Flashcards
Welding and Joining
Cards in this deck (32)
- 1Welding- two or more parts are joined together at their contact surfaces by suitable application of heat and/or pressure.
- filler material can be added to facilitate coalescence - 2Welding advantages- Provides a permanent joint
- Joint can be stronger than parent materials if the correct filler is used
- Often cheapest option regarding material usage and fabrication.
- Can be preformed in the field
- Lighter than equivalent mechanical fastening - 3Welding disadvantages-Often carried out manually ( high labour costs)
_ welding points can suffer from defects which are difficult to monitor
- No allowance to disassembly which can lead to maintenance issues.
- High energy (dangerous)
- High level of skill needed to overcome porosity. - 4Fusion Welding-application of heat to melt the base metals.
- Often involves the addition of filler to the molten pool to provide bulk and add strength. - 5Types of fusion welding- Oxyfuel Gas Welding: Heating of the parent and filler metals via a hot flame.
-Arc Welding: Heating of the parent and filler metals via an electrical arc.
- Resistance Welding: Heating via electrical resistance between the parts to be joined (held under pressure)
- Others (electron beam, laser beam...) - 6Fusion Welding – Power Density- time to melt the metal is inversely proportional to power density.
- 7Features- Fusion Zone: a mixture of base and filler metal
- Weld interface: a thin band of base metal that melted during the welding but solidified prior to any mixing with filler
- Heat-affected Zone: exposed to temperatures below the melting point but high enough to cause micro structural changes. Metal damage is caused by the temperature peak distribution.
- 8Weld Quality- Residual stresses: the rapid heating and cooling in localised regions results in thermal expansion and contraction.
- Distortion: Partial alleviation via welding fixtures (clamping), heat sinks, tacking (more rigid initial structure). - 9Fusion Weld Defects- Undercut: part of the base metal is unintentionally melted away
- Underfill: a gap in the weld below the base metal surface
Overlap: the weld metal spills beyond the joint onto the surface but no fusion occurs. - 10Oxyfuel Gas Welding- Relies upon a the formation of a flame from combustion of fuel + oxidant.
- The most common form is oxyacetylene welding
- Acetylene (C2H2) plus pure oxygen is popular as it produces very high flame temperatures (>3000°C in the flame inner core)
- C2H2 + O2 2CO + H2
- 2CO +H2 +1.5O2 2CO2 +H2O - 11Oxyfuel Gas Welding- neutral flame: acetylene and oxygen mixture is 1:1.
- oxidizing flame: used as slag to weld copper/ copper alloys to prevent oxidisation.
- carburizing flame: If the oxygen is insufficient. Suitable for low heat applications: brazing, soldering and flame hardening operations. - 12Oxyfuel Uses- equipment is relatively inexpensive and portable, therefore versatile process.
- low quantity production and repair trades
- Welding, brazing & cutting/loosening
- steels, stainless steel, aluminium, copper alloys - 13Arc Welding- Fusion is achieved by the heat of an electric arc between an electrode and the work
- The arc is a discharge of electric current, sustained by the presence of a thermally ionised column of gas (“plasma”) through which the current flows.
-To initiate the process the column is quickly brought into contact with the work, then separated by a short distance
-In most processes filler metal is added to increase joint strength
-Consumable electrodes may be used to provide the filler. - 14Arc Welding – Shielding- At high temperatures the metal being joined can react with oxygen, nitrogen and hydrogen.
- Mechanical properties of joint can be compromised.
- Shield electrode tip, arc and molten weld pool with a blanket of flux, inert gas or both.
- flux melts over the weld to form a protective layer - 15Shielded Metal Arc (also known as stick welding)- consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding.
- filler metal is usually similar in composition to the base metal.
- As the coating melts it provides the ionised gas, a protective atmosphere and slag for the weld area and also helps stabilise the rate of electrode consumption. - 16Shielded Metal Arc Welding Uses- Equipment is portable and low cost, widely used.
