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Process analysis and application of Pulse MIG (Metal Inert-Gas-Shielded) Pulse Welding in Aluminum alloy fuel tank for vehicles

Introduction

In line with promotion of green manufacturing and structural requirement of light weight, aluminum alloy, as a structural material of light weight, corrosion resistance, good economic efficiency and high recycling rate, is widely used in automobile industry. In particular, its application has been strengthened along with the increased load, engine power and fuel tank of the vehicles. Traditional steel fuel tank is unable to meet the requirements in use. Aluminum alloy fuel tank has a longer life cycle, owing to the corrosion resistance of its passive surface film, and no coating is needed. Currently it is extensively applied in fuel tanks for vehicles.


However, compared with steel, aluminum alloy has a poor welding feature, which readily results in such defects as incomplete fusion and welding bubble, and causes some difficulties in aluminum alloy fuel tank production. DEX PM3000 for metal inert-gas-shielded pulse welding made by Megmeet adopts a wholly-digital software control system, with its inverting frequency reaching 180 KHZ, realizing a stable welding arc, and no oxidation effects to the metal in question. As the welding wire is used as an electrode, there will be a high deposition efficiency. When welding aluminum alloy, double-pulse may be used for less welding deformation and good welding form. Additionally, because the high and low currents of the pulses are used to adjust the welding arcs, the oxidation film on the surface of the welding pool can be broken by the cathode, thus ensuring the welding metallurgical quality. Therefore, metal gas-shielded pulse welding is a welding method widely adopted for the aluminum alloy fuel tanks.


In the automatic MIG welding process for aluminum alloy fuel tank of vehicles, there tends to be some serious quality problems such as incomplete fusion and welding bubble on the top circular weld of the tank. By some thorough analyses, production tracing and on-site technical experiments, we conclude that the main factors causing such welding quality problems are the changed statuses of the welding torch angle of the special automatic welding machine, the arc lengths of high and low currents pulses, duty cycle and frequency. When the welding torch angle is less than 30°or the extension length of the welding wire is beyond 20 mm, there tends to be some welding defects. When the high and low pulses and the duty cycle are not appropriate, the weld tends to be welding through or higher weld and like defects. After the technical parameters for welding process, welding torch angle and equipment structure have been properly adjusted, one-time conformity rate of the welded aluminum alloy fuel tanks has been improved from 75% to 98% or more. 


1. Manufacturing process and quality problems for aluminum alloy fuel tank

An aluminum alloy fuel tank features a square structure, and its main body consists of a square cylinder and two square end caps welded together, with its dimensions of 1,100 mm × 700 mm × 700 mm, its rated volume being 500 L, and the thickness of the body material 2.5 mm 5052 aluminum and magnesium alloy. The square cylinder of aluminum alloy is formed by rolling and welding. The butt weld for square cylinder is currently done by the automatic special machine using MIG double-pulse welding method, able to ensure the welding quality. However, when welding the square end cap with the same method, there tends to be such problems as incomplete fusion, welding bubble and welding through. As shown in Figure 1, one-time conformity rate of fuel tank end cap and circular weld of the cylinder body is 75%, and non-conformity rate of 25%, which needs much additional argon arc welding, and leads to a high risk of failed weld forming and secondary leakage.

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   Figure 1. Welding quality problems on the fuel tank end cap and cylinder body

   

The rolling and opening necking techniques are adopted for the end cap, and after forming process the end cap is welded with the cylinder body, with the necking part being inserted into the cylinder for the circular butt welding.The double-pulse automatic MIG special welding machines are used for the circular welding, and the single layer and single weld is welded simultaneously when the circular welding of both sides are being performed at the same direction, with the specific welding parameters being shown in Table 1. during welding, the welding torches are fixed, and the fuel tank shall be moved according to the position-changing device. As the fuel tank is of square structure with round corners, during the revolving process, the welding torch angle and arc length will change at anytime; therefore, the operator shall duly, skillfully and manually adjust the torch angle and height to ensure the welding torch angle and arc length are within the proper ranges.


