1. Introduction
Additive manufacturing technologies, despite being invented in the 1980s, are not sufficiently well known and refined yet. However, this is not due to the small number of studies conducted, but rather the dynamically developing market of new technologies and materials. Currently, 3D printing has evolved to a degree that a new discipline referred to as 4D printing [
1,
2,
3] is being developed. This emerging field combines the advantages of 3D printing with a modern “smart” structure and materials, enabling minimization of manufacturing time and creation of objects with unprecedented properties, such as shape memory (SMA); e.g., magnetic (SMM) or temperature memory (SMT). The properties of such materials are known for cast alloys such as NiTi, CuZnAl, and CuAlNi; however, in the case of additive technologies with a high degree of anisotropy and properties dependent on the printing direction, studies of such materials seem to be necessary. The creation of composite materials seems to be a very prospective application of 3D printing, either through the use of composite-based materials or via printing on existing objects. The results of imaging testing conducted on the printing of existing objects or via the use of the electrospinning technology to build composite models are presented in [
4,
5,
6]. The 3D/4D printing technologies are currently using three main types of materials when considering the division in terms of the input material form. In the case of layered technologies, the input material can be supplied in the form of a liquid, solid, or powder. When analyzing the available scientific literature and observing the industrial market demand, it can be stated that the technologies utilizing metal-based powder materials are one of the most promising and quickly developing 3D printing applications. This is related especially to a wide range of materials used and their very good properties. Many methods can be currently classified as 3D printing powder technologies utilizing metal powder-based materials. The ISO/ASTM52900–15 international standard [
7] specifies the terminology concerning the additive methods and the technologies classified as 3D printing laser methods that allow for melting or sintering of metal powders. The most often used include: selective laser sintering (SLS), selective laser melting (SLM) [
8,
9], and laser engineering net shaping (LENS). The selective laser melting technology, which was used in this study to build physical sample models, is one of the most popular powder-bed metal-additive technologies allowing for the production of geometrically complex models from steel-based materials, such as: stainless steel 316L and17–4PH, maraging steel M300, aluminum alloys AlSi10Mg and AlSi7Mg, Nickel 718, Nickel 625, Titanium Ti6Al4V ELI Grade 23, and Cobalt CoCrMo [
10,
11]. In addition, tool wear [
12] during the machining process is highly dependent on the material and its properties, which in the case of 3D printing, especially with metal powder-based materials, can cause excessive cutting-edge wear. As demonstrated by multiple studies, the quality of the technological surface layer and the strength of model manufactured using the SLM technology depend on many technological parameters, such as laser scanning speed and power, layer thickness, building chamber temperature, protective atmosphere, and the printing direction. Due to the layered nature of model building, it is the layering direction referred to as the printing direction (Pd) that seems to be an essential technological parameter when analyzing the dimensional and shaping accuracy, the technological quality of the surface layer, and the surface microstructure. Studies on the technological surface layer of models manufactured using the SLM technology and other laser technologies with consideration of the technological parameters’ impact are presented in many research papers [
13,
14,
15,
16,
17,
18,
19]. Calignano et al. [
16] used the Taguchi statistical method to assess the impact of technological parameters on the surface layer’s quality. The paper features an analysis of the laser speed and hatch distance, as well as designation according to which a reduction in the melted surface’s laser beam scanning speed is advantageous to the surface layer’s quality. Read et al. [
17] analyzed the impact of technological parameters, such as laser speed, hatch distance, laser power, and model position on the building platform (in two planes), on the manufactured models’ porosity. The critical energy density was designated in the paper, giving a minimum pore fraction amounting to 60 J/m
3. Hitzler et al. [
18] mainly analyzed the impact of the laser speed and power, hatch distance, and energy density on the surface roughness expressed as parameter Ra. Furthermore, the samples were positioned in different variants on a platform in relation to planes X-Y; i.e., the printer working table. Calignano [
19] analyzed the surface texture and accuracy of samples produced by laser powder bed fusion technology. It was shown that the choice of parameters of conversion from the CAD model to STL file and the setting of process parameters can affect the accuracy. Moreover, it was stated that the surface roughness was mainly caused by the process parameters and building orientation.
