Welding Processes - Warsaw University of Technology
Welding Processes A Brief History of Welding Late 19th Century Scientists/engineers apply advances in electricity to heat and/or join metals (Le Chatelier, Joule, etc.) Early 20th Century Prior to WWI welding was not trusted as a method to join two metals due to crack issues 1930s and 40s Industrial welding gains acceptance and is used extensively in the war effort to build tanks, aircraft, ships, etc. Modern Welding
the nuclear/space age helps bring welding from an art to a science Types of Welding Fusion Welding Homogeneous Gas Welding Electroslag High Energy Beam Electric Arc Pressure Welding
Heterogeneous Brazing Friction Welding Soldering MIG TIG Shielded Metal Arc Stick Weldability of a Metal Metallurgical Capacity Parent metal will join with the weld metal without
formation of deleterious constituents or alloys Mechanical Soundness Joint will be free from discontinuities, gas porosity, shrinkage, slag, or cracks Serviceability Weld is able to perform under varying conditions or service (e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.) Fusion Welding Principles Base metal is melted
Filler metal may be added Heat is supplied by various means Oxyacetylene gas Electric Arc Plasma Arc Laser Fusion Welding
ELECTRODE COATING CORE WIRE WELDING ATMOSPHERE ARC STREAM ARC POOL SOLIDIFIED SLAG PENETRATION DEPTH WELD BASE METAL Weld Metal Protection
During fusion welding, the molten metal in the weld puddle is susceptible to oxidation Must protect weld puddle (arc pool) from the atmosphere Methods Weld Fluxes Inert Gases Vacuum Weld Fluxes Typical fluxes SiO2, TiO2, FeO, MgO, Al2O3 Produces a gaseous shield to prevent
contamination Act as scavengers to reduce oxides Add alloying elements to the weld Influence shape of weld bead during solidification Inert Gases Argon, helium, nitrogen, and carbon dioxide Form a protective envelope around the weld area Used in MIG
TIG Shield Metal Arc Vacuum Produce high-quality welds Used in electron beam welding Nuclear/special metal applications Zr, Hf, Ti Reduces impurities by a factor of 20 versus other methods Expensive and time-consuming
Types of Fusion Welding Oxyacetylene Cutting/Welding Shielded Metal Arc (Stick) Metal Inert Gas (MIG) Tungsten Inert Gas (TIG) Oxyacetylene Welding Flame formed by burning a mix of acetylene
(C2H2) and oxygen TORCH TIP Inner Cone: 2700-3500 deg C 1250 deg C Combustion Envelope 2100deg C Fusion of metal is achieved by passing the inner cone of the flame over the metal Oxyacetylene can also be used for cutting metals Shielded Metal Arc (Stick)
An electric arc is generated between a coated electrode and the parent metal The coated electrode carries the electric current to form the arc, produces a gas to control the atmosphere and provides filler metal for the weld bead Electric current may be AC or DC. If the current is DC, the polarity will affect the weld size and application Shielded Metal Arc (cont) Process: Intense heat at the arc melts the tip of the
electrode Tiny drops of metal enter the arc stream and are deposited on the parent metal As molten metal is deposited, a slag forms over the bead which serves as an insulation against air contaminants during cooling After a weld pass is allowed the cool, the oxide layer is removed by a chipping hammer and then cleaned with a wirebrush before the next pass. Inert Gas Welding For materials such as Al or Ti which quickly form oxide layers, a method to
place an inert atmosphere around the weld puddle had to be developed Metal Inert Gas (MIG) Uses a consumable electrode (filler wire made of the base metal) Inert gas is typically Argon CONSUMABLE ELECTRODE DRIVE WHEELS POWER
SOURCE SHIELDING GAS BASE METAL ARC COLUMN PUDDLE Tungsten Inert Gas (MIG) Tungsten electrode acts as a cathode A plasma is produced between the tungsten cathode and the
base metal which heats the base metal to its melting point Filler metal can be added to the weld pool TUNGSTEN ELECTRODE POWER SOURCE TUNGSTEN ELECTRODE (CATHODE) ++
SHIELDING GAS BASE METAL ARC COLUMN PUDDLE ++ --BASE METAL (ANODE) Welding Positions
A wide T will exist between base metal and HAZ. Preheating and cooling methods will affect the brittleness of the metal in this region Blowholes Are cavities caused by gas entrapment during the solidification of the weld puddle. Prevented by proper weld technique (even temperature and speed) Weld Defects Inclusions Impurities or foreign substances which are forced into the weld puddle during the welding process. Has the same effect as a crack.
Prevented by proper technique/cleanliness. Segregation Condition where some regions of the metal are enriched with an alloy ingredient and others arent. Can be prevented by proper heat treatment and cooling. Porosity The formation of tiny pinholes generated by atmospheric contamination. Prevented by keeping a protective shield over the molten weld puddle. Residual Stresses
Rapid heating and cooling results in thermal stresses detrimental to joint strength. Prevention Edge Preparation/Alignment beveled edges and space between components to allow movement Control of heat input skip or intermittent weld technique Preheating reduces expansion/contraction forces (alloys) and removes moisture from the surface Peening help metal stretch as it cools by hitting with a hammer. Use with care since it may work harden the metal Heat Treatment soak the metal at a high temperature to relieve stresses Jigs and Fixtures prevent distortion by holding metal fixed
Number of Passes the fewer the better. Joint Design BUTT JOINT FILLET JOINT STRAP JOINT LAP JOINT CORNER JOINT Generalized Welding Symbol
FAR SIDE DETAILS Weld Geometry Electrode Material D L1-L2 D L1-L2
ARROW SIDE DETAILS Field weld symbol Weld all-around for pipes, etc. D = Weld Depth (usually equal to plate thickness) L1 = Weld Length L2 = Distance between centers for stitched welds The Field Weld Symbol is a guide for installation. Shipyards
normally do not use it, except in modular construction. Example Welding Symbol Geometry symbol for V-groove One-sided welds are max 80% efficient Two sided are 100% efficient 1/2 1/2 1/2 1/2
Weld Symbols (Butt Joints) Backing Weld Symbol (Fillet Joints) Weld Symbol (Corner Joints)
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