Preface
A specific model of solid rocket motor casing is made from 30CrMnSi high-strength steel, which is produced through a combination of spinning and vacuum electron beam welding.
Vacuum electron beam welding offers a stable welding process and a high degree of weld purity, but is limited by its long pumping time and low efficiency, making it challenging to meet growing production demands.
Laser welding, on the other hand, is a highly precise and efficient high-energy beam welding method that features high energy density, fast welding speed, minimal welding deformation, and outstanding weld quality. It is commonly utilized in the manufacturing of automobiles, aerospace equipment, medical devices, and other industries.
Compared to vacuum electron beam welding, laser welding is more efficient and the welding process can be easily integrated, automated, and made flexible.
However, during the arc extinguishing phase of laser welding, particularly in the case of laser deep penetration welding, the molten metal may solidify without backfilling, resulting in large, deep arc extinguishing pits at the arc extinguishing position. These pits can negatively impact weld surface quality, weaken the strength of the entire joint, and cause cracks and other defects.
For most welding structures, the arc crater can be eliminated by adding filler material, but this is not possible in the case of solid rocket motor shells, which are typically circular and thin-walled pieces. In this case, the size of the arc crater can only be controlled through technological means to minimize its adverse effects and meet welding quality standards.
Yu Hongjiang successfully prevented the formation of crater cracks in the girth weld of TC11 titanium alloy by utilizing the laser energy subsection attenuation mode.
Cao Haichun effectively controlled the power output by gradually increasing and decreasing it through adjustments. This prevented the weldment from suddenly opening and closing, which would have led to porosity and excessive crater problems.
In this study, the single variable method was employed based on existing laser welding specifications to investigate the impact of arc extinguishing time, welding speed, and defocusing amount on the depth of the arc crater.
The causes of the formation of the crater were analyzed, and the welding process was optimized. Radiographic inspection, microstructure observation, and mechanical property testing were conducted on the optimized welds to determine the welding process parameters that meet the design requirements of an engine housing model.
1. Test materials and equipment
The TruDisk8001 laser is utilized in the test, with a maximum power of 8000 W, beam quality of ≤ 4.5 mm·mrad, a wavelength of 1030 nm, a photoelectric conversion rate of ≥ 25%, and a 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 cylinder, which has a diameter of 200 mm, a wall thickness of 2 mm, and a length of 80 mm, is used as the welding object and is made of thin walls. The material used is 30CrMnSi high-strength steel, with its chemical composition displayed in Table 1.
Table 1 Chemical composition of 30CrMnSi high-strength steel（wt.%）
| C | Si | Mn | Cr | Ni | S+P | Fe |
| 0.27~0.34 | 0.90~1.10 | 0.90~1.10 | 0.90~1.10 | ≤0.030 | ≤0.025 | Rem. |
2. Test process and results
2.1 Causes of arc crater formation
It has been discovered that the formation of the weld can be influenced by adjusting the starting and stopping power of laser welding.
The laser output waveform can be divided into three segments, as illustrated in Figure 2.
The macro-structure of the weld is displayed in Figure 3, and it can be observed that there is a bump at the point where the arc starts. This is due to the fact that the weld is not fully penetrated as the laser energy rises, causing the metal at the front of the molten pool to continuously solidify and form a bump.
At the point where the arc is extinguished, an arc crater is present, which is the result of the reduction of laser energy and the decrease in the quantity of molten metal at the front of the molten pool.
As the laser deep melting keyhole disappears, meaning the laser transitions from deep penetration welding to laser thermal conduction welding, the backfilled molten metal cannot completely fill the holes created by the deep melting keyhole, resulting in the appearance of an arc crater during arc extinguishing.
These phenomena will lead to bulges and indentations in the weld when the arc is started and stopped, with the crater at the arc stop position having a particularly negative effect on the quality of the weld.
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
The “QJ 20659-2016 Technical Requirements for Laser Welding of Structural Steels and Stainless Steels” clearly outlines the specifications for incomplete welding.
According to the first level weld standards, single incomplete weld defects are acceptable.
Figure 4 provides a schematic representation of incomplete weld defects.
The depth of a single incomplete weld defect must not exceed 8% of the base metal’s thickness (δ), with the sum of local defects along any 100mm length of the weld being no more than 10mm.
With a base metal thickness of 2mm, the standard requires that the maximum depth of the pit should be less than 0.16mm.
Fig.4 Schematic diagram of not fully welded
The 30CrMnSi cylinder underwent a laser welding butt test, and the single variable method was employed to determine the effect of welding speed, arc quenching time, and defocusing amount on crater depth.
In the initial stages, a large number of laser welding process tests were conducted to identify the process parameters for a stable welding process and good weld formation. The resulting parameters were found to be: a laser power of 2400W, a defocusing amount of +5 mm, a welding speed of 1.2 m/min, an arc extinguishing time of 2s, a shielding gas of 99.99% argon with a flow rate of 20 L/min and a pressure of 0.2 MPa.
After the tests, the crater depth was measured using a depth dial indicator.
The impact of the process parameters on the arc crater depth is illustrated 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 constant, and investigate the impact of varying arc extinguishing time (0-4 s) on pit depth.
As shown in Figure 5a, when the arc time is 0, the maximum depth of the arc crater reaches 0.43 mm, which fails to meet the standards. This is because the laser energy abruptly disappears, causing the laser deep melting hole to close instantly and preventing the molten metal from completely filling the hole, resulting in a large arc crater.
As the arc extinguishing time increases from 0 s to 2 s, the crater depth decreases noticeably. This is due to the fact that as the arc extinguishing time increases, the gradient of laser power reduction decreases, allowing the molten metal at the front of the laser deep melting hole more time to flow back and forth and fill the hole, reducing the crater depth.
