1
Q
Welding
A
- joining procedure, permanent union of metallic surfaces by establishing atom to atom bonds between the surfaces
- filler material has a composition similar to the base material
- has replaced riveting and bolting in structural steel work for bridges and buildings ( Automobile spot welds )
2
Q
adhesive joining
A
usually makes use of organic adhesives often containing plastic fillers and inorganic solvents that fuse the surfaces of the plastic and adhesives together
- Some plastics a plastic to plastic joint can be formed only by introducing a volatile solvent which melts the interface
3
Q
Mechanical fastening
A
-Most common method of joining two parts together is by mechanically fastening them using hardware like screws and nuts
4
Q
nature of bonding
A
-If two perfectly matched clean surfaces are brought together with suitable atomic spacing, atomic bonds will be automatically established
- Atomic cleanliness requires that atoms exposed on the surfaces. Actually be the atoms of the material to be joined
- Atomic closeness- requires distances between atoms brought into contact between that at which atoms are normally spaced in the structure
- cleanliness may be established by chemical cleaning (fluxing) provided the products may be removed on the surface by melting or fragmentation as a result of plastic deformation
5
Q
Fusion Bonding
A
Surfaces of the pieces to be joined are completely melted no pressure is necessary
- the use of filler is nearly always necessary in welding sheets and structural shapes more than 3mm(1/8) in thickness
Theoretically it is at least possible to produce 100 efficieny in a fusion weld compared to unwelded base metal
6
Q
Pressure bonding
A
- pressure alone may be sufficient to for,m a bond by heat is used for 2 reasons, 1. Close union is established by plastic flow. 2) metals become more plastic and strengths are lowered as temp is raised
Efficiency of pressure bonds based on the original area may be as high as 95 percent
- pressure welded joints may be actually stronger than the original cross section as a result of enlargement that occurs with plastic flow
7
Q
Soldering
A
- melting Temperature of the filler metal is at below 449 Celsius or 840 degrees Fahrenheit
- Greatest use is for providing fluid tightness for electrical connections and sheet metal joint filling in automative assembly work
- soldering for single components done using a hot iron or torch fueled by propane or methyl acetylene propadiene propane gas
7
Q
Flow Bonding
A
- When a filler Material of different composition and lower melting temp than the base metal is used
- Closeness established by the molten filler metal, cleanliness produced by the use of fluxes which dissolves the surface oxides and floats them
- 3 operations that use flow bonds are Braze welding, Soldering, brazing
8
Q
Brazing
A
Done at temperatures abobe 449 C (840 F) using either MAAP gas or aceytylene and oxygen due to the need for higher temps
- use of filler material applied to a close fitting joint by the pre-placement or by capillary action
- Joint thickness critical to strength
9
Q
Diffusion bonding
A
- Diffuses the mating surfaces with the application of immense pressure and heat
- No flux or filler material is required, however surfaces must be prepared to where machining may be required
- Diffusion welding; Improves the establishment of a smooth, clean surface that must be maintained until weld is made, pickling the surfaces in an inert gas environment, low to moderate pressure applied
- used mainly for joining special alloys in aerospace and atomic energy applications
10
Q
Joints
A
Five type of Joints
- Fillet Joint
- Square Groove
- Bevel Groove
- Vee Groove
- J- Groove, U Groove
10
Q
Adhesive bonding
A
- Quick affordable method of joining both similar and dissimilar metal alloys with other materials
- Underlying mechanism responsible for adhesion is surface absorption; involves the formation of relatively weak secondary bonds, requires the joining of large surfaces
- honeycomb panels and carbon fiber can be made through this method
11
Q
Joint types
A
Butt joint
* Tee Joint
* lap Joint
* Corner joint
* Edge joint
12
Q
Weldability
A
metals capability to be welded into a specific structure within certain specific properties and characteristics
* Welded Structure can satisfactorily meet service requirements
* Relative ease in which a sound union maybe be produced between two parts by welding known as weldability of the metal
- vanadium reduces harden ability
13
Q
Pre heat/ Post heat treatment
A
Preheat treatment : By lowering the yield strength of the base metal, provides greater volume through which shrinkage may be distributed, by lowering thermal gradients in the weld zone, reduces the size of stresses by distributing them
Post heat treatment: Relives stresses by permitting yielding to occur at reduces stress levels, can also help restore structure with an improved grain size
13
Q
oxyfuel gas welding (OFW)
A
- Combination of oxygen and fuel usually acetylene flows to create temps of about 3500 C or 6330F
- Oxyacetylene welding temps of 3087C (5589F) ideal for welding steel and cast iron
- Protected from contamination due to the high flow of superheated gas
- no flux is required but surface preparation is crucial
14
Q
Oxyacetylene
A
- the oldest and still most used source of heat based on a chemical reaction is the burning of acetylene and oxygen
- 3 Part oxygen and 1 acetylene the temperature is 3482 C (6300F), 815 C ( 1500F) at the tip of the inner cone of a neutral flame to 3300 C (5972F) in the hottest portions of the outer envelope
- oxyhydrogen used for aluminum welding
14
Q
Recrystalization
A
- materials that have been cold worked automatically recrystalize during welding
- most aluminum begins to recrystalize at about 150 C or 300 F
- causes grain growth
14
Q
Thermal Conductivity
A
- If high thermal conductivity, cannot be fusion welded since it cannot be melted
- aluminum has high thermal conductivity that means high rate of heat input is required to prevent excessive melting of the base metal
- Stainless steel has low conductivity which results in hot spots and very high temperature gradients in the weld zone
15
Q
Percussive welding
A
- used only for making butt joints between the Flat ends of workpieces without filler material
- more closely associated with pressure than fusion
15
Q
arc welding processes consumable electrode
A
GMAW, FCAW welding, SMAW are processes where the welding electrode is consumed
- processes are usually dirtier, break i the current causes splattering to be flung around
- stick and flux core leave a protective layer of slag
