Karteikarten
Joining
Karten in diesem Stapel (70)
- 1Joining FastenersUsed because:
- You need to get the components apart again
- They are cheap
- Mass produced in standard sizes
- No deterioration of parts to be joined during joining process
Join at discrete parts and do not seal against liquids - 2Joining Fusion WeldingMolten metal introduced into gap
Molten metal can come from
- Parts to be joined
- Parts to be joined and a filler metal
Heat source
- Arc
- Electrical resistance
- Chemical
- Surface cleaned by flux / sputtering of electric arc
'Cleaned' - removal of oxide film, parts should be cleaned prior to welding
Maintain cleaness
- Flux
- Inert gas / vacuum
- Maintain surfaces in close contact - 3Consumable Electrode Shield Metal Arc WeldingAC / DC current
Electrode positive (DCRP - reverse polarity)
or negative (DCSP - straight polarity)
Coating decomposes to produced shielding gas - 4Coatings Cellulosic- Large volumes of H, CO, CO2, H2O as shielding
- High H content not good, dissolves into weld and makes it brittle
- Easy to weld - 5Coatings Rutile- Produces shield via slag
- Medium H content
- Slag floats on top of liquid meaning it is easy to remove with a bash of a hammer - 6Coatings Basic- Low H
- CaCO3 -> CO & CO2
- Less gas hence need a short arc length which makes it more difficult - 7Function of CoatingFluxing agent - helps liquid wet metal
Stabilises the arc
Generates gas for shielding
Supplies deoxidants and metal to alloy with molten metal
Produces slag
- Impervious layer
- Prevents oxidation - 8SMAW AdvantagesArc is visable
Weld in all positions (with correct electrode)
Can vary weld composition through coating
Can weld 3 - 19 mm thickness with multiple passes
Mild & low alloy steels
Capital and maintenance cost low - 9SMAW LimitationsManual process - depends on skill of welder
Limits on current & deposition rates
Electrodes are of finite length
Welds have to be deslagged - 10Electrode negative or positive?A negatively charged electrode gives a higher melt off rate but less penetration
- Having a high melt off rate means a larger weld pool, not good for overhead - 11Submerged Arc WeldingHave electrode on a spool
- SAW separates electrode and coating
Shielding provided by granular flux which submerges arc & molten zone
High currents - High deposition rates
Positive polarity recommended to achieve deep penetration - 12Submerged Arc Welding Advantages & Disadvantages(Semi) Automatic but long set up time
Arc is invisible
- More confortable for welder
- Can't see whats happening
Deposition rate 4 - 10 x SMAW
- Increase penetration
- Decrease need for edge preparation
High speed welding
High weld quality
- Little oxygen reaches weld
- Deoxidises weld pool
Limited to flat position
Deslagging required
- Can be reground and used as flux
Shielding provided is substantial and not sensitive to air drafts like gas shielding processes - 13Flux Cored Arc WeldingSimilar to SAW but flux is not contained inside the tubular electrode
Tubular electrode supplied from coil
Process can be self shielding, additional gas shielding (CO2, Ar) may be provided - 14Flux Cored Arc Welding AdvantagesContinuous welding
More tolerant of rust & mill scale than gas shielded process
High deposition rates
Deeper penetration than SMAW
Weld metal chemistries developed by changing flux composition - 15Flux Cored Arc Welding DisadvantagesDeslagging required
More fumes than gas shielded process
Equipment more complex than SMAW - 16Metal Arc Gas Shielded (MAGS) Metal Inert Gas (MIG)Wire on spool fed into torch
Shielding gas provided by torch
Welding torch must be bigger than single electrode
DC electrode positive
- Ionises argon gas
- Gas becomes positive
- Attracted to negative parent metal - 17Heat Transfer MechanismDip
- Low I
- When electrode comes into contact with the metal it short circuits and 'spatters' onto the surface
Globular
- Higher I
- Already have a 'glob' of molten metal at end of wire
- Bigger than wire, gravity causes droplet to transfer to metal
Spray
- Higher I still
- Fine droplets are projected across arc
- Well shaped spatter free weld - 18Problems with a high welding currentOverhead - don't want a large weld pool caused by high weld currents
Thin sheet - A high current may burn through the metal - 19Pulsed CurrentCurrent pulses super-imposed on a low background current
Stable transfer with low current - 20MIG Advantages & DisadvantagesAutomatic - used in industry
Suitable for many ferrous & non-ferrous alloys especially Al
Continuous wire feed enables long welds to be deposited continuously
Productivity is 2x SMAW but equipment is more expensive
No deslagging required
- No post process
- Gas shield must be protected against air draughts limiting outdoor applications - 21Non - Consumable Electrode Tungsten Inert GasIndependently control metal & heat input
Allows overhead welding - can control size of weld pool while still forming stable arc
Don't want electrode to melt hence electrode is negative
- Causes problems (no longer ionises argon)
- Used in AC, half cycle spent heating, half cycle spent cleaning - 22TIG Advantages & DisadvantagesClean
Produces excellent welds
- Control of weld pool, weld shape etc.
