Keywords

1 Introduction

In Laser Metal Deposition (LMD) a laser source is used to create an energy beam focused to melt the metal which is deposited through a nozzle. The deposited material could be powder or filament and it is deposited layer by layer only in the needed areas to achieve the final geometry of the part. Through LMD technology, medium or large preforms and final parts may be manufactured, mainly depending on the real printing volume of the printing equipment employed [1]. Compared to traditional repair technologies such as TIG or WAAM welding processes, LMD allows the material deposition with the following benefits: (i) low heat transference which means less distortion and reduced thermal stresses on the substrate; (ii) higher material deposition rate compared to other AM technologies, e.g. powder bed fusion [2].

The interest in the Inconel 718 (IN718) superalloy is highly increasing in additive manufacturing (AM) due to the wide use of this kind of alloy in the aerospace sector or the field of repair components. The IN718 material is well known for applications where high temperatures, excellent mechanical properties, and corrosion resistance are required [3]. However, the presence of niobium as an alloying element in this alloy tends to segregate, which strongly influences the precipitation of hardening phases. Some research studies have employed different strategies to control and minimize the niobium-enriched eutectoids in both welding and AM processes [4].

Friction Stir Welding (FSW) is a solid-state joining process [5, 6]. It uses a non-consumable tool to join two facing workpieces without melting the workpiece material. FSW is carried out by a rotating cylindrical tool composed of a pin and a shoulder. The tool is inserted and moved forward between the two work pieces to join. The generated frictional heat together with that obtained by the mechanical mixing process turns the stirred materials to soften without melting.

Due to its high energy efficiency, environment friendly and versatility, the joining FSW process is considered one of the most innovative developments for metal joining within the last decades [7]. Although the FSW tool is defined as a non-consumable tool, it can be considered when very high-resistance materials or difficult-to-weld materials are joining. In this case, different FSW tools are needed since they are changed when are damaged or worn. These FSW tools are also replaced when the rest of the softened materials remained adhered to the tool. Using damaged FSW tools creates a non-reproducible join or low-quality welding [8]. The cost of these commercially available FSW tools is around 1000 €/unit for Inconel superalloys which can limit their use.

As an alternative, in this work, the advanced metal AM LMD technology is used for the processing of tools for an innovative joining process through FSW technology. The material selected to be processed by LMD was the nickel-based IN718 superalloy due to its excellent mechanical properties. The IN718 superalloy is found in powder and wire material, thus a wide range of technologies can be employed to process this alloy.

This research results in relevant benefits for the industry using process-energy saving, material reduction, and higher affordable FSW tools together with a faster method of production. A high value is added to this research study through the possibility of customization of these FSW tools by the employ of the LMD technology.

2 Experimental Procedure

2.1 Design and Manufacturing of the FSW Tools

IN718 tools for the robotic FSW process have been manufactured. The selection and design study of the tools was performed using CATIA V5. A holistic approach to the tool design specifically adapted to the thickness of the material to be welded and to the FSW head was conducted. The commercial FSW spindle employed was a CYSTIR model from CYTEC. The FSW head was placed in a KUKA robot.

The used LMD system was an M450 device with a multi-laser metal deposition printhead (Meltio). The used process parameters were: Laser speed 450 (mm/min), layer height 1.2 (mm), laser power 900 (W), hatching distance 1 (mm), current 2 (A) and gas flow 10 (L/min). These optimized parameters were defined in a previous work.

The Nippon Gases (NIPPON M-218) Inconel 718 superalloy wire material was used. Its chemical composition (in wt. %) was: C (0.05), Mn (0.2), Si (0.2), Cr (19), Mo (3), Fe (20), Ti (0.9), Al (0.5), Nb+Ta (5.2), Ni (balance). The FSW tool was designed for its manufacturing by the M450 LMD technology. A machining post-process was needed to achieve its final geometry (Fig. 1). Before the machining process, the tools were separated from the build plate and thermally treated.

Fig. 1.
figure 1

a) 3D-model of the FSW tool, and b) IN718 FSW tool manufactured by LMD.

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2.2 Heat Treatments

To find the optimum heat treatment for the material, four blocks of dimensions 120 mm × 25 mm × 50 mm were manufactured in the IN718 material. The heat treatments performed consisted of TT1-initial heat treatment for stress relief and TT2-additional heat treatment for ageing. Of the four blocks manufactured by LMD, two of them underwent TT1 and the other two underwent TT1+TT2. Details are shown in Table 1.

Table 1. Heat treatment conditions of the IN718 specimens manufactured by LMD.
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2.3 Characterization of the Manufactured Material

The material was characterized by optical microscopy (OM) to obtain the level of densification/porosity and the quality of the manufactured tool. A statistical study of defects and densification value was determined. Image analysis was carried out using ImageJ software and the mean value represented corresponds to the analysis of four images for each manufacturing plane. IN718 tensile samples were machined from the original blocks for mechanical characterization in the different heat treatment states. The mechanical anisotropy between the XY plane (direction parallel to the material deposition) and XZ plane (direction parallel to the manufacturing direction) was considered, therefore tensile specimens were obtained in the two different directions for complete characterization. Cylindrical specimens of 5 mm diameter × 32 mm length were tested in an INSTRON universal tester at room temperature (ISO 6892-1:2020B).

