Latest developments in coaxial multiwire high-power laser cladding

Laser cladding technology is widely used to apply tailored surface coatings, as well as three-dimensional deposits for the repair and additive layer-by-layer fabrication of metallic parts.¹ The main applications are corrosion, wear, or oxidation protection for oil and gas, automotive, aircraft, shipping, or heavy industries.^2–5 The most common laser cladding method is using a coaxial nozzle powder system: the powder from the feeding nozzle is injected by the carrier gas and is melted synchronously by the laser beam to form the melt pool of the cladding layer. High-precision deposition of the materials, low dilution, high level of adhesive strength, low porosity, and a smooth interface with the substrate, which prevents stress concentration at the clad-substrate interface during operation, are beneficial for the long life and high performance of the clad components. However, the processing of larger components is economically challenging mainly because of low deposition rates. In the past, deposition rates of powder laser cladding were too low (up to 1.6 kg/h at laser power up to 4 kW) and specific costs were too high, especially for large-area claddings.^6,7

As a result of the dynamic development of the laser sources, as well as the cladding processes, the conditions have changed right favorable even for the high-performance processes. Fraunhofer IWS has been developing the HICLAD® process family for powder laser cladding for several years to enable customized solutions for highly productive coating processes using high-power diode lasers.⁸ Fraunhofer IWS together with the company Laserline GmbH (Mülheim-Kärlich, Germany) has achieved and surpassed deposition rates with 20 kW fiber-coupled diode lasers, which were previously only feasible using plasma transferred-arc (PTA) processes.⁹ As an example, for the Inconel 625 nickel alloy, deposition rates of more than 14 kg/h were obtained using a COAXpowerline powder nozzle.⁷ Moreover, the productivity can be roughly tripled compared to a solution using a conventional 9 kW laser, resulting in a significant reduction in the production cost.^7,9 Recently, high-power laser cladding of carbides has been applied to coat WC-316L on automotive brake discs, where a significant reduction in the total wear of friction was proved in the dyno test rig.¹⁰ 

Coaxial wire laser cladding is a technology that uses wire as the filler material to form the clad. The advantages of wire cladding are that the wires are less expensive, they can be easily directly inserted into the welding zone, and they ensure to produce clads where the wire usage efficiency is near 100%. Further advantages are significantly less porosity and better conditions with respect to occupational safety and health protection compared to powder-based processes.^11,12

Different coaxial wire processing heads have been developed.^13–16 In the COAXwire laser processing optic developed by Fraunhofer IWS, the laser beam is split into three separate beams, which are subsequently focused on a circular focal spot (Fig. 1).^13,17 With a fixed aspect ratio of 1:3, the focus diameter can be changed by selecting the fiber diameter. This enables the wire to be fed precisely into the laser beam axis and, thus, into the center of the laser-induced weld pool and, thus, an omni-directional welding process is provided. The optical elements are protected by a cross jet stream. To avoid oxidation in the process zone, the shielding gas flow displaces environmental air.

The COAXwire system is suitable for lasers up to 6 kW power and ensures the feeding of wires with diameters from 0.4 to 1.6 mm. The application field of the COAXwire optic includes cladding, repair, and additive manufacturing processes. Good examples are wear protective coatings of iron, cobalt, nickel, and copper alloys, the repair of cutting and forming tools or components made by titanium and nickel aerospace alloys or the additive manufacturing of functional metal parts. The deposition rates of COAXwire are ∼3 kg/h for 2D parts and ∼250 cm³/h for 3D build-up parts.

While powder-based high-power laser cladding proves to be a versatile technology for industrial coating production, high-productivity solution for wire-based processes is still challenging.

At Fraunhofer IWS, a high-performance laser cladding head COAXquattro, designed for multi-powder and multi-wire modes, has been developed.¹⁷ The COAXquattro uses a new concept that can be applied both for omni-directional cladding and for high-power lasers up to 20 kW, and it is designed for multiwire modes (Fig. 2).

The COAXquattro head uses a centric laser beam optic, which is usable for up to 20 kW laser power; the laser beam path is closed and a protective gas can be introduced in the upper area to protect the optics and shield the molten bath from atmospheric oxygen. The filler material is fed into the melt pool via multiple wire feeders coaxially around the laser beam. The adjustment is carried out by means of xyz adjustment unit integrated in the adaptation for the coaxial alignment of the wires fed to the laser beam.

