Lec13 Flashcards
Ideal joint
formed such that when you transverse across the joint see no change in material properties
Joining processes x 4
welding
brazing and soldering
mechanical fastening
adhesives
Welding involves
substantial heating often melting of the metal parts to obtain the join
solidification of metals
when cooled liquid metals starts to form ordered crystals
grow until all metal is solid
crystals tod to gorw in diection of temp gradient
rate of cooling affects crystal size structure and distribution of phases
heating metal
changes structure of metal
Why is join choice so critical
poor choice - potential catastrophic defects
changes in properties of parent metal dont want at critical point
weld defects
cracks cavities inclusions oxides slag flux excessive distortion incomplete fusion between weld incomplete penetration unacceptable weld shape or contour
fusion welding
pool of metal created from mixture of parent materials or
parent plus filler (improve joint strength not always req)
Weld zone composition
does not have same properties and characteristics as parent metals, adequate properties can be achieved with right choice of filler metals
Filler metals should
be of similar composition and properties to base metal (similar melting point)
What is the Heat affected zone
adjacent to fusion zone - parent metal not melted but subject to elevated temp for period of time
issues with HAZ
if enough heat cause transformation recrystallisation grain growth - introduce stresses precipitate coarsening embrittlment cracking discolouration
Weld point becomes
weakest region usually where failure originates
Low rate of heat input welds
high total heat content within metal
slow cooling rates
high ductility
large HAZ
high rate of heat input welds
low heat content within metal
fast cooling
lower ductility
small HAZ
Reducing variation in microstructure in welding
preheat entire base metal prior to welding
or heat segments adjacent to joint prior to welding
what does preheating a join to do welding
reduces cooling rate
slow cooling rate produces more ductile structure - more resistant to cracking
Thermally induced residal stress caused by (welding)
weld pool solidifies weld metal and adjacent HAZ cool and contract
surrounding cooler metal resists
weld region is forced to remain stretched residual tension
Types of welding heat sources
chemical or electrical
fusion welds wtihout filler metal are called
autogenous (filler metal may or may not be used)
Types of fusion weding processes
electron beam welding (EBW)
laser beam welding (LBW)
oxyfuel gas welding
arc welding (MIG TIG)
EBW uses
high velocity narrow beam electrons - KE converted to heat
adv of EBW
high quality welds
small heat affected zone
distortion and shrinkage in the weld are minimal
no shielding gas flux or filler metal
disadv of EBW
Generations of xrays
costly equipment
Laser beam welding heat source
high powered laser beam - focus to small area
high energy density promotes deep penetration
adv of LBW
suitable for welding narrow joint
good strength and ductile
minimum shrinkage and distortion
free of porosity
disadv of LBW
laser is extremely hazardous to eyes
costly equipment
applications of LBW
automotive welding of transmission components
electrical industry welding of thin parts
adv of LBW over EBW
laser can go through air so no vacuum
laser can be easily shaped directed and focused with both transmission and reflective optics (EBW need magnetic lens)
