Against the backdrop of modern industry’s shift toward high performance and lightweight design, metallic materials are widely used due to their excellent physical and chemical properties.
For example, aluminum alloys, with their low density and good corrosion resistance, are widely used in lightweight components for the aerospace and automotive industries;
nickel-based alloys maintain excellent mechanical properties under high-temperature and high-pressure conditions, making them suitable for gas turbines and nuclear reactors;
welded structures combining austenitic stainless steel and pearlitic steel balance corrosion resistance with strength and are widely used in chemical process vessels and thermal equipment;
low-carbon steel and low-alloy steel, with their good formability and relatively high strength, serve as base materials for bridges, pipelines, pressure vessels, and other applications.
However, the welding processes for dissimilar metals are relatively complex;
if the process design is unreasonable or the operation is not performed properly, the quality of the joint will be severely compromised.
Therefore, it is necessary to systematically study the welding processes and operational techniques for dissimilar metals, accelerate the development of targeted process parameters and the formulation of operational standards, and effectively improve welding quality.
Analysis of Welding Processes and Operational Techniques for Aluminum Alloys
Welding Processes
Aluminum alloys are widely used in shipbuilding, aerospace, and pipeline engineering due to their excellent properties, such as light weight, high strength, and corrosion resistance.
Welding Characteristics and Requirements of Aluminum Alloys
Nickel-based alloys have poor weld metal fluidity and shallow penetration. The welding process therefore requires low heat input, a narrow oscillation width, and multi-layer, multi-pass welding.
Strict gas shielding and interpass temperature control also help produce high-quality welded joints.
Therefore, understanding and mastering aluminum alloy welding processes and operational techniques is of critical importance.
Common Welding Processes for Aluminum Alloys
Currently, the most commonly used welding processes for aluminum alloys are tungsten inert gas (TIG) welding and gas metal arc (GMA) welding.
Both of these welding processes use an inert gas as the shielding gas to melt the base metal and filler metal under the high temperature of the arc, thereby forming a welded joint.
However, there are significant differences between them in terms of welding efficiency, weld quality, operational difficulty, and application scenarios, so the appropriate method must be selected based on specific circumstances.
TIG and GMAW Process Comparison
Table 1 compares the processes of Tungsten Inert Gas (TIG) welding and Gas Metal Arc Welding (GMAW).
| Comparison Item | Gas Tungsten Arc Welding (GTAW) | Gas Metal Arc Welding (GMAW) |
|---|---|---|
| Electrode Type | Non-consumable tungsten electrode | Consumable welding wire |
| Welding Efficiency | Slow welding speed, low deposition rate | Fast welding speed, high deposition rate |
| Weld Quality | Stable arc, no spatter, good appearance, low porosity | Prone to spatter; parameters require strict control |
| Operating Difficulty | Two-handed operation; requires a high level of skill | Automatic or semi-automatic operation |
| Applicable Thickness | Thin plates, typically < 6 mm, especially advantageous for < 3 mm | Medium and thick plates, ≥ 3 mm, with a clear advantage for thicker plates |
| Filler Metal Feeding | Manual wire feeding | Automatic wire feeding |
| Typical Applications | Aerospace thin-wall components, pipe root welding, decorative/artistic components | Ship structures, aluminum alloy pipes, automotive industry |
Table 1. Process Comparison: GTAW with Non-Consumable Tungsten Electrode vs. GMAW with Consumable Electrode
Operating Tips
When applying the above welding process to aluminum alloy tubes, observe the following key operating points to avoid quality issues:
1. Pre-welding Preparation
Before welding, oil, moisture, dust, oxide films, and other contaminants must be thoroughly removed from the groove and the areas on both sides of the aluminum alloy pipe.
It is generally recommended to use a specialized cleaning agent in conjunction with a stainless steel brush for cleaning.
The use of ordinary wire brushes or grinding wheels is strictly prohibited to avoid scratching the pipe surface or introducing ferrous impurities, which could impair the performance of the aluminum alloy pipe or increase the incidence of defects such as porosity and slag inclusions.
The cleaned workpiece must be kept dry, and welding should be performed as soon as possible.
If the welding environment is relatively humid, preheating is required to remove moisture adsorbed near the groove of the aluminum alloy tube.
