Variable penetration
      3. Surface depression
    7. Metal Transfer
      1. Eight types of transfer in wire feed processes
  10. Resistance Welding Processes
  11. Specialized Processes and Materials
    • (Varying topics from year to year - e.g., bonding of ceramics or polymer, development of a novel process, studies of joining specific materials, etc.)
    • Example: Maglay Process

Bibliography