In the case of finishing of parts manufactured using 3D printing from metals, publications mainly concern the milling of the manufactured models [
20,
21,
22]. In [
20], the authors presented the results of testing the surface texture of SLMed samples made from AlSi10Mg powder with the use of a TruPrint 1000 printer. After manufacturing using the additive technology, the models underwent milling, and then their surface roughness was measured to quantify the Ra and Rz parameters. The test results demonstrated a 20% improvement in the models roughness after milling when compared to the roughness immediately after printing. Furthermore, the paper featured an analysis of the impact of the SLM 3D printing technological parameters on the quality of the built surface. In [
21], the authors tested parts manufactured from Titanium Ti6Al-4V powder-based material using Wire Arc Additive Manufacturing (WAAM). Then, they also analyzed the surface quality and the samples’ tensile strength. They demonstrated that the mechanical properties of the manufactured models were very high, and met aeronautical standards. Furthermore, no differences were demonstrated in the strength of the models cut from sample models at various angles, which corresponded to the models manufactured at other set printing directions. The paper also featured an analysis of the influence of the printing parameters on the surface quality; e.g., pockets. In [
22], the authors studied the impact of milling on the surface quality of models manufactured from Ti6Al4V titanium powder using SLM technology. The paper features a comparison of the obtained results with results for samples obtained with conventional manufacturing technologies. The analyzed parameters concerned both 3D printing and milling, including their impact on the quality of the manufactured surface (Ra), hardness, and microstructure. It was shown that the samples manufactured with the SLM technology demonstrated greater hardness after milling than samples manufactured with conventional manufacturing methods and then processed mechanically.
There is an abundance of methods to describe the topographies of engineered surfaces. The most common way is to characterize measured texture by ISO 25178–2 areal parameters [
23]. These characterizations focus on the properties of an entire analyzed region and are based on statistical measures like mean, standard deviation, or root mean square of heights, slopes, area, volume, or curvatures. The most basic height parameters were originally developed to study conventionally manufactured surfaces; i.e., formed by machining or other subtractive technologies. However, in the case of metal additive manufacturing, the geometric complexity of the created surface topographies failed to be captured by standard characterization approaches [
24,
25]. A potential remedy would be to describe process-specific topographic features that are inherent to their fabrication, such as pits and hills, ridges, and valleys, which are often present in the microscopic images of additively manufactured metal parts [
26]. Some feature-based parameters, included in the ISO standard, are potentially relevant for characterization of the formations created by AM, but they are limited only to hills and dales. More general feature-based characterization of laser powder bed fusion was shown by Senin et al. [
25], and for electron powder bed fusion by Newton et al. [
27]. Those involved characterization of other topographic features that can be decomposed and described separately by applying a pruning algorithm. However, they did not provide an insight into the scales of those features. This becomes an important aspect, as topographic features of a particular size are best discernible when observed and analyzed at particular scales. This is the very essence of multiscale methods, which were successfully applied to study the additively manufactured surface topographies [
28,
29]. The scale is an important factor to be considered, as physical interactions between the manufacturing process and formed surfaces can occur at multiple scales during fabrication. In terms of additively manufactured surfaces and multiscale analysis, only geometric approaches were noted. This includes area-scale methods [
30,
31] and curvature tensor estimation for multiple scales [
29]. Other multiscale methods that could be potentially applied to study the complexity of AM textures include sliding bandpass filtering, structural function, and wavelet transform [
28]. The last one is particularly interesting, as it has a wide spectrum of mother wavelets that can be selected based on the specific nature of the information that is being sought. It is particularly important in the case of additively manufactured surfaces that are characterized by irregular distribution of surface irregularities in particular directions. Furthermore, the method allows both the detection of defects formed on the surface and the identification of their locations. Individual, untypical surface features, which are common in AM, are highlighted in particular scales of such analysis, which is an advantage in comparison with classical concepts of surface evaluation. Studying the multiscale effects of the finishing on the resulting surface topography of additively manufactured parts is not possible when using traditional characterizations. Therefore, scale-dependent methods appear to be of great potential to capture and understand the nature of changes in additively manufactured surface textures made by postprocessing. This paper addresses that issue.
As we observed, publications concerning machining of AM parts focus mainly on the milling of models manufactured by 3D printing, and unfortunately mainly concern a narrow range of roughness parameters (usually Ra and Rz). Due to the above and the geometric complexity of 3D printed surface topographies, the present study, which focuses on the impact of the printing direction and subsequent milling on the quality of the surface texture, seems to be justified and required for a full understanding of the 3D printing and machining processes. The analysis covers an in-depth evaluation of profile (2D) and areal parameters (3D), and it is accompanied by two multiscale analyses: wavelets and curvature. The latter is especially important to determine the scales with the strongest interactions between the processing and the obtained surfaces.
