A certain model of solid rocket motor housing is made of high-strength steel 30CrMnSi, which is formed by strong spinning and vacuum electron beam welding.
Vacuum electron beam welding has the advantages of stable welding process, fast welding speed and high weld purity.
However, due to the long pumping time and low welding efficiency, it is difficult to meet the growing production demand.
Laser welding is a precise and efficient high-energy beam welding method, which has the characteristics of high energy density, fast welding speed, small welding deformation, and excellent weld quality.
It is widely used in automobile manufacturing, aerospace, medical equipment and other fields.
Compared with vacuum electron beam welding, laser welding has high efficiency, and the welding process is easy to be integrated, automated and flexible.
However, in the arc extinguishing stage of laser welding, especially under the keyhole effect of laser deep penetration welding, the molten metal has solidified without backfilling, and large and deep arc extinguishing pits will be formed at the arc extinguishing position, while the existence of arc extinguishing pits will bring adverse effects, such as failing to meet the requirements of weld surface quality, weakening the strength of the entire joint, and inducing cracks and other defects.
For the general welding structure, the arc crater can be completely eliminated by adding the arc crater, but the solid rocket motor shell is generally a circular thin-walled piece, which cannot be added due to the limitation of the weld structure.
Therefore, the size of the arc crater can only be controlled by technological means to reduce the adverse effects of the arc crater and meet the welding quality requirements.
Yu Hongjiang effectively avoided crater cracks of TC11 titanium alloy girth weld by using laser energy subsection attenuation mode.
Cao Haichun can effectively gradually increase and decrease the power through the adjustment to prevent the weldment from suddenly opening and closing due to the influence of the laser power switch, which will lead to porosity and ending crater problems in the weld.
In this article, based on the existing laser welding specifications, the single variable method is used to explore the influence of the arc extinguishing time, welding speed, and defocusing amount on the depth of the arc crater, and analyze the causes of the formation of the arc crater, optimize the welding process, and carry out radiographic inspection, microstructure observation and mechanical property test on the welds obtained from the optimized process, finally obtain the welding process parameters that meet the design requirements of a model of engine housing.
1. Test materials and equipment
TruDisk8001 laser is used in the test, with a maximum power of 8000 W, beam quality ≤ 4.5 mm · mrad, the wavelength of 1030 nm, photoelectric conversion ≥ 25%, and focal spot diameter of 0.4 mm.
The laser welding system is shown in Fig. 1.
Fig.1 High power flexible laser welding system
The thin wall cylinder with a diameter of 200 mm, a wall thickness of 2 mm and a length of 80 mm is used as the welding object.
The material is 30CrMnSi high-strength steel, and its chemical composition is shown in Table 1.
Table 1 Chemical composition of 30CrMnSi high-strength steel（wt.%）
2. Test process and results
2.1 Causes of arc crater formation
It is found that the weld formation can be controlled by changing the starting and stopping power of laser welding.
The output waveform of the laser can be divided into three parts according to time, as shown in Fig. 2.
The macro morphology of the weld is shown in Fig. 3. It can be seen that there is a hump at the arc starting position.
This is because the weld is not fully penetrated during the rising of laser energy, and the metal at the front of the molten pool is continuously stacked back to solidify to form a hump;
The arc crater appears at the arc extinguishing position, which is due to the reduction of laser energy during arc extinguishing and the reduction of the amount of molten metal in the front of the molten pool.
When the laser deep melting keyhole disappears, that is, when the laser transforms from deep penetration welding to laser thermal conduction welding, the amount of molten metal backfilled in the front of the molten pool cannot completely fill the holes generated by the laser deep melting keyhole, so the arc crater appears during arc extinguishing.
The above phenomena will cause the bulge and depression of the weld when the arc is started and stopped, especially the crater at the arc stop position, which will seriously affect the weld quality.
Fig.2 Waveform diagram of laser power output
Fig.3 Macro view of laser welding seam
2.2 Influence of process parameters on the depth of arc crater
QJ 20659-2016 Technical Requirements for Laser Welding of Structural Steels and Stainless Steels clearly stipulates the situation of incomplete welding.