When the arc time exceeds 2 s, the depth of the crater remains relatively stable at around 0.12 mm.
(2) The influence of defocusing amount on pit depth.
Keep the arc extinguishing time at 2 seconds and the welding speed at 1.2 meters per minute unchanged and examine the effect of varying defocusing amounts (-1 to 9mm) on pit depth.
As shown in Figure 5b, the pit depth decreases as the defocusing amount increases.
This is because a larger defocusing amount leads to an increased laser spot area on the weld surface, causing the weld to widen and the molten pool to enlarge.
When the laser’s deep melting keyhole disappears, the amount of molten metal that can be backfilled around the keyhole increases, resulting in a reduced depth of the crater.
(3) The influence of welding speed on the pit depth.
Keep the arc extinguishing time at 2 seconds and the defocusing amount at +5mm, and investigate the impact of varying the welding speed (ranging from 0.48 to 1.68 m/min) on pit depth.
As illustrated in Figure 5c, an increase in welding speed leads to a reduction in pit depth. This is because a faster welding speed results in a decrease in laser linear energy density, a reduction in metal vapor and spatter production, a narrower weld pool width but a longer length, all of which contribute to a shallower pit depth.
2.3 Optimization of crater process
The analysis and multiple tests reveal that the arc extinguishing time has the most significant impact on the crater depth. However, once the arc extinguishing time exceeds 2 seconds, the crater depth changes little. An increase in the arc extinguishing time will result in an increased weld overlap area and porosity, so the arc extinguishing time was determined as 2 seconds.
The welding speed also significantly affects the crater depth, especially during the arc extinguishing process where the welding speed increases.
The optimized process parameters are as follows: the shielding gas is 99.99% argon with a flow rate of 20 L/min and pressure of 0.2 MPa. The laser power is set at 2400 W with a defocusing amount of +5 mm. The arc extinguishing time is 2 seconds, and the welding speed in the full power section is 1.2 m/min. The welding speed from the arc extinguishing point is gradually increased from 1.2 m/min to 2.4 m/min at a constant speed.
With these process parameters, it is possible to achieve better welds and smaller arc extinguishing pits.
The lap joint effect of the arc extinguishing section weld is illustrated in Figure 6. The weld is even, full, and has a silver-white metallic luster.
The X-ray film of the arc extinguishing section is shown in Figure 7. The number of weld pores is minimal, and there are no cracks or defects.
The bright part at the front section represents the arc starting and arc extinguishing overlapping position, while the small dark section in the middle section represents the arc extinguishing pit position.
The maximum depth of the arc extinguishing pit, as measured by the depth dial gauge, is 0.08 mm, which meets the requirements for Class I welds according to 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 the weld specimen at the crater after the process optimization is depicted in Figure 8. The upper semicircle’s light-colored area represents the weld seam in the arc-stopping remelting zone.
The reason for this is that the laser power is gradually reduced when the arc is stopped, causing a change in the welding mode from laser deep penetration welding to laser thermal conduction welding. The dark area in the lower part represents the weld seam in the deep penetration welding area, which exhibits 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 displayed in Figure 9.
Figure 9a depicts the microstructure of the deep penetration welding zone, which is primarily composed of a mixture of bainite, pearlite, and ferrite and has a microhardness of around 400 HV.
Figure 9b displays the microstructure of the arc stopping remelting zone, which is mainly composed of a mixture of martensite, bainite, and ferrite, and has a microhardness of approximately 570 HV. The higher hardness of this zone compared to the deep penetration welding zone is due to the laser remelting process, which serves as a quenching treatment and results in increased hardness of the remelting zone.
Figure 9c illustrates the overheating zone near the fusion line, which is composed of coarse bainite, pearlite, and ferrite and has a microhardness of around 370 HV.
Figure 9d presents the microstructure at the junction between the heat affected zone and the base metal, which is composed of pearlite and ferrite, with a microhardness of approximately 240 HV. The ferrite in the base metal area is arranged in a network-like pattern and has a microhardness of approximately 200 HV.
Fig.9 Microstructure of the arc-receiving crater weld
To evaluate the impact of the weld crater after process optimization on the tensile strength of the welded joint, a weld tensile test was conducted. The results are displayed in Figure 10.
The tensile specimens numbered 1, 2, and 3 had no crater in the weld seams, while the tensile specimens numbered 4, 5, and 6 were taken from the arc extinguishing section and contained the crater.
The results indicated that all tensile samples broke in the base metal, with a tensile strength of approximately 1080 MPa. This suggests that the strength of the welded joint at the crater after process optimization is comparable to the strength of the welded joint without a crater, and that the weld quality and mechanical properties are in line with the requirements.
Fig.10 Tensile sample breaks
3. Conclusion
(1) During arc extinguishing, the laser energy is reduced, resulting in the inability of the amount of molten metal backfilled at the front of the molten pool to fully fill the holes created by the laser’s deep melting. This leads to the formation of arc craters.
(2) The duration of arc quenching and the welding speed greatly affect the size of the arc quenching pit. By increasing the welding speed from 1.2 m/min to 2.4 m/min while keeping the arc quenching time constant at 2s, it is possible to produce better welds with smaller arc quenching pits.
(Note: The depth of the arc quenching pit is 0.08 mm.)
(3) The microstructure of the remelted arc stop weld is primarily composed of a mixture of martensite, bainite, and ferrite, resulting in a microhardness of approximately 570 HV.
In comparison, the original weld microstructure was primarily composed of bainite, pearlite, and ferrite, with a microhardness of approximately 400 HV.
After optimizing the process, the strength of the welding joint at the arc stop crater has been improved to match that of the non-arc stop crater, ensuring that the weld quality and mechanical properties meet the necessary requirements.
