Fundamentals of simultaneous machining and coating (SMaC) through combination of extreme high-speed laser material deposition (EHLA) and turning

Extreme high-speed laser material deposition (EHLA) has established itself in the coating industry as a resource-efficient and highly productive further development of the conventional LMD (DED-LB/M) process. EHLA is characterized by very high surface speeds of up to 1000 m/min, which, in addition to high coating rates of up to 20 m²/h, also results in a low dilution and minimal heat-affected zone.^1,2 Common applications include wear and corrosion resistant coatings and functional surfaces for tribological contacts in a variety of industries, such as automotive, petrochemical, aviation, tooling, and many others.³ Despite the comparatively high surface qualities achievable with EHLA reaching Ra values below 4.6 μm,⁴ subsequent mechanical processing of the deposited coatings by turning, milling, or grinding is still necessary for most applications and represents a significant cost driving factor. Against this background, Fraunhofer Institute for Laser Technology ILT developed a novel technology called simultaneous machining and coating (SMaC), which enables simultaneous mechanical processing of the applied layers by combining EHLA with machining in a single process step (Fig. 1).⁵ This is possible because the surface speeds of EHLA and turning are largely congruent. Besides improvements in productivity, further advantages are expected in terms of tool life and improved machinability of high-hardness coatings. This is attributed to the decrease in hardness and brittleness of most materials at higher temperatures—a phenomenon that is also exploited in laser assisted machining (LAM).^6,7 Residual heat of the EHLA process is, therefore, expected to have a significant impact on the combined process, rendering the horizontal offset between the tool center points (TCPs), a particularly vital parameter (Fig. 2). The greater the distance between the TCPs, the smaller the influence of residual heat on the machining process.

Integrating the subprocesses into a simultaneous operation raises several questions, e.g., how the parallelization of the two process steps affects the surface finish, whether the heat generated by the deposition process leads to dimensional deviations during machining, and how simultaneous machining influences tool wear. A fundamental investigation into the interactions between the two subprocesses and their effects on the coating result are the integral focus of this study.

Simultaneous processing requires a specialized system technology that incorporates additive manufacturing by EHLA and subtractive machining by turning in a single fixture. One such system was developed in cooperation between the Fraunhofer Institute for Laser Technology ILT and J.G. Weisser Söhne based on the Weisser ARTERY M-2 TM mill-turn center (Fig. 3). A Laserline LDF 8000-30 diode laser with a maximum output power of up to 8.7 kW and a beam quality of 30 mm × mrad serves as the beam source for the EHLA process. The beam is formed to a circular spot with a top-hat intensity distribution and a focus diameter adjustable between 1.4 and 4.6 mm by a Laserline OTZ-5 zoom optic. The powder material is supplied using a Twin 150 volumetric powder feeder from Oerlikon Metco through a continuous coaxial EHLA nozzle HighNo 4.0 from HD Sonderoptiken. The machine’s main spindle can achieve rotation speeds of up to 5700 rpm and can handle workpieces with a length of up to 1000 mm.

The modified ARTERY M-2 is equipped with a turret providing space for up to 12 live tools that are mounted using a BMT 65 tool holder. For the experiments conducted within this work, cutting tools of type AC6030M by the manufacturer Sumitomo Carbide were used. The tools have a corner radius of 0.8 mm and are designed to operate at surface speeds between 80 and 230 m/min with feed rates between 0.05 and 0.6 mm/rev.⁸ 

Round bars made from AISI 4130 with a length of 350 mm and a diameter of 50 mm were utilized as the substrate material. The coating material used in the experiments is a development product from Höganäs labeled X-Rockit® 431 SR 20-53. This material is a martensitic stainless steel powder with a grain size distribution of 20–53 μm. The nominal chemical composition of the powder material is listed in Table I.

The experimental investigations are divided into three stages. In the first stage, EHLA parameter sets were developed to deposit coatings at two predefined surface speeds—100 and 150 m/min. The objective of the development was to produce crack-free, virtually pore-free coatings with a flawless bonding and a target coating height of approximately 200 μm. The coating parameter sets that resulted from an iterative development process are given in Table II and served as the basis for the following simultaneous processing trials.

