Double-Wire Cladding Techniques: A Deep Dive into Additive Manufacturing

The overlay welding process has high efficiency and low equipment cost , and is widely used in the manufacturing and repair of parts, the modification of material surfaces, and the flexible production of small and medium batches of metal components.

In actual use of underground engineering equipment, the equipment is subjected to complex forces and harsh working conditions.

For example, the tunnel boring machine is a tunneling machine that uses the shield method. During the use of the tunnel boring machine underground, the cutterhead and related components such as cutterhead protection blocks, cutting seat, and edge scraper seat inevitably experience wear and tear.

The overlay welding process is not only applied to the remanufacturing and repair of worn components of the cutterhead, but also to the additive preparation of wear-resistant layers and wear-resistant nets of parts, in order to increase the wear resistance of parts. In actual production, in order to improve production efficiency, a larger welding current is often used.

However, due to the limitations of the overlay welding quality, when the current is too large, the dilution rate will increase, which can cause defects such as segregation of the alloy composition and loss of liquid metal during the overlay welding process.

On the contrary, when the welding current in the overlay welding application is relatively small, it will lead to lower production efficiency.

The double wire welding, as an efficient welding method, has been increasingly concerned by people. The double wire welding can achieve high weld deposition rate, and also improve the composition and crystallization of the weld seam by using the temperature field and thermal cycle of the double wire welding, thus enhancing the microstructure and performance of the overlay welding layer.

Therefore, exploring the application of double wire welding in overlay welding, balancing the forming and quality of the overlay welding, and improving the actual production efficiency, are of great significance for the practical technical application of overlay welding.

Overlay welding additive manufacturing system

The welding power source used in the experiment is the QINEO PULSE 600 from CLOOS. When using the QINEO welding machine to perform small current pulse welding, the spatter is small and the forming is beautiful.

Based on this, a double wire welding process is developed which not only considers the quality of the weld seam but also greatly improves the welding deposition efficiency. The double wire welding equipment adopts a double wire gun structure, in which the two wires are constantly melted in the same pool.

The front wire provides preheating for the rear wire, and the rear wire reheats the front wire, which improves the microstructure and performance of the overlay welding layer.

Since the two wires are insulated from each other, it is possible to use a variety of flexible and diverse combinations.

This not only allows the independent adjustment of the parameters of the two wires but also allows the selection of two different diameters and different materials of wires according to specific application requirements, thus covering a wide range of applications.

The working principle of the shared melting pool double wire welding is shown in Figure 1.

As a comparison with single-wire welding in the experiment, the single and double wires are freely switched through the double-wire welding system, while other welding methods and protective gas conditions remain unchanged.

In the experiment, the QINEO PULSE 600 welding machine from CLOOS is used as the welding power source to melt the overlay welding wire, and the CLOOS robot is equipped with a double wire welding gun to ensure the accuracy of the welding gun movement during welding and to control the welding speed. Some parts of the overlay welding system and hardware are shown in Figure 2.

Experimental methods and processes

The base material used in the experiment is Q235 steel, with a thickness of 12mm, and its main chemical composition is shown in Table 1. The UTP AF ROBOTIC 600 wear-resistant welding wire is used in the experiment, with a model of DIN 8555: MSG 6-GF-60-GP, and its main chemical composition is shown in Table 2. The diameter of the welding wire is 1.2mm. The protective gas used is 80% Ar + 20% CO₂.

Table 1: Chemical composition of base material (mass fraction) (%)

Table 2: Chemical composition of welding wire (mass fraction) (%)

The traditional welding parameters for single-wire welding are shown in Table 3.

Table 3: Welding Parameters for Overlay Welding

The welding effect is shown in Figure 3, with a weld width of 10.64mm, weld height of 3.43mm and fusion depth of 1.13mm.

For the double-wire overlay welding test, the welding method and shielding gas conditions were consistent with those of the single-wire overlay welding. The dry elongation in the experiment was 20mm. Using the orthogonal experimental method, the front wire current, rear wire current, and welding speed were adjusted to conduct a three-factor and four-level orthogonal experiment. Weld width and weld height data were obtained by observing and measuring the weld formation. Some welding parameters and weld size are shown in Table 4.

