Lightweight, high speed, safety, energy efficiency, comfort, and long service life are the symbols of modern railway vehicles.
The key to achieving train speeding is to solve the problems of lightweight and sealing of the train. Lightweight is essential for reducing energy consumption by reducing train traction and braking forces, reducing wheel-rail wear, and increasing train running speed.
Sealing directly affects the operational performance of the train and the comfort of passengers. The use of aluminum alloy materials is an effective measure to reduce the weight of the car body.
The 200EMU car body mainly uses three types of aluminum alloys: 5000 series, 6000 series, and 7000 series.
The 5000 series is an aluminum-magnesium alloy with good weldability.
The 6000 series is an aluminum-magnesium-silicon alloy with good weldability and higher welding strength.
The 7000 series is an aluminum-zinc alloy, but due to the addition of zinc, the weldability decreases and the welding strength also reduces.
According to JIS standards, the 200EMU car body mainly uses 5083 aluminum alloy in the 5000 series, 6N01 aluminum alloy in the 6000 series, and 7N01 aluminum alloy in the 7000 series.
2. Material Characteristics and Welding Features of Aluminum Alloys
2.1 Material Characteristics of Aluminum Alloys
Aluminum accounts for 8% of the Earth’s crust in the form of compounds, making it the most abundant metal on Earth. It is lightweight, with a specific gravity about one-third that of steel.
Aluminum alloys are metal materials obtained by adding alloying elements such as magnesium, manganese, silicon, copper, and zinc to pure aluminum to obtain different properties, making them the preferred material for vehicle lightweighting.
They have a dense oxide film (melting point of 2050℃) on the surface, which provides strong corrosion resistance. The melting point is low (660℃), while the melting point of steel is about 1535℃.
Aluminum has good thermal and electrical conductivity, which is about five times that of steel. However, it has a large coefficient of thermal expansion, which is about twice that of steel.
2.2 Welding Features of Aluminum Alloys
Aluminum alloys have their own welding characteristics due to their special material properties.
1. Extremely susceptible to oxidation
Aluminum has a strong affinity for oxygen and will oxidize at any temperature. Aluminum oxide (A2O3) film is generated on the surface of the base metal, with a thickness of about 0.1-0.2um. It has a high melting point, dense structure, and protects the surface of the base metal.
During welding, this oxide film hinders the melting and fusion of the base metal, resulting in incomplete penetration defects.
The density of the oxide film is high, which makes it difficult to float on the surface of the molten pool and easily form slag inclusion defects in the weld.
2. High thermal conductivity and specific heat of aluminum
Although the melting point of aluminum alloy is much lower than that of steel, the thermal conductivity and specific heat capacity of aluminum are relatively large. During the welding process, a large amount of heat energy is rapidly transmitted to the interior of the body metal.
To obtain high-quality welding joints, a heat source with concentrated energy and high power must be used.
3. Large coefficient of linear expansion
The coefficient of linear expansion of aluminum alloys is about twice that of steel, and the volume shrinkage rate during solidification is 6.5% to 6.6%, which makes it prone to welding deformation.
In addition to selecting reasonable process parameters and welding sequences, using suitable welding fixtures is also essential, especially when welding thin plates.
4. Prone to porosity
Porosity in the weld joint is a common defect in aluminum alloy welding, especially anti-rust aluminum alloy welding.
Hydrogen is the main cause of porosity during aluminum alloy welding. Hydrogen mainly comes from moisture in the arc column atmosphere, welding materials, and moisture adsorbed on the base material.
Among them, the adsorption of moisture on the surface oxide film of the welding wire and base material plays a prominent role in the formation of welding pores.
5. Evaporation and burn-off of alloying elements
Some aluminum alloys contain alloying elements with low boiling points, such as Mg and Zn.
These elements are easily evaporated and burned at high temperatures, changing the chemical composition of the weld metal and reducing the performance of the welding joint.
6. No visible color change, making welding operations difficult
During aluminum alloy welding, when it changes from solid state to liquid state, there is no obvious color change, which makes it difficult for operators during the welding process.
3. Choosing the Welding Method for Aluminum Alloys
There are many welding methods for aluminum alloys, each with its own characteristics and applicable scenarios.
Commonly used welding methods include gas welding, stick arc welding, tungsten inert gas welding (TIG), metal inert gas welding (MIG), resistance welding, plasma arc welding, and brazing.
Considering factors such as the material characteristics of the aluminum alloy selected for the 200EMU car body, plate thickness combinations, joint forms, production conditions, usage requirements, and economic factors, most of the aluminum alloy welding methods used for the 200EMU car body are MIG welding.
4. MIG Welding Process for Aluminum Alloys
4.1 Pre-weld Preparation
Before welding, one or more of the following pre-weld treatments should be applied to the joint area of the base metal to remove surface oxides and other adherents to prevent welding defects.
1. Surface degreasing with relevant organic solvents.
2. Machining method
Use a clean stainless steel fine brush to vigorously grind, or use other corresponding methods.
3. Chemical method
Immerse in a 5-10% sodium hydroxide (70℃) solution for 30-60 minutes, then rinse with water; then soak in about 15% (room temperature) nitric acid solution for about 2 minutes and rinse with water, followed by sufficient drying.
4.2 Groove Forms
Mechanical machining, grinding, and other methods can be used for groove processing.
Most of the aluminum alloy car bodies are butt joints, and the groove forms for butt joints are shown in Table 1.
