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JOINING PROCESS.PART3

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JOINING PROCESS.PART3 Powered By Docstoc
					Lesson Outcomes:

After today’s lecture, students are
  expected to:
• Differentiate many types of solid state
  welding processes; cold welding,
  Friction welding, Resistance welding,




                   BMM 3643          Page 2
Solid-State Welding
     Processes
Solid State Welding
• The process of joining takes place without
  fusion (melting) of the workpiece.
• There is no liquid (molten) phase present
  in the joint.
• The basic principle of solid state welding is
  when two clean surfaces are brought into
  atomic contact with each other under
  sufficient pressure, they form bonds
  (strong joint).
                     BMM 3643           Page 4
Solid State Welding (cont.)
• Applying external heat improves the
  bond by diffusion.
• It may improve the strength of the
  bond.
• Heat may be generated by friction.
• Filler metal is not added



                   BMM 3643             Page 5
SSW Advantages over
Fusion Welding Processes

• If no melting, then no heat affected
  zone, so metal around joint retains
  original properties
• Many SSW processes produce welded
  joints that bond the entire contact
  interface between two parts rather than
  at distinct spots or seams

                   BMM 3643          Page 6
SSW Advantages over
Fusion Welding Processes (cont.)

  • Some SSW processes can be used to
   bond dissimilar metals, without
   concerns about relative melting points,
   thermal expansions, and other problems
   that arise in FW.




                    BMM 3643         Page 7
Cold Welding
• In the process, pressure is applied to
  the workpiece, through rolls.
• Prior to welding, the interface is
  degreased, wire-brushed, and wiped to
  remove oxide smudge.
• Cold welding can be used to joint small
  workpieces made of soft, ductile metals.


                    BMM 3643          Page 8
Cold Welding (cont.)
• The pressure required for cold welding
  can be applied through a pair of rolls.
• Hence, the process is called roll
  bonding.




                   BMM 3643           Page 9
Roll Bonding




 Schematic illustration of the roll bonding or
 cladding process.
                       BMM 3643             Page 10
Cold Welding (cont.)
• The process can be carried out at
  elevated temperatures (hot roll
  bonding).
• E.g. cladding of pure aluminium over
  aluminium-alloy sheet and of stainless
  steel over mild steel, for corrosion
  resistance.


                   BMM 3643           Page 11
Cold Welding (cont.)
• If joining of two dissimilar metals that
  are mutually soluble, brittle intermetallic
  compounds may form.
• These will produce a weak and brittle
  joint.
• E.g. in the bonding of aluminium and
  steel.
• The best bond strength is obtained with
  two similar materials.
                     BMM 3643           Page 12
Friction welding
• The heat required for friction welding is
  generated through friction at the
  interfaces of the two components being
  joined.
• In friction welding, one of the
  components remains stationary while
  the other is placed in a chuck or collet
  and rotated at a high constant speed.

                    BMM 3643           Page 13
Friction welding (cont.)

• The two members to be joined are then
 brought into contact under an axial
 force.




                  BMM 3643             Page 14
      Friction welding (cont.)
    (a)




    (b)




(a) Sequence of operations in the friction welding process:
  (1) Left-hand component is rotated at high speed.
 (2) Right-hand component is brought into contact under an axial force.
 (3) Axial force is increased; flash begins to form.
 (4) Left-hand component stops rotating; weld is completed. The flash can
 subsequently be removed by machining or grinding.
 (b) Shape of fusion zone in friction welding, as a function of the force
                                      BMM 3643                     Page 15
 applied and the rotational speed.
Resistance Welding (RW)
• use a combination of heat and pressure
  to accomplish coalescence
• Heat generated by electrical resistance to
  current flow at junction to be welded




                   BMM 3643          Page 16
Resistive Welding (cont.)
• Heat required is produced by means of
  electric resistance between members to be
  joined
      H(heat) = I2Rt
  –   Heat in Joules
  –   I (current) in Amperes
  –   R(resistance) in Ohms
  –   T(time of current flow) in seconds.
• Often modified by including a factor , K,
  which represents the energy losses through
  conduction and convection (K < 1).
     H(heat) = I2RtK
                           BMM 3643           Page 17
Resistive Welding (cont.)

• Total resistance is the sum of
  – Resistance of the electrode
  – Workpiece-electrode contact resistance
  – Resistance of the individual parts to be
    welded
  – Workpiece-workpiece contact resistance
• High current is required usually because
  of poor resistance
                       BMM 3643           Page 18
     Resistance Spot Welding

    (a) Sequence in
    resistance spot welding.




(b) Cross-section of a spot
weld, showing the weld
nugget and the indentation of
the electrode on the sheet
surfaces. This is one of the
most commonly used process
in sheet-metal fabrication and
in automotive-body assembly.

                                 BMM 3643   Page 19
Example
• Two flat copper sheets (each 1.5 mm
 thick) are being resistance spot welded
 by the use of a current of 7000 A and a
 current flow time of 3 s. The electodes
 are 5 mm in diameter. Estimate the
 heat generated. Assume that the total
 resistance is 200 μΩ.


                 BMM 3643          Page 20
Advantages and Drawbacks of
RW
• Advantages:
  – No filler metal is required
  – High production rates are possible
  – Lends itself to mechanization and
    automation
  – Operator skill level is lower than for
    arc welding
  – Good repeatability and reliability
                    BMM 3643           Page 21
Advantages and Drawbacks of
RW
• Disadvantages:
  – High initial equipment cost
  – Limited to lap joints for most RW
    processes




                   BMM 3643             Page 22