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Advanced Welding Processes Episode 1 Part 8

Tham khảo tài liệu 'advanced welding processes episode 1 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 134 Advanced Welding Processes material. Once the droplet has been lifted in this way an asymmetrical magnetic field is created and the droplet may be rotated or expelled under the influence of the resultant forces as discussed below. This mode of transfer is usually undesirable due to the poor stability and high spatter levels which result. Figure 7.3 Repelled transfer. 7.2.3 Projected spray transfer As the current is increased the size of the droplet usually decreases and the frequency of transfer increases. In addition it is found that the droplets are projected axially through the arc with some force. In some cases e.g. carbon steel in argon-rich gas mixtures and aluminium in argon there is a clear transition between the globular and projected spray modes of transfer as the current is increased figure 7.4 . The current at which this transition occurs is an important process characteristic and is known as the spray transition current. Its value depends on the filler material size and composition as well as the composition of the shielding gas. Typical values for steel are shown in table 7.1. Below the transition current the transfer is either globular or dip and above the transition Gas Metal Arc Welding 135 current the transfer is in the form of a steady stream of small droplets whose diameter is similar to that of the filler wire. Since this mode of transfer only occurs at relatively high currents the heat input is high and the weld pool large. These features are attractive for high-deposition-rate downhand welding but limit the positional capabilities of the process. Current A a b _ Droplet volume ------Transfer frequency Figure 7.4 Transfer characteristics for 1.2 mm diameter stainless steel wire in argon 2 oxygen. After Balraj V and Norrish J 1987 Pulsed MIG welding of stainless steel with controlled metal transfer Proc. Ann. Conf. Indian Institute of Welding Madras Indian Institute of Welding pp. 893-907. Table 7.1 Spray transition currents for plain .