The use of multiple wires couples more energy in the material, which increases the energy efficiency and the deposition rate. In addition, multiple reflection occurs between the wires, which further increases absorption. The position of the wires in the melting pool can be varied by the working distance between the wire nozzle and workpiece or by the angle at which the wires are fed into the laser beam. By using multiple wires, the filler material is fed to the melt pool over a larger area and, thus, more uniformly, which results in a lower degree of mixing with the workpiece material. Different wire materials can be fed into the process at various feed rates in order to adjust mixing ratios or to create gradient clad layers. With wire, the nozzle achieves 100% material utilization and extremely high deposition rates with optimum material efficiency.

A particularity of COAXquattro is the possibility of simultaneous processing of wire and powder. The system guides four separately controllable wires and powder streams coaxially into the laser focus. In doing so, combinations of different wires and powders and variable feeding rates can be used. This new concept makes it easier to create in situ alloys from a variety of wire and powder combinations during the cladding process.

The main technical data of the coaxial multiwire high-power laser cladding COAXquattro head are given in Table I.

Typical applications of the COAXquattro laser system include large-area corrosion and wear protection or repair coatings, and deposition of plain bearing alloys, as well as the direct manufacturing of 3D structures of aluminum, especially for large scaled components.

COAXquattro was applied to clad Inconel 625 alloy wires. High-power laser diode fiber (LDF 20000-200 from Laserline) power and wavelength between 940 and 1060 nm was used as laser beam source. The diameter of wires was 1.6 mm. For multi-wire cladding, different processing parameters—laser power, scanning speed, and wire feed rate—were selected to evaluate the potential of -the COAXquattro head system, Table II.

The investigations show the linear dependence of the wire cladding rate on the laser power (Fig. 3). Thus, the deposition rate increases almost proportionally with the laser power and deposition rates from 9 kg/h at 10 kW laser power to 18 kg/h Inconel 625 at 20 kW could be achieved. For the same laser power, the deposition rates of wire-clad Inconel 625 coatings were even higher than those obtained during the high-power laser cladding of Inconel 625 powder.⁷ 

The cross sections of Inconel 625 wire-clad coatings on a carbon steel substrate are shown in micrographs of Fig. 4.

With only one weld pass, the thickness of the clad increased from ∼1.8 mm at 10 kW laser power to ∼3.5 mm at 20 kW laser power. The clads were well-bonded to the substrate and showed smooth surfaces with no visible cracks. The cross-sectional micrographs are cracks-free with near zero porosity, as previously observed in the coatings obtained from high-power laser cladding of Inconel 625 powder.⁷ In the wire-clad coating, a well metallurgical bonding with no distortion and with low dilution is visible. At the interface area between the workpiece and clad layer, some inclusions and hot cracks could be observed. The heat affected zone (HAZ) decreases with an increase in the wire feed rate or with an increase in the scanning speed.

Due to their superior sliding wear properties and corrosion resistance, tin bronzes are mainly used in the manufacturing of bearings. The COAXquattro system was applied to clad CuSn12 (Cu5410) and alloyed CuSn12-Ni2.2 bronze bearings (Fig. 5). The wire diameter of CuSn12 was 1.6 mm, and the laser power was fixed at 20 kW.

In the cross-sectional micrograph of Fig. 5(b), homogeneous clad coating with no cracks nor dilution was obtained by the wire cladding of CuSn12 tin bronze. The microstructure of wire-clad CuSn12 coatings was similar and even better than that obtained by the laser cladding of powder.¹⁸ 

In addition to the wires, powder materials can be simultaneously injected with the COAXquattro laser head system, too. Exemplarily, a Ni powder (at ∼2.2 wt. %) was fed to the CuSn12 wire to create in situ alloying; the laser power was ∼17 kW. In the CuSn12-Ni2.2 wire-powder clad coating, there were no cracks nor dilution observed [Fig. 6(a)]. Some few cracks were identified in the carbon steel substrate.