4. Discussion
There is no single universal characterization that can fully capture the complexity of surfaces created by additive manufacturing. In this study, we focused on the effects of build-up angle and conventional machining on the resulting texture of 3D printed stainless steel. This was analyzed by the conventional ISO 25178 areal parameters and two multiscale methods: wavelet transform and geometric via curvature. All three characterizations performed well in confidently differentiating between the as-built and as-machined samples. Some exceptions included Std (surface texture direction), which was strongly dependent on how the sample was actually oriented under the microscope while measuring, or was aligned in postprocessing software and average mean curvature (Ha), as well as narrow ranges of scales for average Gaussian curvature (Ka), average maximum curvature (κ1a), and a few scales for Kqabs, which appeared to be random.
For curvature, some parameters allowed differentiation versus build angle only for a narrow range of scales (κ2a, κ2q), or only for medium scales (κ1a, κ1q, Ha, Ka, Kq, κ1q
abs,Hq
abs, Ka
abs, Kq
abs). For larger scales, minimum principal curvature and corresponding parameters could not be used to differentiate between surfaces. Understanding how and at what scales the fabrication process affected the processed material required a group of individual parameters to capture all the important aspects of its geometry that resulted from the manufacturing [
24,
55]. This stood in opposition to what is commonly used in the industry [
56] and scientific community [
57], where, for the analysis of geometrically complex surface topographies, basic profile (Ra, Rz, Rq) or areal (Sa, Sq) parameters are used most often. This paper sought to provide some justification for why more sophisticated characterization should be applied in the description of textures created by additive manufacturing.
The research results presented in [
13] featured an assessment of the impact of the printing direction on the formation of frontal and side waviness in sample surfaces. When analyzing the results for the frontal surface, it was possible to notice a similar trend as seen in the topography parameters of surfaces obtained in our study. An increase in the printing direction caused a reduction in the parameters determining the arithmetical mean height.
When analyzing the parameter Sz, which determines the maximum roughness height, it was possible to notice an increase in its value along with an increase in the printing angle. The milling caused a reduction in this parameter for all of the analyzed samples manufactured at different orientations in relation to the building platform. However, when considering the maximum roughness height of the surface after milling, it was possible to notice an inverse trend when compared to the surface only after printing. In this case, an increase in the printing direction caused a reduction in the parameter Sz. This could derive from the removal of sharp roughness peaks during milling, which was especially visible for the printing angle of 90°, which featured the highest parameter Sz.
It was possible to notice an upward trend in the next surface parameter, Sv. This was caused by the nature of the machining process, in which the cutting edge left clear marks on the surface in the form of pits cut by the cutting edge. This trend was not recorded for additively manufactured surfaces. The parameter value was similar regardless of the addition angle.
The assessment also covered the distinction and similarity of the surface topography based on the distribution of peaks and pits. Analysis of the distribution of surface extremes utilizing the wavelet transformation demonstrated that there was a substantial distinction between such irregularities, depending on the building angle, processing type, and scale at which they were assessed. It was concluded that it was not justified to use all the parameters described herein to characterize such distributions, because the skewness and kurtosis featured no substantial distinction between particular values for nearly the entire range of the assessed scales. Such deliberations could be considered with the use of Sq or Sa, because it was demonstrated that when the model building angle and the machining effect were considered in the full range of the analyzed scales, the parameters were statistically distinct, and therefore the distribution of peaks and pits was also distinct.
This study had some limitations that have to be addressed in further research work. This concerns mostly the number of different built-up angles. The higher variety of built-up orientations could help to establish functional relations for certain surface-characterization parameters and contribute to more in-depth understanding of how the fabrication process governs the creation of certain topographic features on the surface. In addition, machining was performed at the same unchanged technological parameters. For example, manipulating the feed rate and cutting speed could potentially affect the frequency of micropits caused by removing powder that was not fully melted from the material. This might change the values of the surface-characterization parameters. Feed rate also affects the distance between valleys and thus alters the surface morphology as well. Further research in this direction should be conducted.