Steels-plain carbon & c-alloy steels-titanium & its alloys, Mechanical Engineering


Plain carbon & C - Mn steels:

Because of low heat input, in as welded condition, the micro structures of the autogenous process welds predominantly show martensitic which characteristically high hardness but generally poor toughness. The heat affected zone can be very narrow & small. Various research studies shows What it is possible to modify & improve the weld composition by adding filler materials in the form of wire or foils to the weld. Under that circumstances, acicular ferrite can be formed which could lead to improvement in toughness. For obtaining adequate electron beam weld mechanical properties in thick structural steel is to employ a stress relief heat treatment on low heat input welds in steel with limited micro alloy additions. Plain carbon & C - Mn steels are prone to solidification cracking if sulphur & phosphorous levels are too high, particularly at high welding speed. It will affect the toughness, Therefore the sulphur content should be less than 0.005 weight percentage for best results.

Roberts et al have demonstrated that electron beam welds in a steel to SAE 5046, containing 0.47 % C, 0.003 % S & 0.016 % P were after being normalised, oil quenched & tempered virtually indistinguishable from the parent material and even the fatigue performance was excellent where a limit of 230 MPa at 108 cycles was established using stress reversal technique on fully machined weld.

Alloy Steels As with C & C - Mn steels,

The basic metallurgy is similar to that of arc welding, low sulphur & phosphorous contents are beneficial if good mechanical properties are required. Electron beam welded samples of Z1N steel quenched & tempered 0.15 % C, 2.8 % Ni & 1.4 % Cr steel exhibited dramatic change in toughness with relatively small changes in compositions.

Titanium & its alloys

Because of titanium's high strength to weight ratio & exceptional corrosion resistance, it is widely used in the aerospace industry. Welding of titanium is similar to welding of stainless steels with two important considerations. Titanium requires greater cleanliness and auxiliary inert gas shielding / vacuum must be used. Because above 650 °C, Ti is extremely very reactive with nitrogen, oxygen & hydrogen and tend to embritted. Hence, ductile welds cannot be produced by GTAW or other welding process using active gases, coated electrodes or fluxes. EBW is an excellent process for joining titanium, because the normal operating vacuum provide the required purity of atmosphere. Deep penetration is produced by the high power density. Defocused beam, will melt the metal at the surface and produce a shallow fused zone by conduction and convection as in the arc welding. Any weldable titanium alloy can be welded by the electron beam process with joint mechanical properties equal to or better than attainable by the GTAW process. When welding over 12 mm thick electron beam welding is competitive from the cost point, because GTAW involves machining and filler metal requirements with groove welds. Single pass butt welding of 18 mm to 50 mm inches thickness can only be accomplished now by EBW. From properties standpoint, EB welding shows excellent tensile and fatigue properties with reduced fracture toughness for alpha - beta alloys and appears to have almost 100 % joint efficiencies for tensile and fracture toughness in beta alloys.

Copper and its alloys

In order to improve the production rate and automate, the conventional TIG or MIG welding method has been partially applied to thin copper plate with the conventional fusion welding method with low energy density, it is comparatively difficult to weld thick copper plate owing to its high thermal diffusivity and it can melt material instantaneously. Electrolytic Tough Pitch (ETP) copper is not readily weldable by many conventional processes because of the risk of cracking, caused by the evolution of steam by combination of cuprous oxide & hydrogen. Since no hydrogen is present in the vacuum environment, ETP copper is easily weldable by elect ron beam. Deoxidised Low Phosphorous (DLP) copper is almost as EB weldable as Oxygen Free High Conductivity (OFCH) copper.

Posted Date: 9/17/2012 7:36:12 AM | Location : United States

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