WELDING Flashcards
Thermal conductivity, thermal expansion coefficient and affinity to oxygen levels are all what for aluminium?
High thermal conductivity, high thermal expansion coefficient and high affinity to oxygen
What is the melting point of pure aluminium and of AlO3?
Pure aluminium - 649
AlO3 - 1538
Does aluminium undergo microstructural transformation?
No - so there is no strengthening effect
What does aluminium have high affinity to in a molten state?
Dissolved H2 , it is porous
How can aluminium alloys be welded?
Can be arc welded in an inert atmosphere: TIG (with AC current) or MIG (with reverse polarity)
What are the problems with welding of aluminium alloys?
- high melting temp of oxides
- high heat conductance
- softening of the parent alloy
- not all Al alloys are weldable (2000&7000 are not)
- no stress relief (residual stresses remain)
What are the general guidelines for welding aluminium alloys?
- prepare base material
- pre heat
- use rapid welding process
- ensure shielding of the weld pool
- use adequate filler material
- convex shaped welds
- wise power source selection (this has an implication on the mechanics of material transfer)
- appropriate wire feeder and welding gun for handling soft aluminium wire (for MIG)
What are the weld joint constituents? Can you draw this?
Weld metal
HAZ
Parent plates (A&B)
What are the two sections of the heat affected zone?
Coarse grain HAZ: partially re-austenized HAZ, grain growth
Fine grain HAZ: grain growth
Characteristics of a single pass welded joint?
- for thinner parts
- more economical on thin parts, but
- rapid cooling
- hard microstructure with weaker mechanical properties
Characteristics of multi pass welded joint?
- thicker structures
- slower cooling
- annealing effect, but
- time consuming
- non-homogeneous microstructure
- better quality welds
- less distortion
- May be the only option!
What is pre-heating?
Heating parent material locally before welding (to around 300). This gives slower cooling rates, minimises hydrogen induced cracking, lowers shrinkage and hence residual stresses
What is annealing?
Heated below the transformation temperature (723) for an extended period of time and very slowly cooled. This enables recovery (a reduction in dislocation density), recrystallisation (formation of new grains) which are good for toughness. But if heated for too long grain growth occurs and this is bad. Objectives are: relieve stresses,increase softness ductility and toughness and produce a specific microstructure. Commonly used after cold working or welding.
What is normalising?
Heated above the transformation temperature (723 - but depends on carbon content) for an extended period of time and cooled slowly in air. Reformation of existing grains into austenitic grains (BCC TO FCC), controlled cooling in conjunction with a CCT diagram allows any microstructure to be obtained. Used after severe distortion of grains (rolling) or for reformation of microstructure to consist of smaller grains (welding). Virtually eliminates residual stress built up after welding.
What is the multi pass weld microstructure like before normalising? (a + pearlite is the equilibrium phase)
Coarse grains are in non equilibrium phase: martensite , bainite, frozen austenite. In the lower layers they are locally annealed: bainite and troostite. CGHAZ-> FGHAZ bainite and troostite.
What is a multi pass weld structure like after normalising?
Uniform microstructure with finer grains. Non equilibrium phases of pearlite and bainite. Better ductility and toughness of weld.
What does controlling harden ability depend on?
Heat input
Cooling rate (preheating)
PWHT
Chemical composition (carbon equivalent)
Heat input in arc processes equation?
HE = ((UI60)/ S)* efficiency [J/mm]
U= arc voltage
I = welding current
S = travel speed
Efficiency : MMA: 0.75; MIG: 0.9; TIG: 0.8
What happens with an increase in pre heating temp?
Cooling rate decreases
What happens to strength as heat input increases?
Strength decreases
What is the carbon equivalent formula for alloyed steels?
CE= C% + Mn%/6 + Ni%/20 + (Cr% + Mo%)/10 + Cu%/40
Tells us about the hardenability. Materials with CE OK for welding. Materials with CE>0.4% need special controls to prevent cracking, low hydrogen filler material used! higher heat input and preheat .
What are the 4 basic steps for selecting a welding process?
1) joint is analysed in terms of its requirements
2) joint requirements are matched with the available capabilities
3) checklist of variables is gathered to determine ability of selected process to meet the application
4) proposed processes are discussed with an expert or manufacturer for verification
What are the details of step 1 - analysis of joint requirements?
Joint size, joint position and thickness of base material. These determine the fill rate, freeze rate, follow up rate and penetration
What are the details for step 2 - matching?
Look up specifications of individual weld processes in catalogues
What are the details for step 3 - checklist?
Volume of production Weld specifications Operator skill Auxiliary equipment Accessory equipment Base metal conditions
How to reduce welding costs to the minimum?
1) analyse delivery of consumables to the welding point
2) consider effectiveness of material handling
3) avoid over-welding
4) enhance current welding processes
5) optimise joint preparation
6) eliminate extra welds from the design
7) add robotics or automation to the process
8) minimise expense of record keeping
9) examine safety concerns
What are the arc welding safety considerations?
Fumes and gases Arc rays (invisible radiation) Sparks Electric shocks Gas cylinders Visual, hearing and heat protection
What are the five types of joints? Can you draw these?
Butt (girth in a pipe), corner, lap, tee, edge
