Arc-welding based additive manufacturing for body reinforcement in automotive engineering

06 Jan.,2023

 

china arc welder

Welding process

In case of manufacturing thin 3D components, a low-energy welding process is needed. Standard processes can be unstable at the low level of welding energy required, and spatter formation may be a problem. In this study, an advanced waveform controlled short arc welding process with low heat input and very stable arc was investigated and chosen. Special feature of this process is the bidirectional wire motion during welding for a better drop separation, almost no formation of spatter and a more stable arc. In each cycle, one drop separation occurs. The wire electrode is fed forward until the voltage almost reaches the value 0 (short circuit). At this moment, the drop passes into the weld pool. The short circuit acts as a trigger to move the wire backwards in a defined way. The droplet is detached cleanly, which results in significantly fewer weld spatters. The wire is fed back further until a certain (set) arc length is reached and the cycle is repeated (Fig. 3).

Fig. 3

One cycle of drop transfer in case of the welding process “MoTion Control Weld” [6]

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This is realized by a special powerful motor (Fig. 4, “MoTion Drive,” near the welding torch) which provides the small high-frequency movements of the wire. As a result, the arc-on-time and the thermal energy can be reduced to a minimum. The “MoTion Control Unit” (Fig. 4) serves as a wire buffer, from which the motor can pull and push the wire as needed. A “CLOOS Qineo NexT” was used as welding power source (Fig. 4b, Table 1).

Fig. 4

Main components of the welding system with bidirectional wire movement: “MoTion Control Unit” and “Motion Drive” (a) welding power source “Qineo NexT” (b)

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Table 1 Welding machine details

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These experimental welding trials were subdivided into two fundamental topics. The first was to check the feasibility in principle of generating a gusset plate by additive manufacturing for reinforcing a right angle of formed thin steel sheets on car body areas. The second topic was to increase flexural rigidity of the sheets by depositing weld metal in the shape of a grid. A subsequent bending test was used to show whether and how much increase in strength could be achieved.

Gusset plate

Preliminary welding trials (Fig. 5a, b) on uncoated and zinc-coated steel sheets of 2 mm thickness showed that alternating welding in flat position with a break of about 2 s after each weld seam seems to achieve the best results. In these experimental tests, it was found that reversing the weld direction of subsequent beads resulted in a lower warpage. Simulations of temperature profile show that in multilayer welding, the same weld direction of the subsequent beads leads to much greater distortion than the opposite direction [7]. This was therefore applied to the body parts.

Fig. 5

Welding results of additive manufacturing of a gusset plate on steel sheets of 2 mm thickness: uncoated (a and b), zinc coated (c and d)

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The minimum achievable thickness depends mainly on the diameter of the filler metal used. The wire diameter was 1.0 mm, and the thickness of the gusset plate was about 2.8 mm. The welding current was reduced with increasing number of layers, so that the cooling time could remain the same. This procedure was then tested on car body parts (Table 2). These were electrolytically or hot-dip galvanized and 0.7 mm thick.

Fig. 6

Results of a study of shape deposition depending on welding direction [8]: long raster pattern (a), short raster pattern (b), illustration of deflection (c), exp. deflection of long and short raster pattern (d)

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Table 2 Experimental assembly and parameters for welding a gusset plate on car body parts

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Grid

Welding trials on uncoated steel sheets

Preliminary tests on steel sheets of 2 mm thickness showed that alternating welding in flat position with a break of about 2 s after each weld seam lead to less deflection compared with same direction welding.

According to a study of A. Nickel [8], the deflection is also significantly influenced by the orientation of the weld seams on the sheet metal. Depositing a long raster pattern leads to more deflection (Fig. 6) than a short raster pattern.

Fig. 7

Three types of samples for bending test: without grid (a), orthogonal grid (b), diagonal grid (c)

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A bending test according to VDA-238 was used. In order to be able to evaluate the direction of the grid in comparison to the bending axis, additional diagonal grids were welded. Three samples were investigated (Fig. 7): base material, orthogonal grid, and diagonal grid. The distance between each weld seams was kept equal. To achieve excellent penetration at the intersection points, the welding power was chosen to be higher for the second layers. The welding parameters are listed in Table 3.

Table 3 Parameters for welding a grid on uncoated steel sheets

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Fig. 8

a, b Welding results of additive manufacturing a grid on zinc-coated steel sheets of 2 mm thickness

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Welding trials on zinc-coated steel sheets

In order to be able to evaluate the applicability on zinc-coated steel sheets, welding was also tried on these materials (Fig. 8). The short patterns were welded first.

Fig. 9

Experimental assembly for bending test according to VDA-238 [9]

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Welding trials on zinc-coated car body parts

The slicing of the models as known from large components is not necessary, but rather the welding sequence is important, so that the thin body panels do not deform too much. Furthermore, there was no complicated programming needed. Simple lines were sufficient. On the car body parts, the long patterns were welded first. The parameters in Table 4 were set for welding to the body parts (assembly the same as above).

Table 4 Parameters for welding a grid on car body parts

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Bending test

The uncoated steel sheets were examined in a three-point bending test according to VDA 238-100. The experimental assembly is shown in Fig. 9. A flat sheet is bent to an angled sheet until a certain bending angle is reached and the bending force decreased after the maximum. The maximum applied force represents the bending stiffness.

Fig. 10

a, b Welding results of additive manufacturing of a gusset plate on car body area of 0.7 mm thickness

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