Influence of LPBF Process Parameters on the Surface Quality of Stainless Steel on the Adhesion Properties with Thermoplastics ^†

Abstract

Spinodoid metamaterials are artificial, architected materials that offer unique properties superior to the corresponding bulk material. This type of structure enables tailored properties to create new opportunities for applications in, e.g., the mobility, health, and energy sectors. Spinodoid metal structures only can be manufactured by advanced additive manufacturing technologies such as the laser powder bed fusion (LPBF) process. For various application scenarios of metamaterials, it may be necessary to bond or coat them with thermoplastics. The surface quality of the additively manufactured structure is particularly relevant for the adhesion of the metallic to the thermoplastic structure, as the mechanical adhesion, for example, depends on the roughness and surface properties. Therefore, this paper focuses on the investigation of the surface properties of additively manufactured stainless steel components with dependence on the parameters of the LPBF process with regard to subsequent adhesion with thermoplastics.

1. Introduction

Metamaterials are rationally designed structures made of a geometrically tailored composition. Especially for mechanical metamaterials, these properties range from highly tuned elastic to nonlinear, multistable and programmable ones. Even auxetic or chiral mechanical behavior and specific wave propagation [1,2,3] can be achieved. In the modern aviation industry, lightweight construction and efficiency play a central role, which is why functionally integrative lightweight structures are necessary to meet future requirements. One of the key technologies for manufacturing complex, functionally integrative metal components is the LPBF process. Spinodoid structures are an example of biologically inspired, non-periodic shapes, such as those which are characteristic in bones and can only be manufactured with 3D printing processes. In aerospace applications, there is also the challenge of withstanding extreme environmental conditions. The change from large temperature extremes promotes corrosion and leads to the degradation of the material and, as a result, to component failure. To protect against extreme environmental conditions, metal components in aviation are protected with plastic coatings or paintwork or even completely substituted with a fiber-reinforced plastic component [4], which, however, can be extremely cost-intensive. For this reason, metal–plastic joints are of great interest. A metal–plastic joint requires a high level of adhesive strength, which is based on certain adhesion theories. According to Amend [5], adhesion theories are categorized into mechanical and specific adhesion theories. Mechanical adhesion is defined as the interlocking of the joining partners. On one hand, this can be carried out at a macroscopic level by adapting the component geometry [6] or at a mesoscopic level by creating pin-like geometries on the surfaces of additively manufactured components [7]. On the other hand, the joint strength can also be influenced by the surface roughness of the metallic component [8]. The influence of the scanning strategy in the LPBF process on the surface roughness has been investigated in various publications. The most important influencing variables are [9,10,11,12] the volumetric energy density $E^{‴}$, which is composed of the hatch distance $d_H$; the power of the laser $P$; the scan speed $v$; and the layer thickness $t$.

$$E^{‴}={\displaystyle \frac{P}{v\times d_H\times t}}$$

(1)

Another influencing process parameter is remelting, which is defined as the second contour scan of the outer component geometry with an adjusted volumetric energy density of the laser. This leads to reductions in surface roughness and waviness due to balling effects [13,14,15]. Balling effects are the sintering or partial melting of powder particles on the component surface. The characteristics and shape are sufficiently described in [16,17].

Previous investigations mainly focused on the optimization of the process parameters with the aim of achieving low surface roughness and thus an improved component finish. As shown in Figure 1, this work investigates the potential influence of the processing angle and the remelting process parameter on the generation of microscale undercut geometries on the component surface. This allows estimates to be made of possible joining methods, as well as of surface post-treatment processes, for the production of metal–plastic components.

2. Materials and Methods

2.1. Sample Production (Laser–Powder–Bed–Fusion)

Powder of stainless steel 316L (X2CrNiMo 17-12-2) was purchased from SLM Solutions Group AG (Lübeck, Germany) with a particle size range of 10–45 µm. The powder that had already been used was sieved again to ensure a particle size < 63 µm.

The selected test specimen geometry was a sheet with dimensions of 70 mm × 25 mm × 1.6 mm, which can also be used for prospective single-lap shear tests according to the ISO 4587 [19].

