Tham khảo tài liệu 'the welding of aluminum & its alloys part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | MIG welding 135 Welding procedures and techniques A set of outline welding procedures are given in Tables and for butt welding using either argon or helium as the shielding gas and guidance on parameters for fillet welding is illustrated in Fig. . The parameters quoted form a starting point from which to develop a procedure specifically designed for the application. They are not to be regarded as hard and fast rules. Also included as Table are suggested weld preparations for MIG welding of a range of plate thicknesses. Arc starting Because the wire is fed into the arc immediately that the arc is started there can be no preheating of the joint as possible with TIG. This results in shallow penetration and a humped weld bead on starting. Lack of fusion defects are often encountered - a cold start - and weld bead shape may not be acceptable. To avoid these defects the welder should strike the arc some 25 mm ahead of the desired start point and then move back to the weld start before beginning to weld forward at a normal speed. Arc starting may be achieved using a scratch start where the wire is allowed to protrude from the contact tip by 10 mm and brought to within 20 mm of the surface. The trigger is operated and at the same time the welding torch is moved to scrape the wire tip over the work surface. As soon as the arc is established the power source senses the change in voltage and starts the wire feed the weld pool forms and welding can commence. A running start is one where the wire begins to feed as soon as the trigger is operated and is short-circuited when it touches the workpiece establishing the arc. The current surge on short-circuiting may cause arcing within the contact tip and spatter to adhere to the shroud and contact tip. These can lead to wire feeding problems. As mentioned earlier the new inverter power sources have a facility for a highly controlled arc start the trigger is operated the wire is fed at a slow .
