6061-t651 aluminum alloy thick plate structure low defect gas tungsten arc welding process

Abstract

The present application belongs to the low defect fusion inert gas protection welding process method of 6061-T651 aluminum alloy thick plate, and the process steps include (1) welding preparation and bevel processing, (2) welding time control before welding, (3) welding environment control, (4) welding wire and protective gas selection, (5) low temperature gradient temperature management, (6) multi-layer multi-pass welding heat input control, (7) stress relief treatment after welding, (8) quality detection. It is suitable for the welding of 6061-T651 aluminum alloy structural parts with a thickness of 20-40mm in the fields of aerospace, military industry, rail transportation, shipbuilding, power equipment, etc.

Description

Technical Field

[0001] This invention belongs to the field of aluminum alloy welding technology, and relates to a low-defect gas metal arc welding process for 6061-T651 aluminum alloy thick plates. Background Technology

[0002] 6061 aluminum alloy belongs to the Al-Mg-Si system and is a heat-treatable alloy. In the T651 condition, it exhibits high strength, good corrosion resistance, and machinability, and is widely used in structural components in aerospace, military, rail transportation, shipbuilding, and power equipment industries. However, the weldability of 6061-T651 aluminum alloy when welding medium-thick plates using gas metal arc welding (GMAW) is poor, mainly due to the following technical challenges: Firstly, there's the issue of hydrogen porosity. Aluminum can dissolve a large amount of hydrogen in its liquid state, but its hydrogen solubility is extremely low in the solid state. The hydrogen solubility in liquid aluminum can reach 0.7 mL / 100g, but after complete solidification, the hydrogen solubility in solid aluminum drops sharply to below 0.04 mL / 100g, a difference of nearly 18 times between the solid and liquid phases. During the solidification and cooling process of the weld pool, supersaturated hydrogen cannot escape in time, forming hydrogen pores. The main sources of hydrogen include: the oxide film on the base metal and welding wire surface. The porous structure readily absorbs moisture, oil, hydrocarbons, moisture from the protective gas, and water vapor from the ambient air. While the national standard GB50236 specifies a relative humidity of ≤80% for the welding environment, practice shows that the porosity of 6061 aluminum alloy welds increases significantly when humidity exceeds 60%. In gas metal arc welding (GMAW), the continuous wire feed introduces moisture and impurities during the molten wire droplet transfer process, resulting in a higher risk of porosity induction compared to non-GMAW.

[0003] Secondly, the heat-affected zone softens. 6061-T651 is an age-hardening aluminum alloy; welding thermal cycles cause softening in the heat-affected zone. When the reinforcing phase undergoes re-dissolution or softening due to over-aging, the joint strength is typically only 60%–70% of the base metal. If the welding heat input is not properly controlled, the heat-affected zone (HAZ) may become the weakest link in the joint. Under conventional processes, the tensile strength of the joint often drops to 120–140 MPa, and the fracture location is mostly located in the HAZ. Gas metal arc welding (GMAW) has high deposition efficiency, but the heat accumulation effect of multi-layer, multi-pass welding is prominent, making it easier to cause sustained high-temperature over-aging in the HAZ.

[0004] Thirdly, there are inclusions and lack of fusion. With a melting point as high as 2050℃, far exceeding the melting point of aluminum matrix at 660℃, incomplete pre-welding cleaning or excessive post-cleaning time can lead to oxide film regeneration, easily causing slag inclusions and incomplete fusion in the weld. Gas metal arc welding (GMAW) experiences significant arc disturbance, making oxide film breakage and removal difficult. Improper selection of the bevel shape can easily result in sidewall incomplete fusion defects.

[0005] Fourthly, welding deformation and hot cracking. 6-series aluminum alloys have a large coefficient of linear expansion, approximately... Aluminum alloys have a linear expansion coefficient approximately twice that of steel and a solidification shrinkage rate of up to 6%. Medium and thick plates undergo multi-layer, multi-pass welding, resulting in large cumulative heat input, high residual welding stress, and a tendency to generate hot cracks and significant deformation. This is especially true for T651 state tensile stress-relieved plates, which already have pre-existing internal stress. The problem becomes even more pronounced after welding and the superimposed stress.