A SLM280 Generation 2.0 Dual Laser machine (SLM Solutions Group AG, Germany) was used to fabricate the specimens. The machine is equipped with two different infrared lasers, and in this study, the laser (maximum power 700 W) with a Gaussian intensity distribution was utilized. The minimum beam size is approx. 80 µm.

A standard processing parameter set was selected to manufacture the sheet specimens under argon atmosphere. With a layer thickness of 60 µm, the volume scan was conducted with a laser power of 350 W, a scanning velocity of 950 mm/s, a hatching distance of 120 µm, and a strip scanning strategy rotating the hatching angle by 67° after each layer built up. In order to change the surface roughness for some samples directly, a contour scan (remelting) was performed with a laser power of 150 W, a scanning velocity of 350 mm/s, and a beam compensation of 60 µm. The beam compensation represents the offset of the laser beam to shift the melting track so that the size of the sample can be guaranteed.

Another factor influencing the surface topology is the overhang angle, which was set to 50°, 70°, and 90° for the specimens. Furthermore, the substrate plate was not preheated in the build envelope of 100 mm × 100 mm (maximum height: 180 mm). As shown in Figure 2, a new classification of samples was devised to distinguish the top and bottom sides. In the further course of the work, the analysis of the top side with an overhang angle of 50° to 90° was defined. For samples with an overhang angle of 110° and 130°, the undersides were examined.

2.2. SEM Analysis

In this work, the analysis of surfaces was carried out in two different ways. On the basis of scanning electron microscope (SEM) images, the surface structure can be shown visually in two dimensions with high contrast. Shadows and high-contrast edges can indicate a level of depth, which can be understood as out-of-plane undercuts that can be used as interlocking elements when bonding with polymers. In the hybridization process between a metal and a polymer, the polymer can enter these deep interlocking elements better and thus cause higher mechanical adhesion. The images were taken with a desktop SEM TM3000 from Hitachi with a 15 kV detector. The TM3000 system software (version 02-02-03) was used to carry out image analyses of the surfaces at 100× magnification for overview images. Detailed images of balling effects were taken at 300× magnification.

2.3. Laser Profilometry

The second method used to explore the surface topology is the laser profilometry, which is determined by the arithmetic surface roughness $S_a$ and is defined in DIN EN ISO 25178 [20]. Figure 3 shows the measurement setup of the laser profilometer from Nanonfocus with the respective sample alignment. To reduce the measurement time, the scanning direction was set in the layer surface direction (x) with a finer resolution and in the layer growth direction (y) with a reduced resolution. Based on the surface analyses via SEM, the resolution parameters of the laser scanning could be determined. The roughness of LPBF-printed surfaces was determined, on one hand, by the size distribution of the powder particles from 10 µm to 45 µm and the resulting balling effects and, on the other hand, by the hatch distance of 120 µm. These geometric effects should be measured with at least 10 scans. For this reason, the x-axis was scanned at a rate of 15 µm and the y-axis at a rate of 1 µm. The measurement area was 5 mm × 5 mm.

3. Results

3.1. Roughness Measurements

The results of the roughness measurement using laser profilometry are compared for all variants in Figure 4. The stainless steel test specimens produced using the LPBF process exhibit an average roughness of 27.14 µm (90° overhang angle with remelting) to 77.07 µm (130° overhang angle without remelting), depending on the overhang angle. Extreme overhang angles such as 50° and 130° each exhibit the highest roughness in the test series. Vertical overhang angles of 90° show the lowest roughness in the test series with and without remelting. The comparison of the extreme overhang angles shows a higher roughness for 130° samples than for 50° samples. This can be explained by the reduced heat dissipation in the case of downskin surfaces due to the metal powder underneath. When comparing samples with and without remelting, a 20–25 µm higher arithmetic surface roughness is achieved on samples without remelting.

