Wire arc additive manufacturing using high-strength steel tubular and solid wires

Abstract Wire Arc Additive Manufacturing (WAAM) utilizes wire as the feedstock and welding arc as the heat source. While Solid Wires (SW) are common, exploration of tubular wires such as Metal Cored Wires (MCW) in Additive Manufacturing (AM) is limited. MCW offers flexibility for alloy design, but both SW and MCW can create silicon islands on welds, affecting mechanical properties and processability. This study uses Gas Metal Arc Welding (GMAW) in Cold Metal Transferred (CMT) mode to compare SW and MCW deposits with different gases. MCW shows more uniform penetration, potentially reducing lack of fusion in AM layers. A novel approach is then used to modify the MCW to minimize silicate formation, reducing islands on the surface. Comparative analysis shows a significant reduction and change in the location of silicates with modified MCW compared to standard, with mechanical properties in as-welded and after post-weld heat treatment (PWHT) remaining comparable to the standard wire.


Introduction
Wire arc additive manufacturing, WAAM, uses wire as the feed-stock material and an electric arc as the heat source.The WAAM process has recently attracted significant interest because of its higher deposition rate, lower material cost, and higher material usage efficiency compared to AM techniques with powder.Currently, it is the general understanding that the wires produced for welding can be appropriately selected for AM [1,2].However, as AM technology is developing, it would not be surprising that the need for specific modifications, especially in certain alloy systems and applications, is identified [3][4][5][6].
Even though the use of tubular wires especially metal-cored wires has gained a lot of attention in the welding industry and their usage is constantly increasing, AM has been mainly focused on the use of solid wires.The relatively higher cost of tubular wires has been a limiting factor.Compared to solid wire it is more flexible to modify the chemical composition in a tubular wire by adjusting the cored metallic components [7].This flexibility in alloy design is of crucial importance as AM is still maturing and there is a need for a lot of research to fully understand the wire characteristics and composition required for AM.The use of tubular cored wire is even a more economical alternative compared to manufacturing solid wires as the custom ingot production is extremely expensive specially in small-scale manufacturing more suitable for research and development projects.MCW is especially more interesting where there are limited solid wire options for some specific alloys such as tool steels or where it is not possible to have the alloy in solid form such as the case of Stellite alloy.Higher deposition rates can also be achieved by MCW compared to the solid wire.Application of metal cored wire is of special interest in case of repair or build-up of parts where wear resistance is a requirement.Parts like drilling heads and pipe fittings have high attractiveness to be manufactured by WAAM as they have high geometrical complexity [8,9].
FCW has the disadvantage of the need for slag removal after each deposition.This limits deposition rate which is an essential part of AM technology.Even though solid wire and MCW do not produce any slags, the formation of silicon islands on the deposits and their removal may slow down the AM process, like slag removal in FCW but in a much lower extent.Some of the wire manufacturers have recently focused a lot of attention to reduce or even eliminate the silicon islands by modifying the wire formulation.Silicates formation management is more feasible in the case of MCW than solid wire due to the higher flexibility of MCW's manufacturing.For a deeper understanding of wires in AM, readers are encouraged to refer to Chapter 3 of the recently published book on additive manufacturing of high-performance metallic materials [10].
Literature has primarily focused on examining the effect of various process parameters on the microstructure and mechanical properties of parts made using solid wires.Use of mild carbon steel and to some extend use of low alloy high strength wires have been investigated by different researchers [11][12][13][14][15][16][17][18][19].Comparing solid and MCW in WAAM has not been greatly dealt with.No study considering the wire modification to manage silicon islands in WAAM has been reported to the best of the authors' knowledge.
This paper aims to address these gaps by investigating the differences in deposits made using solid wire and MCW and additionally analysing a modified MCW in terms of silicon islands management.Initially, deposits were made using SW and MCW using varying gas combinations were produced with the main aim to compare the penetration profiles.Subsequently, the modified high-strength steel MCW was utilized with the aim to investigating the formation of silicon islands, their location and distribution on the deposit surface.Mechanical properties of the deposits in the as welded and after PWHT were also studied.