Under normal circumstances, the preheating temperature should be maintained at around 90 °C, not exceeding 120 °C, to avoid compromising the mechanical properties of the aluminum alloy tubing or increasing the risk of oxide film thickening during the welding process.
At the same time, strictly control the moisture content of the welding materials to ensure they are dry before use.
2. Assembly Alignment and Protection
During pipe alignment, ensure that the gaps are uniform. The alignment welds should not be too large and must be secure, evenly distributed, and free of defects.
During welding—whether using tungsten inert gas (TIG) welding or gas metal arc (GMA) welding—the effectiveness of gas shielding directly affects weld quality.
Therefore, before welding, gas must be supplied in advance to completely purge the air from the piping and the nozzle;
during welding, the gas flow rate must be strictly controlled to avoid insufficient protection due to a flow rate that is too low, or turbulence caused by a flow rate that is too high, which could introduce air;
After welding, continue to supply shielding gas to prevent oxidation of the high-temperature weld through extended protection.
3. Welding Operation Control
For tungsten inert gas (TIG) welding, ensure that the torch moves smoothly and that the distance between the tip of the tungsten electrode and the molten pool remains constant.
When feeding the wire, keep the end of the welding wire within the protective gas flow at all times, and ensure rapid, continuous, and uniform wire feeding;
Use high-frequency or pulsed methods for arc striking to prevent tungsten inclusion caused by the tungsten electrode coming into contact with the pipe material;
Considering that arc pits and cracks are prone to occur at the arc termination point, it is advisable to use the current decay method or the gradual wire feeding method, and install a lead-out plate if necessary;
for multi-layer, multi-pass welding, strict interpass cleaning must be performed to effectively control the penetration depth at the root.
Analysis of Welding Processes and Operational Techniques for Nickel-Based Alloys
Welding Processes
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Nickel-based alloys have different physical and metallurgical properties from ordinary alloys. These characteristics require engineers to carefully evaluate the material properties and tailor the welding process accordingly.
A DN100 nickel-based alloy pipe provides a practical example. A combined process of tungsten inert gas (TIG) welding followed by shielded metal arc welding (SMAW) or metal-flux-cored gas-shielded pulsed down-welding offers a reliable balance between welding efficiency and quality.
Tungsten Inert Gas (TIG) welding primarily handles the root pass. This process produces good root formation and protects the back side of the joint.
Shielded Metal Arc Welding (SMAW) or metal-flux-cored gas-shielded pulsed down-welding primarily handles the fill and cover passes. High deposition efficiency and stable arc control improve welding efficiency and fusion quality.
High-purity argon (φ(Ar) ≥ 99.999%) serves as the shielding gas at a controlled flow rate of 10–15 L/min. This gas effectively protects the molten pool and high-temperature areas while preventing oxidation and nitriding of nickel-based alloy pipes during welding.
Welding Techniques
Specifically:
(1) TIG Welding Process Control
When performing tungsten inert gas (TIG) welding on DN100 nickel-based alloy pipes, a 2.4 mm diameter cerium-tungsten electrode should be used, with the electrode stick-out length controlled between 3 and 5 mm.
During welding, ensure that the torch is perpendicular to the workpiece surface and that the interpass temperature is maintained below 100 °C.
This not only effectively prevents grain coarsening and the tendency for hot cracks but also improves the fluidity of the weld metal, thereby reducing defects such as lack of fusion and porosity.
During on-site operations, first introduce argon through a hose to purge the air from the pipe, then wrap tape around the groove to create a sealed protective space, ensuring that the back of the weld is not oxidized during welding.
Use high-frequency arc striking to prevent contamination and damage caused by the tungsten electrode coming into contact with the workpiece.
(2) Welding Parameters and Techniques
During shielded metal arc welding, the welding current should range from 80 to 120 A, and welding should be performed quickly, evenly, and smoothly at an arc voltage of 10 to 14 V.
During the welding process, do not stop moving the welding torch, do not stir the molten pool laterally, do not swing the electrode excessively, and do not strike the arc outside the groove to prevent welding defects.
When performing small-amplitude oscillation, pause briefly upon reaching the limit position on each side to allow the molten weld metal sufficient time to fill any potential undercut areas, thereby achieving a full and continuous weld bead profile.