Can weld in all positions
Can weld thin sheet
Good shielding
Low currents
- Not very productive
- Used for very tricky, carefully performed welds
Difficult stages of multiple pass weld
- Root run of butt weld
Can improve productivity by preheating wire but TIG isn't used when high productivity rates are needed - 23Keyhole WeldingAt high energy densities, metals will vaporise in microseconds and a deep narrow weld bead can be produced.
Allows for large thicknesses to be welded in a single pass
Just because the power density is high, does not mean the heat input is high
Volume of weld deposit is less and hence, less distortion occurs - 24Resistance Welding Process Spot, Seam & Projection weldingHeat generated as current is applied to two, in contact, metal sheets
Sheet < 6mm thick
Pressure applied
- Air excluded from weld zone
- Electrode to workpiece contact resistance reduced
- Molten metal and oxide expelled from interface
Cleanliness of surfaces is vital!
Spot weld - sheets welded at one spot
Seam weld - pressure and current applied through rollers forming continuous seam weld
Projection weld - Small prepared projections on workpiece surface are melted and collapse when current applied - 25Flash WeldingTwo pieces of metal in contact
High current passes through
Flashing from removal of oxide - 26Upset WeldingSame as flash but with high pressure
- 27Solid State Welding'Bashing' metals together
Joining by plastic flow - 28Solid State Welding AdvantagesNo metling
- Cast structure from fusion welding avoided
No filler metal
- Physical / mechanical properties of joint similar to parent metal
Size of weld zone & thermal distortions less than fusion welding - 29Solid State Welding LimitationsComponent size & shape must be convenient for application of forging forces
If temperature is increased to enhance ductility and increase plastic flow, some advantages are lost - 30Cold Pressure WeldingRequires a ductile metal
- Al, Cu
Pressure applied to workpiece through dies and rollers
Requires careful preparation of surface
Increasing temperature can reduce problems of surface contamination - 31Hot Pressure Welding (Friction Welding)2 axially aligned symmetrical parts
1 part rotates
The other part is advanced to make pressure contact
Axial forces increased to achieve frictional heat
No melting
Flash must be removed post process - 32Hot Pressure Welding Advantages & DisadvantagesNarrow heat region hence less distortion
Wide range of dissimilar metals can be joined
Both components need to be rigid enough to resist torque being applied
Process is automated giving good joint reproductivity - 33Friction Stir WeldingUseful for materials which are difficult to weld
Used for welding plates
Shoulder and probe cause extensive heating and plastic flow
Tool rotated and lowered into metal when it is plastic enough
Probe features screw to achieve mixing in joint and prevent voids - 34Friction Stir Welding ApplicationsJoin all Al alloys previously unweldable
Can be used in any position
Can produce butt & lapp joints
Superior properties as avoids melting - 35Diffusion BondingTwo surfaces in contact
Heated to roughly 2/3 of the melting temperature
Lightly pressed together
Left for considerable time
Diffusion occurs across joint
Requires an alloy capable of dissolving oxides - 36Diffusion Bonding AdvantagesAl, Ti, Zr and many dissimilar metals can be bonded
Can make ceramic - metal joints
Interlayers can be used to improve mating between joints
- Minimised intermetallic formation
- Promote sintering by higher diffusion coefficient and so a lower welding T
Used to produce complex shapes
- Combined with super plastic forming to fabricate sheet metal structures with a high stiffness - weight ratio
- Stop off regions where no bond is required
- Super plastically form the unbonded regions into mould using air pressure - 37Diffusion Bonding DisadvantagesDifficult to apply uniform process
Long cycle times - 38Weld MicrostructuresSolid State - Wrought
- Deformation that occurs around the joint squeezes out contamination
Fusion - Cast - 39What causes gas porosity in welds?Thermally disassociate water vapour leading to hydrogen absorption and cold cracking
Oxygen reacts with Al, Mg & Ti alloys producing inclusions which nucleate gas pores.