[0006] In existing technologies, conventional processes for welding 6061 aluminum alloy have the following shortcomings: 1) Environmental humidity control is lax, often adhering to the national standard of ≤80%, but not tightened to ≤60% for medium and thick 6061 plates; 2) No strict welding time window is specified after pre-weld cleaning, resulting in the oxide film reabsorbing moisture after cleaning; 3) The preheating temperature range is wide, with some processes recommending 100-150℃ or even higher, and excessively high preheating temperatures exacerbate the over-aging softening of the heat-affected zone; 4) Inaccurate interpass temperature control leads to heat accumulation during multi-layer welding, causing the heat-affected zone to remain at a high temperature; 5) Emphasis is placed on single-pass / few-pass welding of thin or medium-thick plates, lacking the design of multi-layer, multi-pass system parameters for 20mm-40mm medium-thick plates; 6) There is a lack of methods for the coordinated control of environmental control, cleaning and welding time, preheating and interpass temperature, heat input, and post-weld heat treatment as a system engineering approach.

[0007] Although the existing non-consumable electrode inert gas shielded welding technology produces high-quality welds, it has low deposition efficiency. For plates with a thickness of 20-40mm, the welding time increases exponentially, making it difficult to meet the mass production needs of medium and large-thickness components. Summary of the Invention

[0008] To address the problems of high porosity, severe softening of the heat-affected zone, and unstable welding quality in the welding of 6061-T651 aluminum alloy medium-thick plates in the prior art, this invention aims to provide a low-defect welding process. Through the coordinated efforts of environmental temperature and humidity control, U-shaped groove structure optimization, pre-weld cleaning and welding time control, low-temperature gradient temperature management, and multi-layer and multi-pass welding heat input control, a high-quality welded joint with X-ray flaw detection grade II or above and a tensile strength ≥215MPa can be achieved.

[0009] The technical solution of the present invention: The process for low-defect gas metal arc welding of 6061-T651 aluminum alloy thick plates includes the following steps: (1) Pre-welding preparation and beveling: Select 6061-T651 aluminum alloy plate with a thickness of 20-40mm and use gas metal arc welding; specially process U-shaped beveling with a beveling angle of 30°+R8, blunt edge of 1-2mm, assembly gap of 0-1mm, and clean burrs after beveling. (2) Pre-welding cleaning and welding time control: First, perform mechanical cleaning, use a stainless steel wire brush or milling cutter to thoroughly grind the oxide film on the bevel surface and within a range of 30-50mm on both sides until the silvery-white metallic luster is exposed; then perform chemical cleaning, wipe the bevel and the area to be welded with acetone or anhydrous ethanol to remove oil and dust; start welding operation within 1 hour after cleaning and complete welding operation within 4 hours. (3) Welding environment control: Welding is carried out indoors, with the ambient temperature controlled at 15-30℃, the relative humidity controlled at ≤60%, the wind speed in the argon arc welding area ≤2m / s, and doors and windows closed to prevent drafts. (4) Selection of welding wire and shielding gas: ER5356 aluminum-magnesium alloy welding wire is selected and matched with the wire feeding mechanism of the gas metal arc welding machine. The diameter of the welding wire is φ1.6mm. The shielding gas is high-purity argon with a purity of ≥99.999% and a gas flow rate of 15~20L / min. (5) Low temperature gradient temperature management: 60-80℃ during the preheating stage. Before welding, use a heating pad or flame to preheat the area of ​​100mm on both sides of the bevel. For multi-layer and multi-pass welding, use an infrared thermometer to monitor the temperature between layers. Control the temperature between layers in the range of 60-80℃. After welding, heat treatment is carried out at 80-100℃. After welding, immediately cover with insulation cotton or perform post-heat treatment in a low temperature furnace and keep warm for 1-2 hours. (6) Multi-layer multi-pass welding heat input control: The consumable electrode inert gas shielded welding is adopted, AC power supply is used, and the welding position is flat welding; the consumable electrode welding wire is continuously fed, and the 20mm thick plate is welded in 11 passes; the heat input is controlled within the range of 7.2 to 11.4kJ / cm throughout the process, and interlayer cleaning is performed after each pass. (7) Post-weld stress relief treatment: After welding, stress relief treatment is carried out by furnace annealing. The workpiece is placed in the heat treatment furnace; the annealing treatment is heated to 170-190℃, held for 5-7 hours and then cooled in the furnace. (8) Quality inspection: After welding, visual inspection shall be carried out, X-ray flaw detection shall be qualified according to NB / T 47013.2 standard II, tensile test, bending test and metallographic inspection shall be carried out.