To avoid systematic errors due to inaccurate placement of the workpiece and to ensure a consistent cutting depth, the substrates were machined to a diameter of 49.6 mm before carrying out the SMaC experiments in the same clamping. Coatings with a length of 15 mm were then deposited and simultaneously turned using the parameter sets given in Table II. For this, the feed rate of the turning tool was synchronized with the feed rate of the EHLA deposition head, and the surface speed was shared between the two subprocesses. The TCP offset Δz was systematically varied by the following increments: 2, 3, 5, 7, 9, and 11 mm. The target diameter for the turning process was set to 49.8 mm in each test, resulting in a cutting depth of around D = 100 ± 35 μm. For both predefined surface speeds, coating samples were also produced without machining and with sequential machining to provide reference points for the following analysis. For sequential machining, the workpiece was cooled down to room temperature after the deposition process and then machined using the same surface speed and feed rate. For each test series, consisting of six simultaneous and one sequential coating and machining processes, a new cutting insert was used.

In the third investigation stage, the influence of simultaneous machining on tool wear was explored. For this purpose, multiple coating sections with a length of 90 mm each were deposited. In one experimental series, six such sections were machined simultaneously with a constant TCP offset of Δz = 7 mm, and in another experimental series, six further sections were machined sequentially. A new cutting insert was used for each test series to allow for conclusions regarding tool wear.

The evaluation of the coating samples was accomplished using various optical analytical techniques. A general assessment of the coatings was initially conducted by microscopy imaging of etched cross sections. The images were used for a qualitative examination of the coatings regarding the height, defect-free deposition, and bonding and served as the basis for the initial process development. To determine the geometry and roughness of machined coatings, the surfaces were examined with a Zygo Ametek NX2 white light interferometer (WLI). On samples from the second and third investigation stages, full coating areas as well as additional 5 mm of the substrate surface on each side of the coating were captured. The curvature of the workpiece was extracted using a cylindrical fit and referencing the adjacent substrate surfaces in Zygo Mx 7.3 software. Further processing of the raw data was accomplished in MountainsMap version 7.4. This involved the generation of total profiles of the coatings along the workpiece axis and the extraction of discrete waviness and roughness profiles using a Gaussian filter, as specified in DIN EN ISO 4287 (see Fig. 4). In accordance with DIN EN ISO 4288:1998, a cut-off wavelength of λ_c = 0.8 mm and a measuring distance of lt = 4.8 mm were selected for the determination of the surface roughness. The same approach was applied to the unmachined EHLA coatings, whereby λ_c = 2.5 mm and a measuring distance of lt = 12.5 mm were used to comply with DIN EN ISO 4288:1998. For the qualitative assessment of the cutting tool wear, the cutting edges were scanned using a Keyence VR-5200 3D optical profilometer before and after operation.

Crack-free and virtually pore-free X-Rockit 431 SR 20-53 coatings were successfully deposited on AISI 4130 substrates using parameter sets A and B with both EHLA and SMaC. Cross sections of selected coatings are provided in Fig. 5. A closer inspection of the profiles obtained from the sequentially and simultaneously machined coatings (Fig. 6) reveal clear differences between the resulting geometries. In comparison to sequential machining (Fig. 6, top row), SMaC introduces a characteristic geometric deviation to the finished contour. The shape of the waviness profiles, thereby, is indicative of a causal relationship between the dimensional deviation and the TCP offset Δz.

Consequently, a strong linear relationship exists between the geometric features in the waviness profiles and the programmed tool offset Δz (Fig. 7).

Sequential machining effectively reduces the average roughness of the deposited EHLA coatings from Ra = 12.5 μm for parameter set A and Ra = 11.1 μm for parameter set B to Ra = 0.74 μm and Ra = 0.67 μm, respectively (see Table III).

The roughness profiles of sequentially and simultaneously machined coatings generated using parameter set A are depicted in Fig. 6. The average roughness values of the illustrated coatings are within the range of Ra = 0.79 ± 6 μm and show no signs of interdependence with the TCP offset. The roughness profiles also indicate no clear pattern in relation to Δz. Minor smoothing of the roughness profile can be attributed to the progressive degradation of the cutting tool.

In contrast, a comparison of the roughness values for simultaneously machined coatings produced with parameter sets A and B reveals noticeable differences. As shown in Fig. 8, the surface roughness of SMaC coatings deposited at a surface speed of 150 m/min is substantially lower than that of the coatings deposited at 100 m/min. This correlation between the surface speed and the surface quality is consistent with the findings from conventional CNC turning processes.^9,10 Notably, at 150 m/min, the surface roughness of the SMaC coatings is up to 63% lower than that of the sequentially machined EHLA coating processed at the same surface speed and using identical turning parameters. This aligns with the observations from the laser assisted turning processes, where a lower workpiece hardness promotes a smoother chip removal and yields an improved surface finish.⁷ 

Although further investigations need to be concluded to confirm this claim, these findings suggest that the process conditions established by SMaC are particularly effective at high surface speeds regarding achievable surface qualities. Furthermore, the dependence of surface roughness on the distance Δz is apparent at high speeds, with a larger TCP offset leading to a better surface finish within the observed parameter range.