Table 4: Welding Parameters for Double-wire Welding

By analyzing the welding current, weld width and weld height data of the double-wire welding, it can be seen that when the welding current and welding speed change, considering the fluctuation of weld width and height caused by variations in weld formation and measurement errors, the change in weld height is not significant, while the change in weld width is more prominent.

When the welding speed is kept constant at 35cm/min, 40cm/min, and 45cm/min respectively, the relationship between weld width and front/rear wire current is fitted with a surface equation.

The established surface equation model function is:

In the formula:

y – weld width (mm);
x1 – current of front wire (A);
x2 – current of back wire (A);
a0, a1, a2, a3, a4, and a5 – coefficients.

When the welding speeds are 35cm/min, 40cm/min, and 45cm/min, the coefficients a3, a4, and a5 in the equation are approximately 0. When the speed is 35cm/min, the surface fitting equation is:

Thus, it can be inferred that the terms x1x2, x12, and x22 in the fitting equation have a relatively small impact on the value of y.

By using the fitting formula to test the experimental data at the speeds of 40cm/min and 45cm/min, and inputting the current values of the front and back wires to obtain the value of y, the calculated values of y and the actual weld width exhibit a fairly uniform error.

The relationship between the weld width and the current of double-wire welding can be obtained from formula (2), as shown in figure 4.

According to formula (2), the weld width is positively correlated with the current of the front and back wires, and approximates a linear relationship, with the effect of the back wire current being greater. In the actual welding process, the front wire has a preheating effect on the back wire, while the back wire has a significant effect on the molten pool.

The molten pool is influenced by the arc force of the back wire and the continuous heat, which increases the tendency of metal liquid flow in the molten pool and ultimately leads to an increase in weld width.

When the welding currents of double-wire surfacing are 140A and 120A, and the welding speed is 30cm/min, the weld width is 10.73mm, the height is 3.23mm, and the penetration depth is 0.82mm. The surfacing effect is good, as shown in figure 5.

At this time, the size of the double-wire surfacing is similar to that of single-wire surfacing, and the penetration depth of the double-wire surfacing is shallower. The heat affected zone is reduced, the reaction degree with the base metal is reduced, and the dilution rate is decreased, which is beneficial to improve the quality of surfacing.

The welding speed is increased by more than 50% compared to traditional single-wire surfacing, greatly improving the production efficiency.

Microstructure Testing and Analysis:

For the double-wire surfacing and single-wire surfacing samples, a 20mm×10mm×10mm surfacing specimen was obtained by cutting, and its performance was tested and analyzed. The welding parameters are shown in Table 5.

Table 5 Main Welding Parameters of Specimens

Micro-hardness testing

A 600HVS-1000AVT type image micro-hardness tester from China was used to perform micro-hardness testing on the samples. The Vickers indenter was a four-sided pyramid-shaped one. The load was 300g (2.94N) and 100g (98N), and the holding time was 15s.

The double-wire welding and single-wire welding samples were measured along the fusion line, starting from the weld surface as the initial measuring point, with a 1mm interval for dot measurement.

Multiple measurements were made at each measurement point, and the average value was obtained, resulting in a cross-sectional average micro-hardness distribution curve (see Figure 6).

From Figure 6, it can be seen that at the weld surface position, the hardness values of the double-wire welding and single-wire welding samples are similar.

From the weld bead surface to a distance of 3mm away from the weld bead surface, the hardness value of the double-wire welding sample remains basically unchanged, with the hardness value of double-wire welding sample 2 slightly increasing, while the hardness value of the single-wire welding sample drops gradually.

At a distance of 3-5mm away from the weld bead surface, the hardness values of the double-wire welding and single-wire welding samples rapidly decrease until they approach the matrix hardness (140HV0.2).

From the microhardness testing, it can be seen that the surface hardness of the double-wire welding sample’s surfacing layer is above 700HV0.2, meeting the hardness requirements of actual surfacing applications.