3 Results and Discussion

Figure 2 shows the optical microscopy (OM) images of the IN718 material fabricated by LMD with TT1+TT2 heat treatment in both planes XY and XZ. In general, high densification has been obtained in the analysed samples, however small defects associated with the AM process itself have been found. Small circular pores could be distinguished, which are usually associated with gas bubbles trapped during the process. Irregular defects were also observed, which may be evidence of certain points with a lack of fusion in some regions. However, the overall densification is above 99%. The values obtained are 99.30% ± 0.28 for the XY plane and 99.03% ± 0.24 for the XZ plane.

Fig. 2.
figure 2

OM images of the IN718 material processed by LMD after stress relief heat treatment-ageing. Densification of the material obtained in a) XY and b) Z.

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Table 2 shows the results of the mechanical properties of the material manufactured by LMD. It shows the average values obtained in the different tensile tests performed. The IN718 material manufactured by LMD was submitted to different heat treatments (TT1-stress relieved and TT1+TT2-stress relieved + ageing) for the XY and XZ manufacturing planes. Table 2 also shows the reference values for forged and cast IN718.

Table 2. Tensile mechanical properties of the IN718 material manufactured by LMD with different heat treatments and their comparison with the standards.
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In both planes, XY and XZ, good mechanical properties: tensile strength (UTS), yield strength (yield) and elongation; were observed for the LMD manufactured material.

Comparing the results of the mechanical tests between the different heat treatments, it has been observed that the combination of TT1+TT2 significantly improves the UTS and yield compared to TT1 performed individually. This behaviour has also been found by other authors [9]. The values compared were those obtained for the same plane, XY or XZ. The UTS values indicate an increase of 23.6% and 30.6% for the XY and XZ planes, respectively, for the IN718 with TT1+TT2 compared to TT1.

In the XY plane, a yield increase from 631 MPa to 980 MPa was identified. After the combination of TT1+TT2, the strength increased up to 55.3%. The elongation, however, is slightly higher for the material with the TT1 treatment compared to TT1+TT2. Values of 18% and 15% were obtained in the TT1 treatment, for the TT1+TT2 treatment which showed elongations of between 11% and 10%, in XY and XZ planes, respectively. One possible reason could be the extra hardening process induced with heat treatment TT1+TT2 for TT1. In metals, it is known that an increase in the strength (UTS and yield), directly related to microstructure, causes a decrease in ductility. Because of the deformation mechanisms operating during deformation, which dependent on grain size, solutes and distance between precipitates. This behaviour was already noted in the Selective Laser Melting (SLM) for the same alloy (IN718) [10].

TT1 stress relieving provides reasonable UTS properties with exceptional ductility values for this material as well as a lower yield value compared to the reference values. The TT1+TT2 treatment increased the strength from 20% to 50% with excellent UTS and yield properties, maintaining a similar ductility to the forging values. Elongation was higher than 10% in all the conditions. This means that the LMD process for IN718 retains or improves the reference values, as also was found in similar studies [9, 11].

Regarding XY and XZ building directions, UTS and yield values were higher in the XY plane. For the optimized heat treatment (TT1+TT2), the average strength of UTS and yield was about 4% higher in the XY plane compared to the XZ plane. The ductility was very similar between the two build directions, indicating that the anisotropy did not have a relevant impact on this LMD-processed material. This anisotropy in mechanical properties between the different XY and XZ build planes relates to the AM process, due to the layer by layer deposition, thermal and microstructural gradients are created. This is fundamental for the optimization of the processing by AM [12].

In general, the IN718 alloy manufactured by LMD presented mechanical properties equal to or better than the standard forging values and far superior to those of the as-cast alloy, especially for the LMD-manufactured and TT1+TT2 heat-treated material. The ductility was equal or improved depending on the case. With these results, the LMD AM technology with wire material has been validated for this application.

Fig. 3.
figure 3

Robotic FSW lab at Cetemet. a) The machined IN718 tool, b) the tool inserted in the holder, c) the tool holder with the spindle and d) the Robot positioned for FW.

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The LMD-manufactured and machined IN718 tool was installed in the tool holder to be used in the robotic FSW application. The spindle, tool holder and tool were installed on the heavy-duty robot located at the facilities of Cetemet in Spain (Fig. 3).

4 Conclusions

The nickel-based IN178 superalloy was successfully additively manufactured by Laser Metal Deposition (LMD) technology for Robotic Friction Stir Welding (RFSW).

The LMD process parameters allowed good overall material properties. The densification of the processed material was above 99%. It was 99.30% ± 0.28 and 99.03% ± 0.24 for the XY and XZ build planes, respectively. The mechanical properties of the manufactured and heat-treated material were even higher than the standard IN718 forged alloy. An average maximum strength (UTS) of 1256 MPa was achieved in the XY plane. The ductility values were the same or slightly higher compared to the standard values of IN718. The TT1+TT2 (stress relieving-ageing) heat treatment for IN178 highly improved the mechanical behavior. The yield strength increased up to 55.3%.

The use of the LMD processing has allowed: a) cost reduction of the FSW tool, b) decrease of the whole manufacturing time, including processing and machining, c) reduction of the lead time from weeks/months to hours/days, d) the dynamic tool customization to fit specific welding needs by only re-designing and 3D printing.