Welding课件.ppt

Welding Processes EN358 – Ship Structures 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 1930’s and 40’s 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 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 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