[0010] Furthermore, in step (6), the multi-layer, multi-pass welding heat input control parameter gradient is designed as follows: One pass for the root pass, current 170-190A, voltage 11.0-12.5V, welding speed 14-15cm / min, heat input 8.13-9.5kJ / cm; 2-5 fill passes with increasing current (260-320A, 12.5-16.0V), welding speed (26-27cm / min), heat input (7.56-9.45kJ / cm). 6-8 fill passes, peak current 310-330A, voltage 14.5-16.3V, welding speed 28-30cm / min, heat input 8.72-9.22kJ / cm; 9-11 fill passes, with decreasing current, 235-250A, 14.5-16.0V, welding speed 26-28cm / min, and heat input 7.5-7.92kJ / cm.

[0011] Advantages of the present invention: (1) High welding filling efficiency, suitable for mass production of thick components, continuous wire feeding of the welding wire of the gas metal arc welding, large amount of filling metal per unit time, and significantly shortened welding time, suitable for 20-40mm thick aluminum alloy, rail transit, and mass production of ship structural parts. (2) The U-shaped groove is specially designed to solve the defect of non-fusion on the side wall of MIG welding. The circular arc R8 structure allows the molten metal to spread fully, reducing the defect of non-fusion on the side wall of the groove. Through strict control of ambient humidity ≤60%, time window constraint of welding within 1-4 hours after pre-welding cleaning, low temperature preheating and heat preservation at 60-90℃, and hot dehydrogenation at 80-100℃ and protection with 99.999% high purity argon gas, the macroscopic metallographic structure is free of porosity, slag inclusions, and non-fusion defects, and the weld X-ray inspection stably reaches the Class II qualification. (3) The joint has excellent mechanical properties. It adopts low-temperature preheating of 60-80℃ and strict interpass temperature control of 60-80℃, which avoids the aging coarsening of Mg2Si strengthening phase caused by traditional high-temperature preheating above 100℃, and ensures the consistency of microstructure transformation. Multi-layer and multi-pass welding is adopted, and the heat input throughout the process is limited to a reasonable window (MIG=7.27-9.5kJ / cm), which avoids the excessive softening of the heat-affected zone caused by excessive heat input in a single pass in the traditional process. No cracks were found in the bending test, and the tensile strength reached 220-228MPa, which is significantly better than the 120-140MPa level of the conventional process. (5) In-furnace annealing is adopted to relieve stress, and the stress relief effect is thorough. The heat input of MIG welding is relatively large, and the overall residual stress is relatively high. In-furnace annealing and heat preservation at 170-190℃ are adopted to fully release the residual stress. (6) The process has good stability. Environmental control, cleaning and welding time, preheating and interpass temperature, heat input and post-weld heat treatment are treated as a system engineering and quantitatively controlled. The parameters of each link have clear ranges, which reduces the dependence on the welder's experience and welding environment, and the welding quality is consistent. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of U-shaped bevel processing and connection, where the U-shaped bevel angle of the 6061-T651 aluminum alloy sheet is 30°+R8.

