Simultaneous machining and coating (SMaC) is a novel hybrid technology developed by the Fraunhofer Institute for Laser Technology ILT that combines additive manufacturing through extreme high-speed laser material deposition (EHLA) with a simultaneously engaged turning process. In the present work, the parallelization of the two subprocesses is successfully demonstrated. For the first time, systematic investigations into the influence of residual heat from the deposition process on the machining process and the properties of the resulting coating, respectively, are conducted. The study specifically examines geometric deviations, surface roughness, and tool wear. One of the main parameters affecting the residual heat introduced into the turning process is the distance between the tool center points of the EHLA deposition head and the turning tool Δz. This parameter is identified as a primary factor influencing the dimensional accuracy of the coating geometry. The results show that SMaC not only offers potential for increasing the productivity of the process chain but also has a positive effect on the service life of the cutting tools involved, potentially improving the workability of hard coating materials. Improvements in terms of the attainable surface roughness are also observed. These investigations provide a basis for research into further aspects of the SMaC process, such as adaptive tool path control methods to enhance dimensional accuracy and the influence of induced compressive stresses in processing highly brittle coating materials.

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.

FIG. 1.

Schematic representation of the operating principle behind simultaneous machining and coating (SMaC).

FIG. 1.

Schematic representation of the operating principle behind simultaneous machining and coating (SMaC).

Close modal
FIG. 2.

Visualization of the TCP offset parameter Δz.

FIG. 2.

Visualization of the TCP offset parameter Δz.

Close modal

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.

FIG. 3.

Depiction of system technology used for coating and machining experiments.

FIG. 3.

Depiction of system technology used for coating and machining experiments.

Close modal

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.

TABLE I.

Nominal chemical composition of the X-Rockit 431 SR 20–53 powder material used for EHLA and SMaC trials.

FeCCrNiOthers
Bal. 0.18% 16.5% 1.75% <5% 
FeCCrNiOthers
Bal. 0.18% 16.5% 1.75% <5% 

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.

TABLE II.

EHLA parameter sets developed for the simultaneous processing trials.

#Surface speedLaser powerLaser spot ØPowder mass flowFeed rate
(m/min)(W)(mm)(kg/h)(mm/rev)
100 4785 1.5 1.44 0.15 
150 6525 1.5 2.52 0.15 
#Surface speedLaser powerLaser spot ØPowder mass flowFeed rate
(m/min)(W)(mm)(kg/h)(mm/rev)
100 4785 1.5 1.44 0.15 
150 6525 1.5 2.52 0.15 
In the second stage, SMaC tests were conducted on AISI 4130 substrates to evaluate the influence of simultaneous processing on the coating result with respect to the programmed TCP offset. The TCP offset Δz is defined as the horizontal distance between the tool center points of the laser beam and the cutting tool,
(1)

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 λc = 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 λc = 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.

FIG. 4.

Profiles obtained from the WLI measurement on the example of SMaC coating: (a) total profile, (b) waviness profile, and (c) roughness profile.

FIG. 4.

Profiles obtained from the WLI measurement on the example of SMaC coating: (a) total profile, (b) waviness profile, and (c) roughness profile.

Close modal

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.

FIG. 5.

Cross section microscopy of EHLA and SMaC coatings etched with Nital: (a) EHLA coating with parameter set A, (b) EHLA coating with parameter set B, (c) SMaC coating with parameter set A and Δz = 7 mm, and (d) SMaC coating with parameter set B and Δz = 7 mm.

FIG. 5.

Cross section microscopy of EHLA and SMaC coatings etched with Nital: (a) EHLA coating with parameter set A, (b) EHLA coating with parameter set B, (c) SMaC coating with parameter set A and Δz = 7 mm, and (d) SMaC coating with parameter set B and Δz = 7 mm.

Close modal
FIG. 6.

Waviness profiles (left) and roughness profiles (right) of sequentially machined (top row) and simultaneously machined (rows 2–7) EHLA coatings applied using parameter set A; zTn is the numerical TCP of the cutting tool and is determined by a local minimum of the waviness profile; zLn is the numerical TCP of the laser beam and is determined by the falling inflection point of the waviness profile.

FIG. 6.

Waviness profiles (left) and roughness profiles (right) of sequentially machined (top row) and simultaneously machined (rows 2–7) EHLA coatings applied using parameter set A; zTn is the numerical TCP of the cutting tool and is determined by a local minimum of the waviness profile; zLn is the numerical TCP of the laser beam and is determined by the falling inflection point of the waviness profile.

Close modal
The heat input into the workpiece generated by the coating process leads to thermal expansion along the radius. As the expansion increases, the turning tool removes progressively more materials from the workpiece radius, causing a gradual reduction in the coating thickness h. Upon completion of the coating process, which precedes the machining process by the offset Δz, the component immediately begins to contract, as no further heat input takes place, and the cooling process sets in. For the remaining machining length, the cutting depth of the turning tool progressively decreases as a result. This phenomenon leads to the formation of a local minimum in the waviness profile of the finished coating. The position of this local minimum is defined in the following as the numeric TCP of the turning tool zTn. The falling inflection point of the waviness profile is determined as a reproducible reference point for the end of the deposition and is, therefore, defined as the numerical TCP of the laser zLn. From this, the numerical TCP offset Δzn can be calculated as the distance between the two numerical TCPs,
(2)
In order to verify the aforementioned hypothesis, the linear correlation ρz, Δzn) between Δz and Δzn is determined as
(3)

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

FIG. 7.

Linear fit between the parameter Δz and Δzn derived from the waviness profiles.

FIG. 7.

Linear fit between the parameter Δz and Δzn derived from the waviness profiles.

Close modal

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).

TABLE III.

Average surface roughness Ra of as-deposited and sequentially machined EHLA coatings.

#Surface speedRa, as depositedRa, sequential
(m/min)(μm)(μm)
100 12.5 0.74 
150 11.1 0.67 
#Surface speedRa, as depositedRa, sequential
(m/min)(μm)(μm)
100 12.5 0.74 
150 11.1 0.67 

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 

FIG. 8.

Average surface roughness Ra as a function of the TCP offset Δz at surface speeds of 100 m/min (parameter set A) and 150 m/min (parameter set B).

FIG. 8.

Average surface roughness Ra as a function of the TCP offset Δz at surface speeds of 100 m/min (parameter set A) and 150 m/min (parameter set B).

Close modal

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.

FIG. 9.

Close-up images of the tool cutting edges: (a) before machining/unused, (b) after sequential machining of six samples, and (c) after simultaneous machining of six samples.

FIG. 9.

Close-up images of the tool cutting edges: (a) before machining/unused, (b) after sequential machining of six samples, and (c) after simultaneous machining of six samples.

Close modal

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).

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