In the high-magnification SEM micrograph, the dendritic structure of the coating could be observed; moreover, Ni was homogeneously distributed in the CuSn12 matrix; the content of Ni was between 2.1% and 3.8% as estimated by energy dispersive x-ray spectroscopy (EDX) analysis [Fig. 6(b)]. For comparison purpose, the microstructure of a CuSn12Ni2 coating obtained by the high-power laser cladding of the prealloyed powder is depicted in Fig. 6(c).

Small addition of Ni improved the mechanical properties of the CuSn12 bearing material, where the hardness values were almost constant along the coating microstructure (Fig. 7).

Thanks to their remarkable properties, carbide-based materials find a wide range of applications for surface protection against abrasion, sliding wear, and corrosion. Because the carbides cannot be produced in the wire form, tungsten carbide (60%) was simultaneously fed to the Inconel 625 wire (Ø1.6 mm) into the COAXquattro system to clad in situ metal matrix composites (MMCs) [Fig. 8(a)]. The resulted MMC coating shows a good bonding with almost no distortion to the substrate. The coating microstructure is pore- and crack-free and contains a homogeneous distribution of nondissolved hard WC particles in the matrix of well-molten Inconel 625 [Fig. 8(b)].

The microstructure of MMC coatings produced by in situ COAXquattro cladding was similar to that observed during the high-power laser cladding of powder mixtures of 60% fused WC + 40% Ni-alloy [Fig. 8(c)]. With optimized parameters, deposition rates up to more than 15 kg/h could be obtained for the COAXquattro of MMC coatings. These values were in the same order as those reached by the high-power laser cladding of powder mixtures.

COAXquattro cladding was applied to generate build-up wall structures with high deposition rates. Al2319 aluminum alloy wires (AlCu6MnZrTi, Ø1.6 mm) were cladded with 20 kW laser power at a wire feed rate of 3000 mm/min and a cladding speed from 700 to 1000 mm/min to produce single tracks with 12 mm width and 3 mm height (Fig. 9).

The clads were well-bonded to the substrate and showed smooth surfaces with no visible cracks. A well metallurgical bonding with no distortion and low dilution is visible in the polished cross-sectional microstructure. The aluminum wire-clad coatings were crack-free with a low content of porosity [Fig. 9(c)].

Preliminary studies on 3D wall structures were carried out using COAXquattro of Al2319 wires at a feeding rate of 2000 mm/min, a laser power of 12.5 kW, and a cladding speed of 800 mm/min [Fig. 10(a)].

The 3D wall shows a crack-free structure but with visible inter-run porosity [Fig. 10(b)]. The presence of the pores could be explained by the high gas affinity of Al-based alloy melt¹⁹; another reason can be gas entrapment during cladding and melt solidification. Fine dentritic structure free of cracks is observed at the transition area between the substrate and clad [Fig. 10(c)].

Deposition rates of ∼4.5 kg/h for 2D structures and ∼1600 cm²/h for 3D structures could be obtained.

At Fraunhofer IWS, a new nozzle for high-power high-productivity laser wire cladding for coating and additive manufacturing, the so-called COAXquattro, has been developed. This system enables to feed at the same time four wires into the melt pool, reaching deposition efficiencies in the same range as a high-power laser powder-based cladding process. For selected materials, the improvement in coating quality compared to powder laser cladding is achieved. Furthermore, with the COAXquattro system, simultaneous feeding of powder particles to wire cladding presents a high versatility and a great potential for in situ alloying and cost-effective production of new compositions on material alloying or hardmetal-reinforced composites for coating application and 3D additive manufacturing. The inter-run porosity of Al-based and other alloys could be reduced with the help of gas shielding devices.

The authors have no conflicts to disclose.

Filofteia-Laura Toma: Conceptualization (equal); Data curation (equal); Visualization (equal); Writing – original draft (lead); Writing – review & editing (lead). Holger Hillig: Conceptualization (equal); Data curation (equal); Funding acquisition (equal); Project administration (lead); Writing – original draft (supporting); Writing – review & editing (supporting). Marc Kaubisch: Conceptualization (supporting); Data curation (supporting); Funding acquisition (supporting); Investigation (equal); Methodology (equal); Writing – original draft (supporting). Irina Shakhverdova: Data curation (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (supporting). Marko Seifert: Funding acquisition (supporting); Project administration (supporting); Frank Brueckner: Funding acquisition (supporting).