Iron oxides formed on liquid - 40How to prevent gas porosity in welds?Envelop weld in shielding gas
Use ceramic flux
- Forms impervious layer on liquid surface
Heat coated electrodes
- Drive out any absorbed water
- Remove organic matter
- Use low hydrogen electrodes - 41What determines the grain growth in a weld?Grain size at fusion line
Longer time spent at high T leads to large grain boundaries
- Varies with weld process
- SAW = coarse grains
- Electron beam = fine grains - 42Why do steel welds have a finer grain structure than welds where grain growth only occurs on heating?New grains are formed each time pass from ferrite to austenite on heating and austenite to ferrite on cooling.
- 43Precipitation Reactions During Cooling 0.15C Steel At Slow Cooling RatesFerrite and cementite are stable phases at room temperature
Austenite stable at high T
Primary ferrite nucleates at austenite grain boundaries
Carbon rejected into grain to achieve low C ferrite
Difficult for ferrite to continue due to high C content, Widmanstatten side plates form
Carbon can fit in between plates hence pearlite is formed (ferrite & cementite)
Widmanstatten don't fill whole grain hence we see intergranular growth - 44Precipitation Reactions During Cooling 0.15C Steel At fast cooling ratesFerrite nucleates at inclusions
Around ferrite is C rich steel
This occurs due to less time for diffusion
Acicular Ferrite - 45Precipitation Reactions During Cooling 0.15C Steel At faster cooling ratesUpper bainite nucleates from areas of the grain boundary with no ferrite
Along with cooling rate, if the alloy additions or austenite grain size increases you get the same evolution of growth
If the inclusions increase the evolution of growth reverses
- Prevents grain growth
- Stop austenite grains from growing hence smaller grains
- More GB area where you can get GB ferrite (slow cooling case) - 46What microstructure do we want in our steel weld?Acicular Ferrite
Provides good strength & high fracture toughness
If you have a directional structure cracks propagate easily
Random growth prevents crack growth
Interlocking nature
0.15C steels do not usually show a hardened HAZ because hardenability and cooling rates are insufficient to form fully martensitic microstructures - 47Effect of alloy additions on the weldability of steelsIncreases hardenability
Displaces diffusional phase transformations to longer cooling rates
Intermediate cooling rates now form martensite (BAD) - 48WeldabilityHow easy is it to avoid the formation of martensite
Directly related to hardenability
Can be assessed from its carbon equivalent (CE) value - 49Carbon Equivalent Value
- 50As CE value increases...Possibility of martensite formation increases
Hardness and brittleness of martensite increases
Possibility of hydrogen induced cold cracking increases - 51How to prevent martensite formationReduce cooling rate by preheating material
- Reduces all traces of moisture from surface
- Reduce distortion and residual stresses in the weld - 52What is the minimum preheat temperature?Depend on:
- Combined thickness of plates to be joined
- Hydrogen content of the filler metal
- Carbon Equivalent
Using an electrode with a lower hydrogen content can lower the preheat temperature required - 53Problem with welding austenitic stainless steelsLow melting temperature constituents formed by segregation of impurities solidifying in the intercellular regions wet austenite grain boundries well and form a continuous liquid film
Residual tensile stresses cause cracks to form - 54Solving the problem with welding austenitic steelsFerrite / austenite phase boundaries are not wetted as easily by low melting point constituents
Weld metal adjusted to have 4 - 10% ferrite
Use of a shaeffler diagram allows selection of the filler metal composition
Have to choose a composition with a higher % ferrite than you want at the end as it becomes diluted by the base metal
Excess ferrite can cause corrosion and precipitation problems - 55Weld decay (Austenitic Stainless Steels)If the metal is heated for a short time to 400 - 800'C chromium carbide precipitates form at grain boundaries
Chromium carbide comes from the grain boundary leading to chromium depletion and sensitification of steel
