Fusion Welding Flashcards
Welding Definition
- Contacting surfaces - flaying surface - are coalesces by application of heat and/or pressure
- A filler material may be added
- The assemblage of parts that are joined are called a weldment
Welding Importance
- Provides permanet joint - parts become one
- Weld joint can be stronger than parent materials
- Economical way to join components in terms of material usage and fabrication costs
- Can join similar or dissimilar metals
- Not restricted to factory environment
Welding Limitations
- Manual process and expensive in terms of labour cost
- Dangerous because of high E involved
- Does not allow for convenient disassembly
- Welded joint can suffer from certain quality defects difficult to detect, which can reduce the strength of joint
The Weld Joint
- Junction of edges or surfaces of parts that have been joined by welding
- Two issues:
- Types of joints - applies to all joining and fastening methods
- Types of welds - used to join the pieces that form the joints
Weld Types
- Fillet - beside two workpieces
- Groove - between two workpieces
- Plug
- Slot
- Spot
- Seam
Fusion Welding
- Fusion is most common
- High density heat E must be supplied to the faying surfaces (resulting Ts cause localised melting of base metals (and filler if used))
- For metallurgical reasons, its desirable to melt the metal with min. E but high heat densities
Power Density
- E source with high power densities - such as laser beams or plasma arcs - can generate high Ts in a small area - which allows for repid melting and fusion of the metal
- This can result in faster welding speeds, which can increase productivity and reduce cost
Fusion Welding Spectrum
- There is a practical range of power density values in which welding can be done
- Too low - heat is conducted into workpiece and melting doesn’t occur
- Too high - metal vaporises in the affected region
PD ~10^3 melting metal <25s
PD ~10^6 vaporises metal in 10^-6 seconds
Fusion Zone
Mixture of filler metal and base metal together homogeneously as in casting - columnar grain grown
Welding Interface
aka. fusion line
A narrow boundary immediately solidified after melting
Heat Affected Zone (HAZ)
Below melting but substantial microstructural change even though the same chemical composition as base metal (heat treating - degradation in mechanical properties)
Unaffected Base Metal Zone (UBMZ)
High residual stress
Heat Affected Zone
- T below melting point, but high enough to cause microstructural change in the solid metal
- Chemical composition same as base metal, but region has been heat treated so that the properties and structure are altered
Effect on mechanical properties in HAZ is usually negative
Its here that welding failures occur
Welding Residual Stress
- Stress that exists in a weldment after all external loads are removed
Caused primarily by nonuniform heat flow during welding
Can lead to many defects - distortion and warping - Avoiding and minimising residual stress and distortion in weldments leads to increase in cost
Heat treatments improve dimensional stability and reduce susceptibility to cracking (e.g. hydrogen, fatigue cracking)
Controlling distortion may require expensive tooling and fixturing and possible post-weld maching
Types of FW
- Arc Welding (AW) - consumable and non-consumable electrod
- Resistance welding (RW) - resistance spot welding (RSW)
- Oxyfuel gas welding (OGW)
- Electron-beam welding (EBW)
- Laser-beam Welding (LBW)
Classification of Fusion Welding by Filler Material
(filler material added to facilitate joining and provide bulk and strength to joint)
- Autogenous weld when no filler is added
- Homogenous weld filler = parent material
- Hetrogenous weld filler is different to parent material
Arc Welding (AW)
- Electric arc is created between an electrode (metal rod or wire) and the work piece - heat generated melts base metal and electrode which forms pool of molten metal - cools to form solid joint
- Filler metal can help increase V and strength of weld joint
- Ts of 6000 degrees sufficiently hot to melt any metal can be produced
Electrodes
Consumable electrodes:
- Provide the source of filler metal
- Available as rods and wires
Non-consumable
- Tungsten (rarely Carbon) which resists melting by the arc
- Gradual errosion (burn-off) can occur during welding (vaporisation is the principle mechanism) - similar to gradual wear of a cutting tool
- Diameter varies 0.5-6.4mm and their length ranges 75-610mm
Arc Shielding
(At high T metals being joined are chemically reactive to oxygen, nitrogen and hidrogen in the air)
- The mechanical properties of the weld joint can be seriously degraded by these reactions
- To protect operation, arc must be shielded from surrounding air
- Electrode tip, arc and molten weld pool are covered with a blanket of gas or flux
- Shielding gasses include argon, helium and CO2
Protection of Welds
- Hot metals are reactive to its environment (air - oxygen)
- N2 and H2 are very soluble in molten metals (H leads to H cracking, nitrides are relatively benign, most become supersatuated solids at high T)
Fluxes: - Consumable e.g. SMAW (Shielded Metal-Arc-Shielding) - CO2
- Seperate flux feed e.g. Submerged Arc Welding (SAW)
Gas shielding - argon and CO2
Flux
A substrate, formulated to serve several addition functions:
- Fluxing agents - promote welding
- Impervious layer on top of formed weld - thermal blanket, seal join from atmosphere (slug)
- Arc stabilisers - stability and directionality of arc
- Gas formers - decomposes to form inert gas (CO2)
Upon cooling, slag solidifes and must be removed later by chipping or brushing
Flux Delivery Techniques
- Pouring granular fulx onto welding operation
