Welding of nickel and nickel alloys

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I. Selection of welding methods: Based on the weldability characteristics of nickel and nickel alloys, the choice of welding process is crucial for successfully welding these materials. ·Production practice has shown that there are various methods for welding this material; different welding techniques can be chosen depending on the specific production conditions and the requirements regarding structural properties. Such as: shielded metal arc welding, submerged arc automatic welding, TIG, MIG, diffusion welding, resistance spot welding, seam welding, and butt welding; plasma arc welding, electron beam welding, and brazing can also be used. However, Tungsten Inert Gas Welding (TIG) and Shielded Metal Arc Welding are more commonly used in production. II. Pre-welding preparation: First, it is necessary to remove grease, paint, oil residues, as well as oxide films and other contaminants from the surface of the workpiece before welding. Surface oxide films and stains, as well as reducing oxides, also form on the surface in a heating atmosphere. Compared to welded steel, fusion welding of nickel-based alloys is characterized by low penetration; it results in a small weld pool and poor fluidity of the filler metal. From the perspective of welding performance, it is not advisable to use a high wire energy to increase penetration, in order to prevent the welding material from overheating, excessive burnout of deoxidizing elements, and poor weld formation caused by excessive agitation of the weld pool. To ensure penetration, a joint design with a large groove angle and a small root gap should be selected. III. Preheating and Post-weld Heat Treatment: Rolled nickel-based alloys generally do not require preheating, but when the temperature of the base material is below 15°C, the area around the joint, over a width of 250–300 mm, should be heated to 15–20°C to prevent moisture condensation from causing pores in the weld. The interlayer temperature should be strictly controlled; in practical production, it is generally kept below 100°C to reduce overheating. Although stabilization treatment is sometimes applied to prevent intergranular corrosion or stress corrosion during use, post-weld heat treatment is generally not recommended. IV. Tungsten Inert Gas Welding Process: Tungsten inert gas welding is the most widely used welding method in the production of nickel-based alloys. It generally employs direct current positive polarity, high-frequency arc initiation, as well as current attenuation and delayed gas shutoff techniques. (1) As a shielding gas, argon must be dry and of high purity, and argon should also be supplied to the back side for protection. (2) Tungsten electrodes are usually cerium-tungsten electrodes, shaped into a pointed form with a tip diameter of 0.4 mm and an angle of 30–60 degrees, which ensures stable arcing and sufficient penetration depth. Care should be taken to prevent the tungsten electrode from contacting the molten pool; any contamination on the tip must be ground off. (3) The selection of welding wire is key to determining the quality and performance of the welded joint. The welding wire used for TIG is mostly comparable to the composition of the base metal. (4) Process characteristics · During welding, a short arc and fast welding speed should be used. ·Slight movements are allowed during operation, but the angle of the welding torch and wire must be controlled properly. During multi-layer welding, the interlayer temperature should be controlled to not exceed 100°C. Make sure to fill the arc crater. The wire energy should be as low as possible while ensuring penetration. Nickel-based alloy weld pools have poor fluidity of the liquid metal, resulting in a shallow penetration depth; therefore, it is necessary to pay attention to observing the weld pool during welding to prevent defects such as pores and incomplete penetration. Rapid cooling should be applied after welding.