If lateral oscillation is required, the oscillation amplitude should not exceed three times the diameter of the electrode, and the surface of the weld bead must be thoroughly cleaned after each pass is completed.
When extinguishing the arc, the arc pit must be filled to prevent arc pit cracks.
(3) Welding Parameters and Process Control
In metal-cored gas-shielded pulsed welding operations, precise control of parameters is critical.
Therefore, it is recommended to use a digital welding machine equipped with a built-in library of specialized welding parameters for nickel-based alloys and featuring pulse waveform control capabilities as the welding equipment.
Generally, the welding current should be 120–180 A, the pulse frequency 80–120 Hz, the pulse duty cycle 30%–50%, and the arc length factor 1.1–1.3.
The welding system automatically matches the welding voltage to the welding current, with the voltage ranging from 18 to 24 V. Operators control the welding speed between 15 and 25 cm/min to prevent defects caused by excessive heat input.
In practice, operators use a mixture of Ar, He, and CO₂ as the shielding gas. This combination helps maintain arc stability and molten-pool wettability while reducing the risk of hydrogen-induced cracking.
Operators control the groove angle between 10° and 15° to achieve an aesthetically pleasing weld bead and root fusion quality that meets applicable standards.
The recommended oscillation amplitude should not exceed five times the wire diameter, with a frequency of 1 to 2 Hz, to prevent loss of control over the molten pool or the formation of porosity and inclusions.
Medium-frequency induction heating equipment is used for dehydrogenation heat treatment to reduce the tendency for cold cracking.
Analysis of Welding Processes and Operational Techniques for Austenitic Stainless Steel and Pearlite Steel
Welding Processes
In the field of dissimilar metal fabrication, welding of austenitic stainless steel and pearlite steel is common.
Due to significant differences between these two types of materials in terms of chemical composition, microstructure, and physical properties, welding operations face numerous technical challenges.
Taking the welding of 304H austenitic stainless steel and 12Cr1MoV (12X1MΦ) pearlitic heat-resistant steel as an example, engineering practice faces several challenges.
A martensitic embrittlement layer can form in the fusion zone. Carbon migration can produce softening and carbon-enrichment zones.
High welding residual stresses can also develop. These factors increase the risk of cracking along the fusion line during high-temperature service.
Therefore, special process approaches must be adopted for welding austenitic stainless steel to pearlitic steel.
The pre-deposited barrier layer method has been proven to be an effective method for controlling the quality of joints between austenitic stainless steel and pearlitic steel.
The pre-deposited barrier layer method primarily involves first surfacing a layer of transition metal with a high nickel (Ni) content onto the groove surface of the pearlitic steel;
this transition layer possesses good plasticity reserves and the ability to form austenite.
Next, a barrier layer is deposited using austenitic stainless steel welding consumables. Finally, the pearlitic steel component with the barrier layer is joined to the austenitic stainless steel component via welding.
In this process, both base metals actually involved in the weld are austenitic materials, thereby avoiding the various problems associated with the direct fusion of dissimilar steels.
The specific process flow is as follows:
Groove inspection → preheating treatment → build-up welding of the barrier layer → post-weld heat treatment → inspection of the barrier layer weld → heat treatment of the barrier layer weld → groove machining → joint welding → quality inspection and documentation.
Operational Techniques
By compiling operational techniques from each process step, we have derived the core process parameters and control requirements for the pre-stacking isolation layer method, as shown in Table 2.