[0013] Figure 2This is a schematic diagram of the sampling location for the welded joint. In the diagram: 1 is the discarded section after removing 25mm, 2 is the welding direction, 3 is the sampling area for tensile / bending / fracture specimens, 4 is the additional specimen area, 5 is the sampling area for tensile / bending / fracture specimens, and 6 is the macro / micro inspection specimen area. Detailed Implementation

[0014] The present invention will be further described in detail below with reference to specific embodiments.

[0015] Example 1: Welding of 20mm thick 6061-T651 aluminum alloy plate by gas metal arc welding.

[0016] (1) Pre-welding preparation and beveling: The equipment is a YD-500GP5 type MIG welding machine with AC power supply; the base material is 6061-T651 aluminum alloy with specifications of δ20mm×155mm×500mm; the bevel is a U-shaped bevel with an angle of 30°+R8, a blunt edge of 1.5mm, and an assembly gap of 1mm.

[0017] (2) Pre-welding cleaning and welding time control: use a stainless steel wire brush to grind the bevel and the area within 40mm on both sides, wipe with acetone, start welding within 30 minutes after cleaning, and complete welding within 4 hours.

[0018] (3) Welding environment control: Welding is carried out indoors at an indoor temperature of 16°C and a relative humidity of 51%.

[0019] (4) Selection of welding wire and shielding gas: Welding wire: ER5356, φ1.6mm; Shielding gas: 99.999% high purity argon; Gas flow rate 17~19L / min.

[0020] (5) Low temperature gradient temperature management: Preheat the area within 100mm on both sides of the bevel to 70-80℃; Interlayer temperature: Control at 70-80℃; Post-heating: Keep at 90℃ for 45 minutes after welding.

[0021] (6) Multi-layer multi-pass welding heat input control: MIG welding is completed in 11 passes. The specific welding parameters are shown in Table 1.

[0022] Table 1 Welding parameters for Example 1 .

[0023] (7) Post-weld stress relief: Anneal at 180℃ for 6 hours.

[0024] (8) Test results: X-ray flaw detection level II qualified (NB / T 47013.2); tensile strength 224MPa, 220MPa (breaks in the heat-affected zone); lateral bending d=60mm, the surface is intact when bent to 90°; no defects such as pores, inclusions, or lack of fusion were found in the macroscopic metallographic analysis.

[0025] Example 2: Welding of 20mm thick 6061-T651 aluminum alloy plate by gas metal arc welding.

[0026] (1) Pre-welding preparation and beveling: Equipment: YD-500GP5 type MIG welding machine, AC power supply; Base material: 6061-T651 aluminum alloy, specifications δ20mm×155mm×500mm; Bevel: U-shaped bevel angle is 30°+R8, blunt edge 1.5mm, assembly gap 1mm.

[0027] (2) Pre-welding cleaning and welding time control: use a stainless steel wire brush to grind the bevel and the area within 40mm on both sides, wipe with acetone, start welding within 30 minutes after cleaning, and complete welding within 4 hours.

[0028] (3) Welding environment control: Welding is carried out indoors at an indoor temperature of 16°C and a relative humidity of 51%.

[0029] (4) Selection of welding wire and shielding gas: Welding wire: ER5356, φ1.6mm; Shielding gas: 99.999% high purity argon, gas flow rate 17~19L / min.

[0030] (5) Low temperature gradient temperature management: Preheat the area within 100mm on both sides of the bevel to 70-80℃; Interlayer temperature: Control at 70-80℃; Post-heating: Keep at 90℃ for 45 minutes after welding.

[0031] (6) Multi-layer multi-pass welding heat input control: MIG welding is completed in 11 passes. The specific welding parameters are shown in Table 2.

[0032] Table 2 Welding parameters for Example 2 .

[0033] (7) Post-weld stress relief: Anneal at 180℃ for 6 hours.