3.2. SEM Analysis

The SEM analyses for an overhang angle of 50° clearly show that a higher number of powder buildups (balling effects) occur without remelting (Figure 5a) than with remelting (Figure 5b). The balling effects in Figure 5a are mainly limited to areas between the melting zones, which are very diffuse and non-directional. There are also significantly more interlocking elements than in Figure 5b. In Figure 5b, fewer balling effects can be seen. Powder adhesion is visible along the melting zone and shows a directional pattern. The interlocking elements are much less distinct than in Figure 5a.

When comparing the SEM images of samples with an overhang angle of 70°, there is also a larger number of powder buildups (balling effects) observed in the case of no remelting (Figure 6a). In contrast to an overhang angle of 50° (Figure 5), these balling effects occur in all areas. The melting zones in Figure 6a are diffuse and unstructured. There are more interlocking elements in Figure 6a compared to Figure 6b but fewer than in Figure 5a. In case of remelting (Figure 6a), powder buildup appears along the melt zones and follows a clear pattern.

In Figure 7, SEM images of samples with an overhang angle of 90° are compared. The surface appears very similar with and without remelting. An equal number of powder buildups can be detected. Similar to Figure 6b, there are little to no interlocking elements.

SEM images of samples manufactured with an overhang angle of 110° are shown in Figure 8. The sample without remelting (Figure 8a) shows a larger number of powder buildups. The balling effects are distributed without any indication of structuring in all areas. The melting zones are diffuse and unstructured. There are more interlocking elements compared to the sample with remelting (Figure 8b). They resemble the samples with an overhang angle of 70° (Figure 6).

When comparing the 110° samples with the 130° samples, the degree of powder buildups changes only slightly. The balling effects are distributed over all areas of the surface. On the samples with a 130° overhang angle (Figure 9), many deep interlocking elements can be seen, with the sample without remelting (Figure 9a) showing deeper interlocking elements.

4. Discussion

In the roughness measurements by profilometry, a high arithmetic surface roughness is measured at extreme overhang angles of 50° and 130°. Therefore, when joining directly with thermoplastics, e.g., by hot pressing or injection molding, a high adhesive strength is expected for extreme overhang angles such as 50° and 130°. Remelting leads to a reduction in roughness, which is expected to result in reduced adhesion to thermoplastics. It can therefore be assumed that samples without remelting and extreme overhang angles should achieve the highest adhesive strengths. However, the SEM images show that samples with overhang angles of 50° and 70° and with remelting do not contain any significant interlocking elements but many balling effects. Due to the high balling effects, a higher anchoring of the polymer is expected, which would lead to high joint strengths. Therefore, further studies on surface wettability with contact angle measurements and single-lap shear tests of the joined samples are planned.

5. Conclusions and Outlook

This work is a preliminary investigation of additively manufactured 3D structures produced for hybridization with polymers, e.g., with hot pressing. Flat test specimens made of 316L stainless steel were produced using the LPBF process with the aim of analyzing the factors influencing possible interlocking elements. The input variables were a variation in the overhang angle from 50° to 130° and the remelting of the outer contour of the sample. The component surfaces were examined by the arithmetic surface roughness using the laser profilometry and by SEM images. The following knowledge was obtained:

• The surface roughness ranges from 27.14 µm (at an overhang angle of 90° with remelting) to 77.07 µm (at an overhang angle of 130° without remelting);
• Extreme overhang angles show the highest roughness, independent of remelting;
• Overhang angles of 90° exhibit the lowest roughness in the test series with and without remelting;
• Remelting increases roughness by an average of 20–25 µm;
• SEM images of the 50° and 130° samples without remelting show the most possibilities for interlocking elements;
• Remelting leads to a homogeneous surface property with fewer balling effects;
• In the case of a 50° to 110° overhang angle, only minor changes in surface topology as a result of remelting are observed.

Based on these results, initial conclusions could be drawn about the strength of the joints. In further investigations, the wettability of the surfaces, as well as the achievable joint strength between the LPBF metal part and thermoplastic components, will be evaluated. The aim is to find out whether the joint properties are mainly influenced by the surface roughness or if balling effects have a significant influence. In addition, the influence of a pre-treatment for better wettability on the joint strength of a hybrid sample will be examined.