- GMAW typically uses an inert gas usually argon and or helium to prevent oxidation, Inert used since they don’t react with the molten metal pool
16
Q
electric arc
A
- All production welding uses electricity as an energy source
- one of the hottest sources of energy available except for nuclear reactions near 6090 C ( 11000F)
- Arc usually exit between the work and the metal rod
- 1o to 30 percent of the melted rod is normally lost through vaporization and spattering outside the molten pool
- when rod is negative called straight polarity when positive called reverse polarity
- Arc welding the most versatile of all welding processes
16
Q
Stud Welding
A
- Developed in the shipyard to attach steel studs to the steel deck of a ship used to hold wood over deck
17
Q
Arc Welding electrodes
A
- Coating provides a gaseous shield that flushes away the atmospheric gas
- siding salts used for direct current welding rods, potassium salts for alternating currents
- Slag protects hot metal
- Coating may add filler
18
modification for arc welding for special purposes
* Speed of manual welding is increased where feasible by work positioners
* repair and maintenance welding, construction of bridges and structures and cross country pipelines do not permit positioning of work
* GMAW Filler-wire diameter may range from .020” to .125”
* argon, helium or mixtures are common shielding gasses
19
Non Consuable electrode ( GTAW)
* GTAW Process the most controlled and preferred of the arc welding processes due to the variety of the weld beads possible
* Pulsed arc power supplies are capable of producing varied pulse characteristics to the arc
20
Automatic welding under flux
* High production process used is submerged arc welding ( SAW)
* Shielding provided by granular flux fed from the hopper to surround the arc completely
* basically a shop process
21
Laser Welding
* uses a highly focused laser with spot sizes typically ranging from .2mm to 13mm
* has high power density and creates a narrow weld seam and small haz
* Particularly dominant in the automative industry
21
plasma Arc welding
* Plasma is gas that has been heated to such temperature that the gas is ionized
* Process has a high intensity and high rate of heat transfer makes it useful for welding high conductivity metals such as aluminum
22
High Energy beam welding
* Flows of electrons from an electron source/gun or photons from a laser beam is focused to power densities large enough to melt and vaporize the metals being joined. High quality, deep penetrating welds with small heat affected zones and minimized distortion can be produced
* Selective laser Sintering welds thin layers of powder from the material, microjoining when weld dimensions less than a millimeter
* Electron Beam welding (EBW)- valuable for welding beryllium, molybdenum, zirconium, barium and other refractory metals
* Electon beam gun: Process carried out in a vacuum high rate of heating restricts the HAZ, fusion depth to width as much as 20 is possible
22
Forge Welding
* Solid state process in which the workpieces are heated to the welding temp and then applied with loads sufficient to cause permanent deformation at the fraying surfaces
* Most commonly applied to Butt welding of steels at temps 80 to 90 percent the melting temp, requires post heat treatment
* Joint surfaces are slightly rounded or crowned to ensure that the centerline region of the components joined will be welded first
23
Ultrasonic Welding
* Small Quickly oscillating motions through a transducer melting surfaces together while a small axial load is applied, friction between the parts melts the surfaces together
- Vibration aids cleaning, upper limit is 2.54MM USW is an important assembly method for plastics
23
Friction Welding
* mechanical energy is supplied not only to facilitate fragmentation but also develop heat
* used almost exclusively to make butt welds in heavy round sections
* process unsuitable for load sensitive parts
24
Cold Welding
* tremendous compressive forces diffuse the contacting surfaces to join the material together
* One uses hydraulics to axially compress the work pieces together another is where tubes are drawn through a dieland mandrel
* Cold bonding : two surfaces on an atomic scale, one anatomically flat and clean ans they are put together a bond is affected
* welds are made by squeezing the metal between two punch faces that cause metal flow normal to the direction of the load
* Copper and aluminum
25
Electroslag welding
* Water cooled copper shoes or slides cover the gap where the welding is in process. Slag protects the heated metal
*may be defined as continuous casting
* Single electrode used for section up to 2” thick for thicker multiple electrodes may be used melting rates up to 18kg per hour
* arc with protective has atmosphere also used
26
Resistance welding
* Sometimes referred to as spot or seam welding. Commonly used in the automative and pipe industries and relies on the resistance of joining the surfaces
* higher resistance at interfaces
* local melting may take place but is not necessary for the process, dissimilar metals and all metals may be resistance welded
P= I squared R
P= power in watts, I= Current in ampres, R= Resistance in ohms
27
Spot welding
Most important applications of resistance heating are for spot welding, primarily for lap joints, spot welds obtained by concentrating the pressure and current flow with shaped electrodes
* most common variations are seam and projection welding
* Seam overlapping spot welds produce a continuous joint used where pressure or liquid tightness a requirement
* projection welding the joining of electrical contacts to relay and switch parts.
27
Explosion Welding
* uses explosive forces to create an electron sharing metallurgical bond between two metal components
* Suited to joining large areas of two or more metals of different compositions
* greatest use of procedure is in coating, cladding structural metal but more corrosion resistant metal
28
Discontinuities in fusion welds
* AWS categories weldment discontinuities in three general clases
1. Those associated with drawing or dimensional requirements
2. Associated with structural discontinuities in the weld itself
3. Properties of the weld metal or welded joint
29
Stresses
* no stresses higher than the yield strength can exist in a material at any given temperature
* if an entire unrestrained body is cooled uniformly from any given temp, no increase in stress will occur
* normalizing provides stress relied and increases uniformity of the grain structure, heated to about 1050C ( 1200F)
Asnt Studycards Basic Material And Processes
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