Table 1. MIG pulse welding parameters for the circular weld of aluminum alloy fuel tank

Joint typeWelding current (A)

Welding voltage (V)

Welding speed (mm/min.)Weak pulse current (%)Weak pulse arc lengthDuty cycle (%)

Frequency (HZ)

Overlapping8017.56040%050%1.5

Notes: 1.2 mm welding wire is applied; the material is ER 5356 Aluminium-Magnesium alloy; the protective gas is 99.99% of Argon.


2. Technical analysis of aluminum alloy fuel tank

        

2.1. Before the tank cylinder being welded with the end caps, the end caps will be rolled and necked to certain form. After this rolling and necking process, the wrinkles at the necking places of end caps will centralized and distributed near the round corner of the end caps, and when being overlapped with the cylinder, the space in assembly near the round corner will be the largest one. After our tracing the production process, we find that the forming wrinkles normally take place at the inner necking part, which has a limited affect on assembly space, and the biggest assembly space will not be more than 2 mm. Upon our thorough technical verification, a space not more than 2 mm will not have a notable affect on the welding quality. When the assembly space between the end cap and the cylinder is controlled within 2 mm, the wrinkles out of the rolling and necking process shall not be a main factor for the current weld defects.When the cylinder and end caps have been assembled, they will be fixed on the position-changing device of the special automatic welding machine. Before welding, the operator will normally wipe the area to be welded with a piece of gauze dipped by absolute ethanol in order to remove oil stains on such area. However, when the welding is started, it cannot be ensured that all ethanol has completely evaporated, especially the ethanol flowing into the assembly space; when there is any residue, the welding bubbles tend to occur in the weld. Therefore, it is doubtful that applying the ethanol on the area to be welded before welding will improve the welding quality. Although there is a plastic protective film on the surface of the aluminum alloy plate used to make the fuel tank, such film is usually damaged during the manufacturing process, as before the fuel tank being assembled and welded, such file will be removed. Before welding, the surface of the aluminum alloy plate has been oxidized and stained to certain extent. Before welding the circular weld of the fuel tank, the area on the circular weld to be welded and the meeting part of the circular weld and the straight weld on the cylinder have not been mechanically treated, and both ends of the straight weld of the cylinder will feature relatively higher or there may be some arc craters with relatively thicker oxide skin. All these adverse factors may affect the welding quality. During the automatic welding for the fuel tank, as the special welding machine and the position-changing device are separate, namely the movements of the two are not relevant, and the fuel tank features a square structure, the linear velocity, the rotating radius and the normal of the touching point of the fuel tank and the welding torch are constantly changing when the fuel tank is rotating at a normal speed with the position-changing device, namely the actual welding speed, the welding wire extension length (Extension length) and the welding torch angle ( the angle of tangent line at the touching point of the welding wire and the work piece) are constantly changing. All these uncertain technical parameters will surely affect the welding quality. In order to lessen the influences of all these technical factors, during the welding process, the operator will manually adjust the welding torch height and angle, but the advantages of automatic welding will be lost.

2.2. Analysis of welding parameters

Besides the parameters listed in Table 1, among all the main parameters the welding torch angle and the welding wire extension length have not been confirmed. With respect to the automatic MIG special welding machine that is currently used, its welding torch is fixed with a stance being set in advance, irrelevant to the movement of work piece, thus we can infer that the inclination angle of the welding torch α, the welding wire extension length L and the linear welding speed V will change periodically with the movement of the work piece (fuel tank), as simplified and shown in Figure 2. When the position-changing device fixed with the fuel tank to be welded moves from P position to P" through P′, the touching point of the welding torch and the fuel tank will move from Point A to Point C through Point B. In this process, the welding wire extension length L will be shortened first, and then be lengthened; while the inclination angle of the welding torch α will be increased, and then be decreased (α→α′→α") accordingly; the linear welding speed V will also be changing. Considering that the section of the fuel tank is not strictly square, and the four straight sides will feature some slightly intruding “drum” shapes, the distance from each point on the sides of the edges to the center will not as big as that shown in the schematic drawing, namely the welding speed V will change in a limited way. During the welding, the inclination angle of the welding torch α and the welding wire extension length L will change greatly, resulting in that it is difficult for the protective gas to shield the welding, thus greatly affecting the welding quality.