[0034] (8) Test results: X-ray flaw detection level II qualified (NB / T 47013.2); tensile strength 228MPa, 224MPa (breaks in the heat-affected zone); lateral bending d=60mm, the surface is intact when bent to 90°; no defects such as pores, inclusions, or lack of fusion were found in the macroscopic metallographic analysis.

Claims

1. A low-defect gas metal arc welding process for thick aluminum alloy plates of type 6061-T651, characterized in that... Includes the following steps: (1) Pre-welding preparation and beveling: Select 6061-T651 aluminum alloy plate with a thickness of 20-40mm and use gas metal arc welding; specially process U-shaped beveling with a beveling angle of 30°+R8, blunt edge of 1-2mm, assembly gap of 0-1mm, and clean burrs after beveling. (2) Pre-welding cleaning and welding time control: First, perform mechanical cleaning by using a stainless steel wire brush or milling cutter to thoroughly grind the oxide film on the bevel surface and within a range of 30-50mm on both sides until a silvery-white metallic luster is exposed; then perform chemical cleaning by wiping the bevel and the area to be welded with acetone or anhydrous ethanol to remove oil and dust. Welding operations will begin within one hour of cleaning and be completed within four hours. (3) Welding environment control: Welding is carried out indoors, with the ambient temperature controlled at 15-30℃, the relative humidity controlled at ≤60%, the wind speed in the argon arc welding area ≤2m / s, and doors and windows closed to prevent drafts. (4) Selection of welding wire and shielding gas: ER5356 aluminum-magnesium alloy welding wire is selected and matched with the wire feeding mechanism of the gas metal arc welding machine. The diameter of the welding wire is φ1.6mm. The shielding gas is high-purity argon with a purity of ≥99.999% and a gas flow rate of 15~20L / min. (5) Low temperature gradient temperature management: 60-80℃ during the preheating stage. Before welding, use a heating pad or flame to preheat the area of ​​100mm on both sides of the bevel. For multi-layer and multi-pass welding, use an infrared thermometer to monitor the temperature between layers. Control the temperature between layers in the range of 60-80℃. After welding, heat treatment is carried out at 80-100℃. After welding, immediately cover with insulation cotton or perform post-heat treatment in a low temperature furnace and keep warm for 1-2 hours. (6) Multi-layer multi-pass welding heat input control: The consumable electrode inert gas shielded welding is adopted, AC power supply is used, and the welding position is flat welding; the consumable electrode welding wire is continuously fed, and the 20mm thick plate is welded in 11 passes; the heat input is controlled within the range of 7.2 to 11.4 kJ / cm throughout the process, and interlayer cleaning is performed after each pass. (7) Post-weld stress relief treatment: After welding, stress relief treatment is carried out by furnace annealing. The workpiece is placed in the heat treatment furnace; the annealing treatment is heated to 170-190℃, held for 5-7 hours and then cooled in the furnace. (8) Quality inspection: After welding, visual inspection shall be carried out, X-ray flaw detection shall be qualified according to NB / T 47013.2 standard II, tensile test, bending test and metallographic inspection shall be carried out.

2. The low-defect gas metal arc welding process for 6061-T651 aluminum alloy thick plates according to claim 1, characterized in that... Step (6) Multi-layer, multi-pass welding heat input control, parameter gradient design is as follows: One pass for the root pass, current 170-190A, voltage 11.0-12.5V, welding speed 14-15cm / min, heat input 8.13-9.5kJ / cm; 2-5 fill passes with increasing current (260-320A, 12.5-16.0V), welding speed (26-27cm / min), heat input (7.56-9.45kJ / cm). 6-8 fill passes, peak current 310-330A, voltage 14.5-16.3V, welding speed 28-30cm / min, heat input 8.72-9.22kJ / cm; 9-11 fill passes, with decreasing current, 235-250A, 14.5-16.0V, welding speed 26-28cm / min, and heat input 7.5-7.92kJ / cm.

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