Porosity Formation in Weld Metal (Report 1)
Effect of hydrogen on porosity formation in pure aluminum at non-arc melting
1973 Volume 42 Issue 3 Pages 225-235
Details
1973 Volume 42 Issue 3 Pages 225-235
In practical welding of aluminum, there are many factors causing porosity in aluminum weld metal such as welding speed, arc voltage, arc current, and shielding gas composition.
This study has been done to establish the relationship between the solubility of hydrogen in molten aluminum and the formation of porosity under the given conditions of the temperature of molten aluminum and partial pressure of hydrogen.
The results are summarized as follows:
1) Pure aluminum was melted at various hydrogen partial pressures and temperatures by using the levitation melting apparatus, molten aluminum was allowed to fall into a specially designed copper mold, which gave the similar cooling rate as weld specimen. Porosity of this solidified specimen was checked by X-ray, section, and gravity inspections respectivly. Porosity concentration and its size increased with an increasing hydrogen partial pressure and temperature. Porosities were observed in the final solidification part, in the grain or at the edge of grain.
2) It was found that a driving force for the porosity formation in aluminum was strongly correlated with the solubility gap between the solubilities of hydrogen near the melting point (liquid state) and at an arbitrary temperature of molten aluminum. That is, when this solubility gap was large, many porosities were observed. For instance, porosities amounted to about 30% of the bulk of aluminum under the condition of 100% hydrogen and 1100°C, but no porosity was observed under the condition of 100% hydrogen and 700°C. From this fact, it is concluded that gas bubbles are not formed at solid-liquid interface but are already formed (corresponding to its solubility gap) in liquid aluminum under a rapid cooling condition.
3) Floating velocity of gas bubbles in liquid aluminum was calculated by assuming the size of gas bubbles. Form this calculation, it was found that the small size of gas bubbles had not given enough time for them to float out from the bulk to atmosphere under a rapid cooling condition.