Single incomplete weld defects are allowed in the first level weld standards.
The schematic diagram of incomplete weld defects is shown in Fig. 4.
The maximum depth of a single incomplete weld defect shall not be greater than 8% δ (δ is the thickness of the base metal), and the maximum cumulative length of local defects on any 100 mm length weld shall be less than or equal to 10 mm.
The thickness of the test base metal is 2 mm. According to the standard requirements, the maximum depth of the pit shall be less than 0.16 mm.
Fig.4 Schematic diagram of not fully welded
The laser welding butt test of 30CrMnSi cylinder was carried out, and the single variable method was used to explore the influence of welding speed, arc quenching time and defocusing amount on the crater depth.
A large number of laser welding process tests were carried out in the early stage, and the process parameters of stable welding process and good weld formation were obtained: laser power is 2400W, defocusing amount is+5 mm, welding speed is 1.2 m/min, arc extinguishing time is 2 s, shielding gas is 99.99% argon, shielding gas flow is 20 L/min, and shielding gas pressure is 0.2 MPa.
After the test, the pit depth is tested with a depth dial indicator.
The influence of process parameters on the depth of arc crater is shown in Fig. 5.
Fig.5 Influence of process parameters on the depth of arc crater
(1) The influence of arc extinguishing time on the pit depth.
Keep the defocusing amount at+5 mm and the welding speed at 1.2 m/min unchanged, and explore the influence of different arc extinguishing time (0~4 s) on the pit depth.
It can be seen from Figure 5a that when the arcing time is 0, the maximum depth of the arcing crater can reach 0.43 mm, which does not meet the standard requirements.
This is because the laser energy disappears suddenly, the laser deep melting hole is closed instantly, and the molten metal cannot completely fill the hole, so a large arc crater is formed;
When the arc extinguishing time increases from 0 s to 2 s, the crater depth decreases significantly.
This is because with the increase of arc extinguishing time, the attenuation gradient of laser power decreases, and the molten metal at the front of the laser deep melting hole has more time to fill the laser deep melting hole back and forth, so the crater depth decreases;
When the arcing time is more than 2s, the variation of the depth of the crater tends to be stable, about 0.12 mm.
(2) The influence of defocusing amount on pit depth.
Keep the arc extinguishing time 2 s and welding speed 1.2 m/min unchanged, and explore the influence of different defocusing amount (- 1~9 mm) on the pit depth.
It can be seen from Fig. 5b that with the increase of defocusing amount, the pit depth decreases.
This is because the defocusing amount increases, and the laser spot area on the weld surface increases, which leads to the widening of the weld and the enlargement of the molten pool.
When the laser deep melting keyhole disappears, the amount of molten metal that can be backfilled around the keyhole increases, which reduces the depth of the crater.
(3) The influence of welding speed on the pit depth.
Keep the arc extinguishing time for 2s and defocusing amount+5mm unchanged, and explore the influence of welding speed (0.48~1.68 m/min) on the pit depth.
It can be seen from Figure 5c that with the increase of welding speed, the depth of the crater decreases.
This is due to the increase of welding speed, the decrease of laser linear energy density, the decrease of metal vapor and spatter generated, the narrowing of the weld pool width but the increase of the length, resulting in the reduction of the depth of the crater.
2.3 Optimization of crater process
It can be seen from the above analysis and multiple tests that the arc extinguishing time has the greatest impact on the depth of the crater, but the depth of the crater changes little after the arc extinguishing time exceeds 2s, and the increase of the arc extinguishing time will lead to the increase of the weld overlap area and porosity, so the arc extinguishing time is determined as 2s.
The welding speed also has a great influence on the crater depth, considering that the welding speed increases in the arc extinguishing process.
The optimized process parameters are as follows: the shielding gas is 99.99% argon, the gas flow is 20 L/min, the shielding gas pressure is 0.2 MPa, the laser power is 2400 W, the defocusing amount is+5 mm, the arc extinguishing time is 2 s, the welding speed in the full power section is 1.2 m/min, and the welding speed from the arc extinguishing point is increased from 1.2 m/min to 2.4 m/min at a constant speed.