| Process Step | Key Parameters / Requirements | Purpose and Function |
|---|---|---|
| Groove Preparation | V-groove; single-side angle 35° ± 1°. Mechanically grind and clean the groove and all contaminants within 50 mm on both sides until the metallic luster is exposed. | Ensure accurate groove geometry, minimize impurity introduction, and ensure uniform interlayer thickness. |
| Preheating | Use electrical heating for preheating; preheat temperature: 100–150°C; interpass temperature: ≤150°C. | Reduce susceptibility to cold cracking and prevent the formation of high-hardness martensitic structures. |
| Pre-Deposited Interlayer (Layer 1) | Deposit using high-nickel welding material; thickness: (3 ± 1) mm. Manually perform tungsten inert gas arc welding; pure tungsten electrode Ø2.5 mm. | Provide austenite-forming elements and suppress carbon migration. |
| Pre-Deposited Interlayer (Layer 2) | Use austenitic stainless-steel welding material; build up to a total thickness of (9 ± 1) mm; use low current, multiple passes/layers, and rapid welding. | Establish an interlayer that meets the specified thickness requirement. |
| Postheating | 100–150°C × 4 h; insulation layer thickness ≥50 mm. | Slow the cooling rate and prevent cold cracking and hardened microstructures. |
| Post-Weld Heat Treatment (PWHT) | 700–720°C × 1 h; heating/cooling rate ≤220°C/h. | Eliminate welding residual stress and improve microstructural properties. |
| Fit-Up Welding | No preheating; interpass temperature ≤150°C; use segmented symmetrical welding, with each segment ≤100 mm. | Reduce thermal stress and avoid hot cracking and restraint stress. |
| Weld Inspection | 100% visual inspection (Grade I weld); 100% liquid penetrant testing + radiographic testing (Grade I acceptance). | Ensure weld quality meets applicable standards, with no defects such as cracks or slag inclusions. |
Analysis of Welding Processes and Operational Techniques for Low-Carbon Steel and Low-Alloy Steel
Welding Processes
Low-carbon steel and low-alloy steel are widely used in industrial manufacturing due to their good machinability, weldability, and cost-effectiveness.
In engineering practice, CO₂ gas-shielded arc welding has become one of the mainstream welding methods for joining these two types of materials, thanks to its advantages of controllable heat input, high deposition efficiency, low welding costs, and ease of operation.
Using CO₂ gas as a shielding gas isolates the weld zone from the atmosphere during the welding process, preventing the molten metal from reacting with elements such as O and N in the air.
Simultaneously, the heat from the arc melts both the filler wire and the base metal, fusing them to form a strong welded joint.
Operating Techniques
When performing CO₂ gas-shielded arc welding on low-carbon and low-alloy steels, the following operational precautions should be observed:
1. Proper Selection of Welding Wire
Low-hydrogen welding wire should be used to reduce the diffusion hydrogen content in the weld metal and suppress the tendency for cold cracks at the source.
2. Scientific Setting of Preheating Temperature
Since low-alloy steel has a significantly greater tendency to harden than low-carbon steel, preheating should be based on the properties of the low-alloy steel.
The carbon equivalent method typically helps determine the required preheating temperature. Plate thickness correction factors further refine the calculated value.
For example, a carbon equivalent above 0.45% requires a preheat temperature of 100–150 °C.
3. Precise Control of Heat Input
Hydrogen-induced cracking is a relatively common quality defect in the welding of low-carbon and low-alloy steels.
Engineering practice has shown that this can be prevented by controlling heat input, specifically by using a moderately low heat input—while ensuring good fusion—and implementing appropriate interpass temperature control measures.
4. Standardized Control of Operating Procedures
Before welding, thoroughly clean the groove and the 20 mm area on either side of it to remove oil, rust, scale, dust, and other contaminants.
Adjust the torch angle and wire stick-out length appropriately based on the groove type and assembly gap;
Generally, the torch should be tilted forward at an angle of 10° to 15° relative to the normal of the workpiece. During assembly, strictly control the misalignment.
For plate thicknesses ≤20 mm, the misalignment should be less than 1/8 of the plate thickness; for plate thicknesses >20 mm, the misalignment should be less than 1/6 of the plate thickness.
During welding, use a small-amplitude sawtooth oscillation for flat welding, with an amplitude of 4 to 6 times the diameter of the welding wire;
for vertical and overhead welding, appropriately reduce the oscillation amplitude to prevent the molten pool from sagging or falling off;
maintain an appropriate distance between the contact tip and the workpiece, and control the gas flow rate at 15 to 20 L/min. When striking the arc, the retreating arc method is recommended; when extinguishing the arc, fill the arc pit and delay the gas shut-off.
Conclusion
Gas-shielded arc welding is widely used to join metals and dissimilar metals.
The process has also produced good results when welding aluminum alloys, nickel-based alloys, austenitic stainless steel, pearlitic steel, low-carbon steel, and low-alloy steel.
To fully leverage its advantages, operators must handle every process step with rigor and attention to detail.
Standardized operating procedures also help maintain consistent and reliable weld quality.