Materials and methods
High-strength steel wires of 1.2 mm diameter and S355 carbon manganese steel were used in this paper as feedstock and substrate, respectively.The chemical composition of the wires and the base metal is shown in Table 1.The chemical composition of wires is measured using Optical Emission Spectroscopy (OES) and is the actual value, but the plate chemistry is a typical value.The high-strength solid wire (SW) has AWS A5.28 classification of ER100S-G and the metal cored wire (MCW) is AWS A5.28 classified as E110C-K4 H4.Modified metal cored wire (MCW-m) is the modified version of the standard MCW with silicon islands management.The wires are high strength with typical yield strengths of 725 MPa and 750 MPa for the solid and the cored wires, respectively.
In the first stage, single-layer deposits were made using SW and standard MCW with GMAW process in CMT mode assisted with an ABB robot.The main aim here is to compare solid wire and metal-cored wire profiles produced using different shielding gas combinations (Table 2).Deposition parameters are shown in Table 3.
In the second stage, the semi-automated GMAW method in spray arc mode was used to first produce fillet welds in the flat (PA), horizontal (PB) and vertical up (PF) positions.The welding parameters used are shown in Table 4.The aim here is to compare silicon islands formation and distribution in the standard metal cored wire (MCW) and the modified version  (MCW-m).Welding parameters were chosen to achieve high-quality welds without any defects.Using same parameters in the flat position, V-joints were then produced to compare mechanical properties between the two MCW, in as-welded and post-weld heat-treated condition.PWHT holding time and temperature used were 570 °C and 2-hr holding time.Tensile test was done at room temperature, and Charpy V-notch was done at different temperatures of −40 °C and −60 °C.Table 5 summarizes all mechanical tests performed in this part.

Comparing solid and metal-cored wire profiles
Figures 1 and 2 illustrate the cross-sectional profiles of deposits created using solid wire and the standard metal cored wire, respectively.It is evident that the solid wire produced a finger-like penetration profile, whereas the metal cored wire produced a more uniform penetration profile.This effect is not very dependent on the shielding gas combination as it is for the deposits made using the solid wire.The uniform profile made by the MCW is particularly advantageous in layer-by-layer additive manufacturing as it promotes a more even melting of the previous layer, thereby reducing the likelihood of inter-layer lack of fusion.

Silicon islands management using modified MCW
The modified metal-cored wire resulted in fewer silicon islands.The location of them was also changed from the deposit toe to surface making them easier to remove.Figure 3 shows the results in the PB position.As it is obvious from the figure the standard wire (MCW) produces more silicates distributed over the entire deposit surface while in the modified wire (MCW-m) the silicates are localized in few  locations over the deposit surface making them easier to remove.The same behaviour was seen in the PA and PF positions.Table 6 presents the chemical analysis of MCW-w in as-welded and after PWHT.The chemical composition of the standard MCW is also given in as-welded condition for comparison.A slight variation in the alloying elements is observed when comparing the two wires.However, both chemistries conform to well-known norms and standards.It is important to note that the primary objective in the development of the modified wire was not to change the alloy chemistry but rather to control the formation of silicon islands by adjusting the formulation without changing the overall alloy chemistry.
Table 7 Presents the tensile and Charpy V-notch results in as-welded and after PWHT.The results for the standard wire is also shown in as-welded condition.
The results of mechanical testing validate the acceptable tensile and toughness properties that are within AWS and ISO norms for both the standard and modified wires.The slightly higher mechanical properties observed in the case of MCW-m + As-welded, compared to the standard wire, can be attributed to the minor change in chemistry.It is important to emphasize that the primary focus is not into the variations in values among different wires or treatments, but rather on affirming that the management of silicon islands through wire formulation modification does not adversely impact the mechanical properties.

Conclusion
This study investigates the application of metal-cored wire in additive manufacturing, with emphasis on penetration profiles and silicon island management.Single deposits were made using solid wire, metal-cored wire, and a modified metal-cored wire designed for silicon island control.Deposition techniques employed  included Spray Arc and Cold Metal Transfer (CMT).The key findings are summarized as follows: • Metal-cored wire with modified chemistry exhibited a reduction in silicon islands, relocating silicates partly from the deposit toe to the deposit surface, also making them easier to remove.• Tensile and Charpy V-notch values for the modified Metal Cored Wire (MCW) in both as-deposited and post-weld heat treatment (PWHT) at 570 °C for 2 hours demonstrated that the modification of wire's formulation did not affect the mechanical performance adversely.• MCW showed a more uniform penetration profile than solid wire, suggesting potential benefits in additive manufacturing by minimizing the lack of fusion between layers.

Table 5 .Figure 1 .
Figure 1.Cross-sectional profiles of single-layer deposits produced using solid wire with different combinations of ar and Co 2 gases.

Figure 3 .
Figure 3. Silicon islands formation in the deposits produced using standard and modified metal cored wires in the horizontal position.note the change in location and amount of silicates.

Figure 2 .
Figure 2. Cross-sectional profiles of single-layer deposits produced using metal-cored wire with different combinations of ar and Co2 gases.

Table 1 .
Chemical composition (in wt.%) of the wires and base plate.

Table 2 .
different combination of ar and Co 2 gases for single layer deposition with solid (SW) and metal-cored (MCW) wires.

Table 3 .
Parameters used for deposition of single-layer deposits.

Table 4 .
Welding parameters used for evaluation of silicon islands in the metal cored wire.

Table 6 .
Chemical composition (in wt.%) of the original and modified metal cored wires.

Table 7 .
tensile and Charpy results in as-welded and after PWHt for the MCW-m and MCW wires.