Why do pores appear
1.1 The inside of the laser welded hole is in a state of unstable vibration, and the flow between the hole and the molten pool is very intense. The metal steam inside the hole explodes outwards, causing steam vortex at the opening of the hole, which will inject the protective gas (Ar) into the bottom of the hole. As the hole moves forward, the protective gas will enter the molten pool in the form of bubbles. Because the solubility of Ar is very low, coupled with the cooling speed of laser welding is very fast, the bubbles can not escape and are left in the weld, forming pores. In addition due to poor protection caused by. During the welding process, nitrogen intrudes into the molten pool from the outside, and the solubility of nitrogen in liquid iron is very different from that of nitrogen in solid iron, thus during the cooling and solidification process of the metal. Since the solubility of nitrogen decreases with the decrease of temperature, when the molten pool metal is cooled to the beginning of crystallization, the solubility will suddenly drop sharply, and a large number of gases will precipitate to form bubbles. If the floating speed of the bubbles is less than the crystallization speed of the metal, pores will be formed.
Porosity suppression by laser welding
1 Inhibit welding pores by surface treatment before welding
The surface treatment before welding is an effective method to control the metal pores in laser welding of aluminum alloy. The surface treatment methods usually include physical mechanical cleaning and chemical cleaning.
經過對比，采用化學方法處理試板表面（金屬清洗劑清洗-水洗-堿洗-水洗-酸洗-水洗-干燥）的流程處理最好。其中堿洗用25%NaOH（氫氧化鈉）水溶液去除材料表面厚度，酸洗用 20% HNO3（硝酸）+2% HF（氟化氫）水溶液中和殘留的堿液。試板表面處理后在24小時內實施焊接，試板處理后停留時間較長時焊前裝配再用無水酒精擦拭。
After comparison, the process of chemical treatment of the test plate surface (metal cleaning agent cleaning - water washing - alkali washing - water washing - pickling - water washing - drying) is the best. Among them, 25%NaOH (sodium hydroxide) aqueous solution is used to remove the surface thickness of the material in alkaline washing, and 20% HNO3 (nitric acid) +2% HF (hydrogen fluoride) aqueous solution is used to neutralize the residual lye in pickling. After the surface treatment of the test plate, welding shall be carried out within 24 hours. If the test plate stays for a long time after treatment, the assembly shall be wiped with anhydrous alcohol before welding.
2. Inhibit welding porosity through welding process parameters
The formation of weld porosity is not only related to the quality of weldment surface treatment, but also related to welding process parameters. The influence of welding parameters on the porosity of the weld is mainly reflected in the weld penetration, that is, the influence of the weld back width ratio on the porosity.
As shown in the figure above, Ar (argon) and He (helium) are used to protect the weld surface. In the process of laser welding of aluminum alloy, the degree of ionization of Ar and He to the laser is different, resulting in different weld forms. The result shows that the porosity of the weld obtained by using Ar as the shielding gas is lower than that by using He as the shielding gas.
At the same time, we should also note that a large amount of plasma generated by welding with too small gas flow (<10L/min) cannot be blown away, making the welding pool unstable and increasing the probability of porosity formation. The gas flow is moderate (about 15L/min), the plasma is effectively controlled, and the protective gas plays a good anti-oxidation effect on the solution pool, with the least porosity. Excessive gas flow accompanied by excessive gas pressure causes part of the protective gas to mix into the solution pool, resulting in increased porosity.
Affected by the properties of the material itself, the phenomenon of non-porosity can not be completely avoided in the welding process, and the porosity can only be reduced.