Table 1. Bevel Types for Butt Joints
|Types||Joint Shape||Plate Thickness |
|Root Height |
|Root Gap |
|Root Angle |
|V-shape||No Backing Plate||3.2-9.0||1||0||70|
|No Back Gouging||10.0-32.0||+0.5/-1||+2/-0||60|
|L-shape||No Backing Plate||3.2-11.0||1||0||55|
|No Back Gouging||12.0-32.0||+0.5/-1||+2/-0||50|
|X-shape||No Backing Plate||6.0-16.0||1+0.5/-1||0+2/-0||70|
|No Back Gouging||17.0-32.0||60|
|K-shape||No Backing Plate||6-22.0||1+0.5/-1|
|No Back Gouging||23.0-32.0||50|
4.3 Welding Rods and Wires
In principle, welding rods and wires should be selected according to the provisions of JSZ3232 “Aluminum and Aluminum Alloy Welding Rods and Wires” based on considerations such as the type of base metal, plate thickness, and other necessary conditions to ensure good welding quality.
The selection of welding rods and wires shall be in accordance with Table 2.
Table 2. Selection of Welding Rod or Wire Based on Base Metal Combination.
(1) A1200 can also be used.
(2) A4043 can also be used.
(3) A5356, A5556 or A5183 can also be used.
(4) When discoloration is not allowed after anodizing treatment, A5356 is preferred.
4.4 Welding Standards
The main welding parameters for MIG welding of aluminum alloys are welding current and welding speed. The range of MIG semi-automatic welding conditions for butt joints under different welding positions is shown in Figure 1.
Figure1. Welding Conditions for Butt Joint
- Visual inspection (VT)
- Radiographic testing (RT)
- Ultrasonic testing (UT)
- Magnetic particle testing (MT)
- Penetrant testing (PT)
- Eddy current testing (ET)
- Air tightness testing with compressed air (gas leakage testing)
5. Defects in Aluminum Alloy Welding and Prevention Measures
Due to the low melting point, high thermal conductivity, and large coefficient of thermal expansion of aluminum alloys, it is easy to produce defects such as cracks, poor fusion, and porosity during welding.
There are two main types of cracks that occur during aluminum alloy welding: solidification cracks and liquation cracks at grain boundaries.
Solidification cracks are caused by the segregation of alloying elements at the crystal interface and the presence of low-melting-point inclusions.
Solidification cracks, also known as arc crater cracks, occur when cracks form during the metal solidification process. Pore cracks occur when the arc crater is not completely filled during the cessation of the welding process.
Therefore, when purchasing a welding machine, it is necessary to choose a machine with an arc cessation function. Welders must use a small current and fill the arc crater during the cessation of welding.
One of the main reasons for the occurrence of welding cracks is excessive heat input. Therefore, welding operators are required to strictly follow the standards specified in the process documentation during welding.
Another major reason for the occurrence of welding cracks is impure base metal composition.
Therefore, when domesticating aluminum alloy materials, process departments must first conduct welding process tests on preselected materials when choosing aluminum alloy plates and profiles.
Only then can they select materials that meet the requirements to reduce the tendency for cracking.
Liquation cracks occur in the heat-affected zone of the base metal due to localized melting and solidify at the crystal interface, making them very susceptible to cracking during repair welding.
Therefore, welding operators are required to strictly follow the “Aluminum Alloy Repair Welding Regulations” during repair welding.
The locations where welding cracks are most likely to occur are at the beginning and end of the weld.
Therefore, when welding, arc starting plates and arc cessation plates must be used wherever possible.
In areas where arc starting plates cannot be used, a back-stepping method can be used. This involves welding backwards from 2-3mm away from the start point of the weld, then welding forwards again.
5.2 Poor Fusion
The causes of poor fusion are generally as follows:
a. Large thermal conductivity, difficult to melt.
b. Pulse welding results in a finger-shaped weld shape.
c. If the high-melting-point oxide film on the surface of the plate is not removed, it will form defects and affect the wettability of the welding metal.
Since non-fusion often occurs 20mm~40mm from the start of the weld, the arc starting current must be controlled during welding. The welding machine purchased must have an arc starting function to ensure that the arc starting current is greater than the welding current, thereby avoiding non-fusion.
Al2O3 (aluminum oxide) on the base metal is transparent, hard (second only to diamonds), and grows at an extremely fast rate of up to 10nm (0.000010mm).
Therefore, the oxide film must be removed as required before welding and welding should be carried out as soon as possible.
Hydrogen is the main cause of porosity in aluminum alloy welding, and the sources of hydrogen invasion (protective gas accounts for about 12%, electrode welding wire adhesion accounts for about 24%, base metal adhesion accounts for about 7%, and intrusion from air accounts for about 57%).
The proportion of hydrogen invading from the air is the largest, so the prevention measures for porosity are as follows:
(1) Oil stains and moisture on the surface of the base metal and welding wire should be removed.
After the welding wire is unpacked, it should be used as soon as possible. If the welding wire is not used up that day, it should be removed and stored in a warming box. The procurement department should not purchase too much welding wire at one time.
In principle, aluminum alloy welding wire cannot be used after being stored for half a year.
(2) Remove the moisture in the protective gas (dew point management) and require the use of argon with a purity of 99.999%.
(3) Construct in a low humidity environment.
(4) The wind speed at the construction site should be below 1m/s.
(5) Use flat or vertical welding positions that allow easy expulsion of porosity.
(6) Stabilize the wire feeding speed.
(7) Water on the surface of the base metal must be removed and cleaned before welding. When the humidity is above 90%, the surface of the base metal should be blown with hot air to remove moisture and reduce porosity tendencies.
Aluminum alloy car bodies have the characteristics of strong corrosion resistance, light weight, and beautiful appearance, making them a high-precision and high-tech body structure.
The welding of aluminum alloy car bodies has encountered numerous technical problems during production, with the most severe being the deformation caused by aluminum alloy welding.
Research on the MIG welding performance of aluminum alloy materials is beneficial to improving our company’s ability to manufacture first-class aluminum alloy railway passenger car bodies.