For a qualitative assessment of the tool wear, larger coating sections were investigated. A total of 12 coating segments, each 90 mm long, were produced. Six segments were machined sequentially after deposition and the other six were produced using SMaC. A new cutting insert was used for each of the two scenarios. This corresponds to approximately 845 cm² of the machined surface per process variant contributing to tool wear. In Fig. 9, the cutting edges of the inserts from the described tests are shown. Subfigure (a) features an unused cutting edge prior to the experiments, while (b) and (c) show the conditions after sequential and simultaneous machining, respectively. In the case of sequential machining, significant wear of the main cutting edge is visible, whereas the condition of the cutting edge after the simultaneous process exhibits a considerably lower level of abrasion. These observations indicate that SMaC might contribute to the reduction in tool wear compared to the sequential processing of EHLA coatings.

Within this work, a systematic investigation into the fundamentals of simultaneous machining and coating (SMaC) was carried out. Alongside the first-ever demonstration of this simultaneous process, several observations were made in this work, pointing to some unique characteristics of this technology.

First, the parallelization of the deposition and machining processes appears to have a positive effect on the roughness of the resulting surfaces. This effect was primarily observed at a higher surface speed of 150 m/min, leading to an average roughness as low as Ra = 0.25 μm. At 100 m/min, the surface roughness reached Ra = 0.79 ± 6 μm for both simultaneous and sequential machining. The investigation of longer coating sections provides evidence to suggest that simultaneous processing can reduce wear on the cutting insert and, therefore, has the potential to extend the service life of the turning tool. Overall, a significant portion of the findings displays considerable parallels with LAM. The combination of long tool life and improved surface quality combined with increased productivity enables SMaC to address a broad range of applications in industrial production.

A primary challenge of the SMaC process is the dimensional deviation caused by the thermal expansion of the workpiece during material deposition. These deviations can extend into the two-digit micrometer range in coating thickness and compromise the dimensional accuracy of the workpiece. One significant conclusion of this study is that the dimensional deviation is directly correlated with the programmed TCP offset. Consequently, this deviation can be mitigated by optimizing the turning process tool path in response to the heat input into the workpiece. The development of methods for such process control will be the subject of subsequent investigations.

To mitigate dimensional inaccuracies in the process, several approaches can be explored. One potential solution involves active control of the tool infeed, utilizing methods such as pyrometer measurement of workpiece temperature or triangulation of workpiece elongation. These measurement values can serve as control variables for a closed-loop system. An alternative approach could involve simulation. By the numerical simulation of heat transfer, the thermal expansion of the workpiece can be predicted beforehand, allowing for the generation of a corrected tool trajectory.

Further investigations are necessary to fully understand the impact of the TCP offset Δz on tool longevity and coating performance. The use of a piezoelectric force sensor for measuring cutting forces at different offsets could yield critical information and assist in identifying optimal SMaC process conditions. Furthermore, alternative machine concepts, in which the distance between the laser beam and the cutting tool is independent from the workpiece diameter, may be considered.

Based on the described findings, SMaC appears to enhance the machinability of high-hardness materials. Consequently, examining the process limitations for simultaneous machining of very brittle coatings, which are not suitable for conventional turning processes, would be of interest. One example of such a material is the low-melting NiCrBSi alloy Höganäs Amperit® 1660-02. In this context, the compressive stresses introduced by the turning tool could potentially contribute to the prevention of cold cracking.

The authors have no conflicts to disclose.

Viktor Glushych: Conceptualization (lead); Data curation (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Methodology (lead); Project administration (equal); Resources (equal); Supervision (lead); Validation (equal); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead). Niklas Dall: Conceptualization (supporting); Data curation (equal); Formal analysis (equal); Investigation (lead); Methodology (supporting); Software (equal); Visualization (supporting); Writing – review & editing (equal). Max Zimmermann: Conceptualization (supporting); Investigation (supporting); Validation (supporting). Thomas Schopphoven: Conceptualization (supporting); Resources (equal); Supervision (equal); Writing – review & editing (supporting). Wilhelm Meiners: Methodology (supporting); Writing – review & editing (supporting). Constantin Leon Häfner: Resources (equal); Supervision (equal).