- Used in construction, pipelines, shipbuilding and repair work (accounts for ~50% of all industrial and maintenance welding)
- Base metals include steels, stainless steels, cast irons and certain non-ferrous alloys. - 17Gas Metal Arc consumer electrode- Metal Inert Gas (MIG)
- The electrode is a bare wire fed continuously from a spool through the welding gun
-Shielding is produced by flooding the arc area with a gas
- Inert gases are used aluminium alloys
CO2 used for low and medium carbon steels
- The gas avoids the need for slag formation – ideal for multiple runs - 18MIG Welding Uses- Faster than other processes
- Automated
- Almost all metals can be MIG welded
- More suited to titanium alloys & other high temp alloys - 19TIG Welding Uses- Manual application due to its difficulty
- Allows a wide range of metals (including some dissimilar metals) to be joined in a wide range of thicknesses (including very thin joints)
- Most common applications are for aluminium or stainless steel - 20TIG Welding pros and cons- Main advantage is ability to produce high quality welds with excellent surface finish
- Relatively slow, high heat input & despite the name the electrodes are still gradually depleted. - 21Laser Beam Welding- Utilises a high power LASER beam to produce the fusion of the metal
- Produces welds of good quality with minimum shrinkage and distortions
- Depth-to-width ratios can be between 3-10
- The method is becoming widely used in automotive industry. - 22Resistance Welding-uses electrical resistance to locally generate heat
- two electrodes supply current and apply pressure to the weld zone
- Requires no shielding, electrodes are non-consumable and typically made of copper-tungsten alloys
- Examples above show spot and seam resistance welding
- Seam welding involves the cyclic passage of AC current but can produce continuous welds
- 23Friction Welding- Coalescence is achieved by friction heating combined with pressure
- Machines similar to lathes: process is suited to automation
- The friction raises the temperature to the hot working range (without melting) - 24Friction Stir Welding- Rotating tool is fed along the join line, generating friction and mechanically stirring the metal.
- The heat softens metal to a highly plastic state
- The shoulder generates the majority of the heat while probe stirs the arising plastic zone
- Used in the aerospace, automotive, railway & shipbuilding industries
- Typically used to for butt joints of large aluminium parts - 25Pros and cons- Good mechanical properties without defects
- An exit hole is produced when the tool exits the work
- 26Cold Welding- Pressure is applied to the workpieces through dies or rolls
- Due to the high plastic deformation rates at least one of the metals must have a high ductility.
- In roll bonding, pressure is applied through a pair of rollers. The process can be carried out at elevated temperatures.
- A typical application is the production of bi-metallic strips for thermostats, by using two materials with different thermal expansion coefficients.
- 27Brazing- Filler metal is melted and distributed by capillary action between the faces of the parts to be joined
- No melting of the work occurs – the filler melts below the melting point of the base metal
- Well designed and brazed joint will become stronger than the filler metal out of which it has been formed.
- due to the metallurgical bonds that form between the small part clearances
- Typical applications include automotive (joining pipes etc.) electrical equipment and jewelry. - 28Pros and cons of brazing
- 29Adhesive Bonding (as gluing or glue bonding)- the application of an intermediate layer to connect substrates of different materials.
- Adhesive may be used interchangeably with glue or cement - 30Cyanoacrylates- Do not fill spaces
- exothermic reaction with natural fibres
- Water resistant
- Can be used as a binder for composite repair (Use baking soda as a filler)
- stems blood loss until hospital treatment.
- Dermabond medical adhesive
- 31Polyvinyl acetate (wood glue, school glue, white glue (PVA))- emulsion in water, PVAc emulsions are used as adhesives for porous materials
- Wood
- Paper
- Cloth
- Porous building stone such as sandstone - 32Epoxy Resin- a class of reactive prepolymers and polymers which contain epoxide groups (structural adhesives or engineering adhesives)
- High-performance
- In a linear epoxy resin, cross linking is achieved which presents a three-dimensional cross-linked structure which has superior mechanical strength.
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- Bachelors Materials & manufacturing Flashcards on Welding and Joining, created by Natalia Rehakova on 13/05/2016.
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- Natalia Rehakova
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