Leads to intergranular corrosion - 56Preventing Weld DecayRestrict chromium carbide formation by restricting the carbon content
Add Ti or Nb to the steel
- Forms carbides more readily than Cr
- Stabilises stainless steels
Welding solution heat treated at 1050'C followed by quenching
- Size problems
- Distortion problems - 57Problem with welding aluminium alloys3 x thermal conductivity of steel
- High intensity energy source
- Weld quickly
Gas porosity (dissolved H)
- Al alloys always contain oxides
- Water vapour
- Remove oxide before the process but it instantly reforms
- Use MIG where sputtering cleans surface
Precipitates in weld coarsen at low temperatures and dissolve at high temperatures
- Softening of HAZ
High strength alloys have long freezing ranges and are prone to solidification cracking
- Can be friction stir welded
- More chance of shrinkage and gas porosity - 58Welding Preparations Double V- Requires access to both sides
- Less weld metal
- Second side cut after first is welded which removes root defects
- U groove for thicker welds - 59Weld Preparations V Groove- V doesn't go all the way to the bottom as this would create a sharp corner which would melt easily.
- Use multipass technique due to depth which also heat treats previous pass - 60Distortion in WeldsCaused by non-uniform T field associated with welding
Resulting thermal strains cannot be accommodated without without exceeding the elastic limit
Assumed to have tensile residual stresses of yield point magnitude
- Otherwise they'd be permanent strains
If you tried to machine it flat again this would disturb the balanced force system, redistribute the residual forces and it would just distort differently - 61Reducing Distortion in WeldsUse a low heat input system
- Electron beam
- Non-oxy-acetylene welding
Use correct edge preperation
Compensate for distrotion
- Preset assembled parts in direction opposite to distortion
Restrain during welding
Symmetrical welding procedures if you have multiple welds to do
- 62Geometric Discontinuities in Welds
- 63Planar Defects in Welds
- 64Volumetric Defects in WeldsOxides, fluxes, slag or electrode coating trapped in weld zone
Porosity from trapped air, gases released during solidification, chemical reactions during welding - 65Hydrogen Induced Cold CrackingH formed from disassociation of H2O in welding arc dissolves rapidly in the molten weld pool and diffuse to the HAZ
- On cooling martensite forms
Solubility of hydrogen is much less in martensite than in austenite and so a supersaturated solid solution of H is obtained
H diffuses to discontinuities in the HAZ and comes out of solution as H2
- Together with high residual tensile stresses from a high restraint joint geometry result in the formation of a minute crack - 66Preventing Cracking in WeldsCracking requires H and Martensite
Use a low H electrode
Preheat to prevent formation of martensite (reduce cooling rate)
Change joint design so less restraint
Bake at 200 - 300'C immediately after weld to eliminate H - 67Lamellar Tear in SteelsJagged cracks outside of HAZ
- Parallel to plate surface
Associated with bands of non-metallic inclusions formed from rolling
- Sulphides
- Oxides
- Silicates
Results in a decrease of through thickness toughness especially in the 200 - 400'C range.
When the FZ boundary runs parallel to these bands and the orientation of the residual stresses is the same as that of the reduced toughness the chance of cracking is increased significantly - 68Prevention of CrackingCracks form when the weld cannot contract freely
Change joint design to minimise weld metal or to make FZ less parallel to the susceptible plate surface
Use a cleaner steel (less inclusions) - 69Solidification CrackingManifest along weld centre line
Low melting point inclusions pushed to last part of weld to go solid
Iron sulphides / eutectics
When it contracts there is still a partially liquid region where these low melting point inclusions have not solidified
Pulls weld apart - 70Prevention of Solidification CrackingAdd Mn (Manganese) so MnS is formed and it has a higher melting temperature and doesn't wet the boundary as well
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- Karteikarten am Joining, erstellt von tom8390 am 08/01/2015.
- Erstellt von
- tom8390
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