Research On Characteristics Of Ultra High Laser Power Welding Of Thick 5A06 Plates Under Subatmospheric Pressures

With the development of industry,nuclear power ships,petrochemical industry and other fields have higher requirements for high performance thick-plate and medium thick-plate structure and welding quality.It is of great significance for the development of industry and the progress of society to realize the reliable connection of large thick wall structural parts.Laser welding under subatmospheric pressure can significantly improve the weld penetration depth,suppress the defects in the weld,suppress the plsma plume,and improve the stability of the welding process.However,in the current research on Laser welding under subatmospheric pressure,the laser power is usually below 10 k W.When the incident laser power exceeds 10 k W or even reaches 30 k W,there are few studies on the system.In this paper,the characteristics of ultra-high laser power welding under subatmospheric pressure are studied with 5A06 aluminum alloy as the research object.Firstly,the characteristics of ultra-high laser power welding under subatmospheric pressure process were studied,and the influences of vacuum degree,welding speed,defocus distance and laser power on weld forming were analyzed.It is found that ultra-deep welds with well-formed surfaces and almost no defects can be obtained by ultra-high power laser welding under high vacuum degree.Defocus distance has a great influence on weld forming.When improper defocus distance is selected,a large number of humps appear on the weld surface,and the optimal defocus distance is different under different powers.It was found that with the increase of vacuum degree,the porosity in the weld was obviously suppressed.When the vacuum degree reached 10 Pa,the porosity was almost not observed in the weld,and the ultra-high laser power welding under subatmospheric pressure process window was obtained.From the perspective of weld penetration depth variation,it was determined that the penetration depth mutation vacuum degree of aluminum alloy ultra-high power vacuum laser welding was 10⁴ Pa.At this time,the weld porosity was relatively high,but it still met the standard under certain circumstances.The plasma plume characteristics under different vacuum degrees were analyzed and the effect on energy transmission was studied.The study found that with the increasing vacuum degree,the average grayscale,height,area and standard deviation of the plume decreased,which indicated that the vacuum environment could significantly inhibit the plume behavior in the welding process and improve the stability of the welding process,and established a linear relationship between the grayscale,height,area of plasma plume and vacuum degree.With the increase of vacuum degree,the intensity,electron temperature and electron density of plasma plume spectral line decrease obviously.When the vacuum degree decreased from 2.5×10⁴Pa to 103 Pa,the plasma plume temperature decreased from 6,500 K to 5,100 K,and the plasma plume electron density decreased from 4.103×1017 cm^-3 to 2.448×1017 cm^-3.The refraction of the plasma plume on the incident laser increases the effective spot area,but does not consume the energy of the incident laser.When the vacuum degree increased from 5×10⁴ Pa to 10 Pa,the average deflection angle of the center of the detected laser spot decreased from 1.19 mrad to 0.097 mrad.The effective spot area was reduced from 9.94 mm2 to 6.01 mm2.Compared with the reduction of laser power,it is more effective to increase the vacuum degree during the welding process.Combined with the above process test and welding process analysis,the welding parameters were optimized to obtain a single 10⁴ mm non-defective super deep weld of 5A06 aluminum alloy,which was in the shape of a slender spindle.Inspired by the improvement of penetration depth and weld quality resulting from laser welding under subatmospheric pressures,laser welding of 130 mm 5A06 aluminum alloy butt joint was implemented with 10 Pa ambient pressure,-35 mm defocus distance,0.3 m/min welding speed,and 30 k W laser power from both sides of the plate.Before welding,the surface of the two plates were smoothed without machining groove.The joint is well formed with few defects.The minimum tensile strength of the weld joint for double-sided welding can reach 95% of the base metal,and the central lap joint area has higher strength and hardness,which is related to the relatively high content of Mg elements in this area.Compared with the grain size,Mg element content has a more significant effect on the weld performance.Studies above provide a more solid theoretical foundation for further study.For the welding of thick-plate structure,studies above provide a new style of thought,make it possible to process large thick-plate structure more efficiently.It provides a theoretical basis for the development and application of local vacuum welding equipment.