4.4. Physical model of the susceptibility of hot cracking for copper GTA welding
Variations of the hot cracking susceptibility during copper GTA welding could be divided into the following five stages combining the internal factor with the external factors, as shown in Fig. 14:
Fig. 14. Schematic of the physical model for the susceptibility of hot cracking during copper GTA welding.
(I). Free crystalline region The actual line of S-L is lower compared with its corresponding theoretical line because of the influence of under cooling and low-melting eutectic. The α-Cu is crystallized from the liquids when the welding pool cools down the actual line of S-L. In this region, the size of α-Cu is smaller and the quantity of α-Cu is fewer. These smaller …show more content…
Quasi-S phase region When the temperature continues to cool down from 1058 °C, the ductility of the welds increases. To illustrate, ductility at 1053 °C is 0.00298, which is higher than the corresponding ductility at 1058 °C. At 1058 °C, Δε during welding is 0.00483. The expansion velocity and susceptibility of hot cracking decrease as (Δε-P) decreases. The increase in ductility is related to the microstructure of the welds at a temperature below 1058 °C. In this temperature range, the amount of liquids among the α-Cu grains is minute, and the composition of the liquids resembles the eutectic of (Cu+Cu2O). Solidification of Cu+Cu2O eutectics at the tip of cracking hinders the propagation of crack between adjacent α-Cu grains, and in some degree leads to the increase of strength. Thus, the strength of the welds increases from 10.3 Mpa at 1058 °C to 12.2 Mpa at 1053 °C. At 1020 °C, which is close to the lower limit of the welds’ S-L range, the ductility of the welds increases to 0.00493, which is very close to the Δε (0.00508) during welding. The above result indicates that (Δε-P) is very small and cannot trigger the continuous expansion of hot cracking. The susceptibility of hot cracking declines …show more content…
13. At 300 °C, Δε declines compared with the corresponding Δε without preheating, and Pmin is exhibited when Δε300°C = 0.75Δεwithoutpreheating at the temperature lever. The variations of Δε (line II) and Pmin (line IV) intersected each other, which indicates that hot cracking inevitably occurs even if the actual deformation of the weld metal preheated to 300 °C declined in some degree. Then, the 10 mm-thick plates were welded at a preheating temperature of 300 °C. Hot cracking is located at the surface and inner part of the welds, as shown in Fig. 18. The surface crack rate is 25.1% and the section crack rate is 51.3%, which are lower than the crack rate without preheating. Δε continuously declines when the preheating temperature is 500 °C compared with the corresponding Δε when the preheating temperature is 300 °C, and Pmin is exhibited when Δε500°C = 0.12Δεwithoutpreheating at the temperature lever. The variations of Δε (line III) and Pmin (line IV) did not intersect with each other, which indicate that the actual deformation of the weld metal in BTR is smaller than the ductility of the inherent material, and the weld metal could afford the deformation. The above result satisfies equation (5) of the optimized hot cracking formation criterion. Therefore, hot cracking is inevitably inhibited and the 10 mm-thick
Variations of the hot cracking susceptibility during copper GTA welding could be divided into the following five stages combining the internal factor with the external factors, as shown in Fig. 14:
Fig. 14. Schematic of the physical model for the susceptibility of hot cracking during copper GTA welding.
(I). Free crystalline region The actual line of S-L is lower compared with its corresponding theoretical line because of the influence of under cooling and low-melting eutectic. The α-Cu is crystallized from the liquids when the welding pool cools down the actual line of S-L. In this region, the size of α-Cu is smaller and the quantity of α-Cu is fewer. These smaller …show more content…
Quasi-S phase region When the temperature continues to cool down from 1058 °C, the ductility of the welds increases. To illustrate, ductility at 1053 °C is 0.00298, which is higher than the corresponding ductility at 1058 °C. At 1058 °C, Δε during welding is 0.00483. The expansion velocity and susceptibility of hot cracking decrease as (Δε-P) decreases. The increase in ductility is related to the microstructure of the welds at a temperature below 1058 °C. In this temperature range, the amount of liquids among the α-Cu grains is minute, and the composition of the liquids resembles the eutectic of (Cu+Cu2O). Solidification of Cu+Cu2O eutectics at the tip of cracking hinders the propagation of crack between adjacent α-Cu grains, and in some degree leads to the increase of strength. Thus, the strength of the welds increases from 10.3 Mpa at 1058 °C to 12.2 Mpa at 1053 °C. At 1020 °C, which is close to the lower limit of the welds’ S-L range, the ductility of the welds increases to 0.00493, which is very close to the Δε (0.00508) during welding. The above result indicates that (Δε-P) is very small and cannot trigger the continuous expansion of hot cracking. The susceptibility of hot cracking declines …show more content…
13. At 300 °C, Δε declines compared with the corresponding Δε without preheating, and Pmin is exhibited when Δε300°C = 0.75Δεwithoutpreheating at the temperature lever. The variations of Δε (line II) and Pmin (line IV) intersected each other, which indicates that hot cracking inevitably occurs even if the actual deformation of the weld metal preheated to 300 °C declined in some degree. Then, the 10 mm-thick plates were welded at a preheating temperature of 300 °C. Hot cracking is located at the surface and inner part of the welds, as shown in Fig. 18. The surface crack rate is 25.1% and the section crack rate is 51.3%, which are lower than the crack rate without preheating. Δε continuously declines when the preheating temperature is 500 °C compared with the corresponding Δε when the preheating temperature is 300 °C, and Pmin is exhibited when Δε500°C = 0.12Δεwithoutpreheating at the temperature lever. The variations of Δε (line III) and Pmin (line IV) did not intersect with each other, which indicate that the actual deformation of the weld metal in BTR is smaller than the ductility of the inherent material, and the weld metal could afford the deformation. The above result satisfies equation (5) of the optimized hot cracking formation criterion. Therefore, hot cracking is inevitably inhibited and the 10 mm-thick