- Using a stick electrode coated with flux material that metls during welding to cover operation
- Using tubular electrodes in which flux is contained in the core and released as electrod is consumed
Shielded Metal Arc Welding (SMAW)
aka. Metal Manual Arc (MMA) Welding
- Uses consumable electrode comprising of a filler metal rod coated with chemicals that provide flux and shielding (no external gas)
- About 50% of all large-scale industrial-welding operations use this process
(not suitable were hygiene is needed)
Advantages of SMAW
(4)
- Portability - can be used in remote locations or places where lectricity is not readily available
- Versatility - used to weld a range of metals (steel, stainless steel, cast iron)
- Economical - power supply, connecting cables and electrode holder available for a few thousand pounds
- High quality welds
Disadvantages of SMAW
(4)
- Consumable electrode - sticks periodically changed increasing cost
- Simple to operate - welder requires minimal training
- High current levels may melt coating prematurely
- Difficult to weld thin materials as it can lead to excessive high input, causing the material to warp and distort
Gas Metal Arc Welding (GMAW) Process
aka. Metal Inert/Active Gas (MIG/MAG)
- Uses a continuous and consumable bare metal wire electrode and a shielding gas to join two metal pieces (shielding gases include argon, helium, CO2 or a blend)
- Used in automotive, construction and manufacturing industries
Advantages of GMAW
(5)
- Versatility - GMAW can weld any metal and most commercial alloys
- High welding speed - GMAW is a high-speed welding process than can produce high quality welds quickly and efficiently (ideal for applications that needs lots of welds in a short time)
- Automation - GMAW can be easily operated and automated
- Better production efficiency versus SMAW since the electrode or filler wire does not need to be continuously replaces
- Minimal post-weld cleaning - produces very little splatter and slag inclusions
Disadvantages of GMAW
(3)
- Equipment more expensive and less portable than SMAW
- Torch is heavy and bulky so joint access might be a problem
- Various metal transfer modes add complexity and limitations
Flux Cored Arc Welding (FCAW)
- Uses a continuously fed electrode coil that has flux core - similar to MIG/MAG but with flux
Self-shielded FCAW - core includes compounds that produces shielding gases making it more portable
Gas-shielded FCAW - uses externally applied shielding gases
Advantages of FCAW
(4)
- Popular choice as a replacement for GMAW and/or SMAW for welding steels and stainless steels over a wide stock thickness range
- Higher deposition rates than other welding processes - means more weld metal can be deposited per unit of time, resulting in faster weld completion
- Self-shielded version is tolerant to drafty conditions
- More tolerant to weld metal contamination than GMAW
Disadvantages of FCAW
(7)
- Filler metal is more expensive than GMAW filler metal
- Requirement to remove slag after welding
- Fume production can be extremely high - especially with self-shielded version
- Limited to steels and nickel-based alloys only
- Gas-shielded version is not very tolerant to drafty conditions
- Spatter expelled from the welding arc can sometimes be significant
- More complex and expensive equipment compared to SMAW
Submerged Arc Welding (SAW)
- Uses a continuous, consumable bare wire electrode, with arc shielding by a cover of granular flux
(arc is submerged under a layer of flux - prevents spatter, sparks and fumes from escaping the weld area) - Commonly used in the fabrication of large structures, such as ships, pressure vessels and offshore platforms
Advantages of SAW
(4)
- Extemely high metal deposition rates, especially when multiple wires are used
- No arc radiation - minimal smoke and fumes
- Significant opportunity to customise weld metal properties through the selection of flx
- Mechanised process (usually) does not depend on welder skill
Disadvantages of SAW
(3)
- Restricted to flat position for grooves welds, and flat and horizontal positions for fillet welds
- Fux handling equipment adds complexity
- Not suitable for thin sections
Advantages of TIG
(4)
- High quality welds and finish
- Minimal post-weld cleaning required and no possibility for a slag defect
- Works well for complex geometries and thin sheets
- Expensive due to the use of inert gas
Gas Tungsten (AW) (GTAW)
aka. Tungsten Inert Gas (TIG)
- Uses a non-consumable tungsten or tungsten alloy electrode held in a torch to produce the weld
- Filler wire could be fed into the weld pool to add material to the joint
- Inert shielding gas is used
- No flux is needed
- Commonly used in aerospace, automotive and other industries where high-quality welds are required
Disadvantages of TIG
(3)
- Possibility of tungsten inclusions in the weld
- Slower and more costly than consumable electrode AW processes
- Requires high level of welder skill and experience
Resisting Welding (RW)
A group of fusion welding processes that uses a combination of H and P to accomplish coalescence
- Heat generated by electrical resistance to current flow at junction to be welded
- Principle RW process is resistance spot welding (RSW)
Resistance Spot Welding (RSW)
Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes
- Used to join sheet-metal parts
- Widely used in mass producion of automobiles, metal furniture, appliances and other products
Typical car body has ~10,000 spot welds
Advantages of RW
(5)
- No filler material
- High production rates possible
- Lends itself to mechanisation and automation
- Lower operator skill level than for arc welding
- Good repeatability and reliability
Disadvantages of RW
(2)
- High initial equipment cost
- Limited to lap joints for most RW processes
