tailieunhanh - Advanced Welding Processes Episode 2 Part 2

Tham khảo tài liệu 'advanced welding processes episode 2 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 254 Advanced Welding Processes Figure Weld bead appearance for tests shown in figure . Good wire ơ gives smoother finish and low spatter level in comparison with poor wire Ố . e Deviation monitors Dynamic resistance. The quality of resistance spot welds may vary due to the surface conditions of the plate being welded electrode wear current shunting and transient changes in the welding parameters. Various methods of monitoring spot weld quality in order to detect the effect of these variations have been proposed but the dynamic resistance technique is probably the most suitable for industrial application 195 196 . In this system the instantaneous welding current and voltage are measured and the resistance of the metal between the electrodes is calculated. If computer-based techniques are used to collect the data the resultant dynamic resistance curve may be plotted on the monitor screen immediately after the weld has been completed. The normal dynamic resistance curve for a satisfactory weld in plain carbon steel is shown in figure . The curve may be divided into three zones i Zone A the area corresponding to the reduction of contact resistance. Monitoring and Control of Welding Processes 255 ii Zone B the area corresponding to resistive heating and increasing resistivity of the material. iii Zone C the area corresponding to a reduction in thickness and weld nugget growth with consequent steady reduction in resistance. ---Dynamic Resistance Figure Typical dynamic resistance curve for a normal spot weld. Changes in shape of the curve indicate deviations from the normal weld nugget formation and in the limit a defective weld. The computer-based dynamic resistance instrument 197 shown in figure may be used to monitor the performance of the process in the following manner. The welding parameters are set and a test specimen is welded with the monitor set in its test mode. The welded specimen is checked visually and by destructive testing and .