Under this process parameter, better welds and smaller arc extinguishing pits can be obtained.
The lap joint effect of arc extinguishing section weld is shown in Figure 6. The weld is even and full, with silver white metallic luster.
The X-ray film of the arc extinguishing section is shown in Fig. 7.
The number of weld pores is small and there is no crack defect.
The bright part of the front section is the arc starting and arc extinguishing overlapping position, and a small dark section of the middle section is the arc extinguishing pit position.
The maximum depth of the arc extinguishing pit measured by the depth dial gauge is 0.08 mm, which meets the requirements for Class I welds in the laser welding standard QJ 20659-2016.
Fig.6 Welding receiving arc segment overlap effect
Fig.7 X-ray film of the arc-retracting section
2.4 Microstructure and mechanical properties of welding seam in arc extinguishing section after process optimization
The section of weld specimen at the crater after process optimization is shown in Fig. 8.
The light color area of the upper semicircle is the weld seam in the arc stopping remelting zone.
The reason is that the laser power is gradually reduced when the arc is stopped, and the welding mode is changed from laser deep penetration welding to laser thermal conduction welding;
The dark area in the lower part is the weld seam in the deep penetration welding area, which is a dendritic structure with central symmetry.
Fig.8 Cross-section of the weld at the arc crater
The microstructure of the weld at the arc crater is shown in Fig. 9.
Fig. 9a shows the microstructure of deep penetration welding zone, which is mainly composed of bainite+pearlite+ferrite mixed structure, with a microhardness of about 400 HV;
Fig. 9b shows the microstructure of the arc stopping remelting zone, which is mainly composed of the mixed structure of martensite+bainite+ferrite.
The microhardness is about 570 HV. The reason why the hardness of this zone is higher than that of the deep penetration welding zone is that the weld has undergone laser remelting, which is equivalent to a quenching treatment, which increases the hardness of the remelting zone;
Fig. 9c The overheating zone near the fusion line is composed of coarse bainite+pearlite+ferrite, with a microhardness of about 370 HV;
Fig. 9d shows the microstructure at the junction between the heat affected zone and the base metal, which is composed of pearlite+ferrite, with a microhardness of about 240 HV.
The ferrite in the base metal area is distributed in a network, with a microhardness of about 200 HV.
Fig.9 Microstructure of the arc-receiving crater weld
In order to test the influence of the weld crater after process optimization on the tensile strength of the welded joint, the weld tensile test was carried out.
The test results are shown in Fig. 10.
The weld seams of 1 #, 2 # and 3 # tensile specimens have no crater, and the weld seams of 4 #, 5 # and 6 # tensile specimens are taken from the arc extinguishing section containing the crater.
The test results show that all tensile samples are broken in the base metal, and the tensile strength is about 1080MPa, which indicates that the strength of the welded joint at the crater after process optimization can reach the strength of the welded joint at the crater without crater, and the weld quality and mechanical properties meet the requirements.
Fig.10 Tensile sample breaks
(1) During arc extinguishing, the laser energy is reduced, and the amount of molten metal backfilled at the front of the molten pool can not completely fill the holes generated by the laser deep melting holes, so the arc crater appears during arc extinguishing.
(2) The arc quenching time and welding speed have a great influence on the arc quenching pit. When the arc quenching time is 2s and the welding speed is increased from 1.2 m/min to 2.4 m/min at a constant speed from the arc quenching point, better welds and smaller arc quenching pits can be obtained.
The depth of the arc quenching pit is 0.08 mm.
(3) The microstructure of the arc stop remelting weld is mainly composed of the mixed structure of martensite+bainite+ferrite, with a microhardness of about 570 HV.
The original weld microstructure is mainly composed of the mixed structure of bainite+pearlite+ferrite, with a microhardness of about 400 HV.
After the process optimization, the strength of the welding joint at the arc stop crater can reach the strength of the welding joint at the non arc stop crater, and the weld quality and mechanical properties meet the requirements.