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Figure 2. Schematic drawing for the fuel tank moving when the welding torch is fixed

       

    

In order to verify how the various technical parameters uncertain will affect the welding quality during the welding of the aluminum alloy fuel tank, we have carried out a series of technical tests. The relevant results show that when the touching point of the welding torch and the fuel tank is located at around the central position of the straight edge of the square tank (see the position of P" in Figure 2), the inclination angle of the welding torch α will be less than 30°, the welding wire extension length L will exceed 20 mm, and the probability of occurring incomplete fusion or welding bubble will be over 50%. While in the appropriate parameters for MIG welding for aluminum alloy,  α shall be in a range of 75° - 80°, and L in a range of 12 mm - 18 mm [ 8-11]. Therefore, incomplete fusion and welding bubble will normally occur during the welding torch moving over the round corner of the fuel tank to the center of the straight edge, and this is where the welding operator manually gives his intervention with the welding torch stance.


3. Equipment improvement and process adjustment

Based on the above analysis, in the welding process for aluminum alloy fuel tank, there are a number of unreasonable things, and the process shall be adjusted accordingly, and the welding machine shall be improved properly. (1) To eliminate the uncertain parameters affecting the welding quality, the automatic welding machine currently used shall be improved. Using the welding robot workstation will be the best way, but considering economic factors and enterprise production scale, we may improve the welding torch structure of the welding machine. We may fix the formerly fixed welding torch on a driven rod which can move upward or downward, and at the lower end of the driven rod there can be fixed a pair of rotatory profiling wheels, and the two wheels shall be positioned at the side lines of the circular area to be welded, and rotate with the rotation of the fuel tank, thus driving the driven rod to move upward or downward. The related movements of the improved welding torch and the fuel tank are shown in Figure 3. After the welding torch structure being improved, during the fuel tank to be welded rotating from P position to P′, the driven rod where the welding torch is fixed will be driven by the profile of the fuel tank, moving freely upward or downward, with the whole welding torch structure moving accordingly. In this process, the welding wire extension length L and the inclination angle of the welding torch α will not change any more. Namely, after the welding torch structure being improved, the two parameters, the welding wire extension length L and the inclination angle of the welding torch α can be certain.

(2) Besides enhancing the management of the welding-related materials stock and dampness prevention, other relevant processes need to be adjusted. After forming is completed, the area to be welded and the necking parts of the end caps of the fuel tank shall be wiped with absolute ethanol to remove the oil stain before being assembled to the cylinder. Assembly shall not be carried out until all ethanol has evaporated. Before welding, the area to be welded shall be treated mechanically for cleanness. Two ends of the straight welds of the fuel tank shall be polished with abrasive paper, ensuring that the meeting part of the circular weld and the straight weld will not be fairly high, and lessening its affecting the welding torch extension length and protective gas flow, and then the area to be welded of the whole circular weld shall be polished one time, and sweep away the debris and dust with a brush before finally performing the welding.


4. Conclusion

After process and equipment improvement, the welding quality for the aluminum alloy fuel and the production efficiency have all been remarkably improved, and the quality problems of incomplete fusion and welding bubble in batches have been solved, with one-time conformity rate of the welded aluminum alloy fuel tanks has been improved from 75% to 98% or more. The production efficiency has been greatly increased. Formerly, one automatic special welding machine should be operated by two welding operators; after our improvement, one welding operator can operate two automatic special welding machines.


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