Lec14 Flashcards
oxyfuel gas welding (OGW) heat source
fuel and oxygen produce flame acts as heat souce
two stage combustion of acetylene gas in a mixutre with oxygen (use more oxygen than acetylene)
max temperature of flame occurs OGW
near end of inner cone, outer envelope of the flame preeats metals and provides shielding from oxidation
adv of OGW
low equipment cost
portable and req no power supply
good for welding thin materials
can be used as heat source to control distortion or shape
disadv
slow process
can only use poor conductors and materials that will not react significantly with flame gases eg steel or cast iron
jetting pure oxygen if not all burned - oxidation
Consumable electrode type process
arc produced between tip of an electrode and workpiece to be welded using dc or ac
Types of consumable electrode welding
shielded metal arc welding
flux cored arc welding
gas metal arc welding
Shielded metal arc welding
use a finite length electrode
shielded metal arc welding bonding coating surrounding wire contains chemicals that
vaporise - protective atmosph isoning elements to stabilise arc act as flux to deoxidide and remove impurities provide protective slag coating add alloying materials add additional filler metal affect arc penetration
shielded metal arc welding process
1 electrode tip touch workpiece (complete circuit)
2 move electrode to dist that will maintain stable arc
3 heat melts tip of electrode wire and adjacent base metal
4 fluxing constituents combine with other impurities in molten metal to make up part of slag
5 slag protects cooling metal from oxidation and slows down cooling - annealing
adv of shielded metal arc welding
simple
versatile
inexpensive
req a few varities of electrode
disadv of shielded metal arc welding
discontinious electrode
shallow welds
req slag removal
flux cored arc welding vs shielded metal arc welding
same as shielded metal arc welding expect tubular continuous electrode filled with flux
Why is flux cored arc welding more expensive
have to make electrode into tube
adv of flux cored arc welding
continuous electrode
can increase penetration depth
self shielding
disadv of flux cored arc welding
slag removal still req
greater cost than shielded metal arc
Gas metal arc welding (or MIG)
most common shielding gas (helium argon) flowing through tough protects arc and molten metal continuous slid uncoated electrode
Types of shielding gas for GMAW
helium - hottest arc deepest penetration
argo intermediate
carbon dioxide coolest arc temperature and shallowest weld
adv of GMAW
fast and economical
no frequent change of electrode
no flux req
no slag formed over welds (little finishing)
disadv of GMAW
more costly than FCAW (Flux core arc welding) SMAW (shielded metal arc welding)
Submerged arc welding difference
no shielding gas instead of thick layer of granular flux is deposited ahead of solid bare wire electrode)
SAW process
arc maintained beneath blanket of flux
some flux melt and remove impurities from pool of molten metal
molten flux solidifies covering over weld
further cooling solidified flux cracks loose
unmelted flux is recovered by vacuum and reused
SAW solidified flux use
good thermal insulation
promotes slow cooling of weld
promotes soft and ductile welds
adv of SAW
High welding speeds
deep penetration
high cleanliness
disadv of SAW
extensive flux handling
contamination of flux by moisture - lead to porosity in weld
large volume of slag
high heat inputs
welding is restricted to horizontal position (flux falls off)
Non consumable electrode welding processes
gas tungsten arc weldling
plasma arc welding
Gas tungsten arc welding (GTAW or TIG in past) heat source
arc provided by non consumable electrode made of tungsten
Environment GTAW
inert environment - better current carrying and electron emission characteristics and longer electrode life
Use of filler metals for GTAW
not req when there is close fit between pieces
where high deposition rate is req filler metals can be preheated
applications of GTAW
butt and lap joints
all types of metal
adv of GTAW
deep penetration and better arc stability
narrow HAZ
no flux is used - no cleaning
very high quality welds
disadv of GTAW
high inital equipment cost
large torches limit accessibility
requires skilled workers
slow welding process
Plasma arc welding
A concentrated plasma arc is produced and aimed at the weld area
plasma arc welding zone shielding supplied by
outer shielding ring by gases such as argon and helium
two methods of PAW
transferred arc workpiece part of circuit arc goes from electrode to wrokpiece
non transferred arc - arc between electrode and nozzle
heat is carried to the workpiece by the plasmc
factors to consider when producing a good joing
choice of filler metal
type of heat input
inclusions/porosity
HAZ