During single-wire surfacing and double-wire surfacing, the alloy elements of the surfacing layer diffuse towards the base metal, and the closer the surfacing layer is to the base metal, the more obvious the hardness decrease.

From the hardness distribution curve, it can be inferred that during single-wire surfacing, the diffusion process is relatively stable, and the hardness value is significantly affected by the distance.

As the surfacing layer approaches the base metal, the hardness value gradually decreases.

In double-wire surfacing, the use of temperature fields and thermal cycles in double-wire welding improves the element diffusion process, optimizes the structure and properties of the surfacing layer, and within a certain range of distance from the surface of the surfacing layer, the hardness value remains basically unchanged.

Friction and wear performance testing

The double-wire surfacing sample and single-wire surfacing sample were subjected to dry sliding wear tests under the same environmental conditions (temperature: 28-30°C, humidity: 60%) on the HT 1000 ball-on-disc machine.

A Si3N4 ball with a diameter of 4mm was selected for the test, and the load was fixed at 10N, the sliding speed was fixed at 59mm/s, and the wear time was 30 minutes. The wear amount was measured using an FA2104 precision balance.

Observing the testing process, it was found that under smaller loads and lower speeds, double-wire welding sample 2 experienced a short period of mild wear combining abrasive wear and plastic deformation, but stabilized after about 1 minute.

The trend of the friction coefficient curve changed similarly to the single-wire welding sample. The friction coefficient of double-wire welding sample 1 fluctuated greatly, and double-wire welding sample 3 entered the friction steady-state stage after a longer period of time.

The friction coefficient of the single-wire welding sample was the smallest, fluctuating around 0.4, and the friction coefficient of the double-wire welding was 0.6-0.8.

Due to the low sample temperature, no melting wear was observed in any of the samples. The friction and wear performance test results are shown in Figure 7.

From Figure 7b, it can be seen that the double-wire welding sample has extremely small frictional loss, while the frictional loss of the single-wire welding sample is approximately 1.5g.

The test results of friction and wear performance indicate that compared with single-wire surfacing, double-wire surfacing results in an increase in the friction coefficient and a decrease in the wear amount.

Figure 7: Results of friction and wear performance testing of the samples.

Wear Surface Structural Performance Test

Material wear is a complex process. To confirm the reason for the loss of wear amount, the morphology and component analysis of the worn surface of the double-wire deposited welding sample and the single-wire deposited welding sample after friction test were conducted using ZeissSigma scanning electron microscope (SEM) and Smartedx energy dispersive spectroscopy (EDS).

The SEM and EDS images of the worn surface of the double-wire deposited welding sample and the single-wire deposited welding sample after friction testing are shown in Figure 8.

It can be seen from Figure 8 that the surface of the double-wire welding sample 1 is mainly composed of shallow and fine plow marks with a small amount of adhesion marks.

At this time, the wear is mainly abrasive wear. The adhesive area of the single-wire welding sample surface increases, and there are many white particles.

Through EDS comparison and analysis, it is determined that the white particles are mainly compounds containing Si elements. The silicon compound is mainly due to the high hardness of the counter friction pair in the dry friction wear process.

The wear particles attached to the surface of the sample, at this time, the wear is mainly abrasive wear and adhesive wear.

It is inferred that the metal crystals that form silicon compounds during single-wire deposition have poor anti-adhesive properties, which increases adhesive wear during friction and increases wear.

During double-wire deposition, the composition and crystallization of silicon compounds are improved, which reduces wear.

Conclusion

In the welding operation, the double-wire deposit welding method is adopted. By adjusting the welding parameters and controlling the forming size of the deposit layer, and utilizing the temperature field and thermal cycle characteristics of double-wire welding, the composition and crystallization of the weld are improved and the dilution rate is reduced.

This improves the organizational performance and wear resistance of the deposit layer to a certain extent, and the efficiency of the deposit welding is greatly improved.

The results of this study have reference value for the application of deposit welding in underground engineering equipment, as well as the application of double-wire welding in the field of deposit welding and arc additive manufacturing.