Improving machinability of additively manufactured components with selectively weakened material

A LPBF Machine RenAM 500Q from the company Renishaw is used for manufacturing of the specimens. The specimens are made from maraging steel M300, also known as tool steel 1.2709. For quality assurance, the powder is examined for particle size distribution and it is found that it is within the range of a typical LPBF powder [24] with the following values: D10  =  24.45  \(\upmu {\text {m}}\), D50  =  35.47  \(\upmu {\text {m}}\) and D90  =  47.79  \(\upmu {\text {m}}\). Renishaw provided settings were used as process parameters for all parts and support structures except for the adapted weakened structure. An excerpt is listed in Table  1.

During the LPBF process the typical goal is to achieve fully dense parts. However, it is also possible to improve the cutting conditions during post-processing and reduce cutting forces by utilizing the possibilities of the process. Several options are considered and the selected one is to create a slightly weakened structure by manipulating the LPBF process parameters in order to reduce the material strength. This manipulation is made by lowering the induced energy during the process resulting in a lower density and strength of the material. To achieve this, the distance between the laser tracks, which is typically 75  \(\upmu {\text {m}}\) for the selected configuration, is increased. This leads to gaps between the current molten metal and the adjacent and solidified laser track. These gaps are partially filled with powder and act as weak spots during the cutting process. The concept of the formation of these structures is illustrated in Fig.  1.

Two versions of this weakened structure are examined (W2 and W3 with a hatch distance of 200  \(\upmu {\text {m}}\) and 300  \(\upmu {\text {m}}\) respectively). In Fig.  2 the surfaces of the two versions in comparison to a surface of a fully dense part ( \(\rho\)  =  8.1  g/cm) can be seen. The voids lead to a reduced density of \(\rho _{W2}\)  =  7.6  g/cm and \(\rho _{W3}\)  =  7.1  g/cm. All density values are measured using the Archimedes principle.

The workpiece geometry for the orthogonal cutting investigations is made out of a beam of maraging steel with a length of 180  mm, width of 5  mm and height of 50  mm. The sample is split into four individual sections with a different density, to directly compare the effects of the weakened structure on cutting forces and chip formation within the same cut. The first and last section, both 35  mm in length, are made out of fully dense material. The two middle sections, both 55  mm in length, are made out of the developed weakened structure W3 and W2, respectively. Twenty orthogonal cuts are performed on one sample.

As an application example and in order to exclude as many influences of the LPBF process on the geometry as possible, the drilling application specimen are designed as horizontal cylinders with the same outer diameter and with a varying central bore geometry. A chamfer is added on one side to aid the tool engagement (diameter of 18 mm). The intense heat that is generated by the laser beam to melt the material during the LPBF process has to be dissipated fast enough to not affect the neighboring loose powder or cause the part to locally overheat. This means that every new layer of fused powder has to be supported by the previous layer. Areas where this is not the case are assisted by so-called support structures. They provide a path to dissipate heat and prevent overheating. It is recommended to use supports for all downward facing surfaces with an angle lower than 45 to maintain part quality [5]. To examine the weakened structure, samples are built with a cylindrical region (diameter 12–5 mm) of intentionally weakened material W2 or W3, referred to as “Ring” samples. Due to the reduced energy input, this region can also serve as a support structure of the bore. There are also fully dense “Ring solid” samples built for comparison purposes. Although this is typically not recommended without support structures, it is still possible with a 12 mm diameter.

To investigate the chip formation and machining forces in comparison to commonly used support structures, specimens with a standard “line” and with a rhombus support structure are produced.

In addition, specimens without a prebuilt bore are produced in fully dense, W2 and W3 variants (up to 14 mm diameter), and compared with conventionally manufactured maraging steel samples, to assess the impact of the weakened structure on a large cutting volume with high machining forces. Three test specimens are built for each variation. The dimensions of the test specimens, the buildup direction during the LPBF process, the different central bore geometries and the proportion of the weakened structures are shown in Fig.  3. Not all produced samples are shown. “Ring solid” samples share the same base geometry as W2 and W3. A photo and section view of the weakened samples can be found in Fig.  4.

The orthogonal cutting is performed on a special test stand that enables relative linear movement between the sample and the tool (see Fig.  5). The workpiece holder is driven by a linear motor while the tool holder is stationary during the cutting process. The tool holder height can be precisely adjusted to set the depth of cut. Samples are clamped on a dynamometer type 9263 by Kistler, in order to measure the resultant process forces: cutting force \(F_{x}\) and thrust force \(F_{z}\). In addition, a structure-borne noise sensor using High Frequency Impulse Measuring (HFIM) is placed on the tool holder to analyze impact of the weakened structure onto the tool behavior. The cutting speed is set to 100  m/min with the depth of cut ae at 0.2  mm. Carbide cutting inserts with a proprietary TI25 coating by Paul Horn GmbH are used for the orthogonal cutting experiments.

Drilling experiments for the application samples are conducted with a Hermle UWF 1202 machining center with emulsion cooling. Process forces \(F_{z}\) as well as the drilling moment \(M_{z}\) about the rotational axis of the drill are measured during machining using a Kistler Charge Amplifier force plate type 9273 using the Workpiece coordinate system (\(F_{x}\),\(F_{y}\),\(F_{z}\)) and the specimens are clamped on top with a three-jaw chuck. The test setup is shown in Fig.  6. A carbide drill with two cuttings edges, internal cooling, diameter of 15.8 mm and proprietary nanoFIRE coating from the company Gühring is used with a selected cutting speed of \(v_{c}\)  =  65  m/min and feed of f  =  0.315  mm/U.

No intermediate process steps are carried out between the additive and subtractive processes. The weakened structure within the bores is completely cut away during machining, to guarantee a smooth surface without any remaining pores. To analyze and evaluate the resulting surfaces after drilling, roughness and cylindricity measurements are performed on all samples. Surface roughness of the machined inner bore wall is measured for each specimen on five lines along the bore axis using a surface measuring device Mitutoyo Surface Scanner SV-C3200. The cylindricity is measured using a CMM retrofitted by Renishaw with a REVO-2 5-axis system and RSP2 scanning head with RSH250-6x10 stylus.

The median axial drilling force \(F_{z}\) and the drilling moment \(M_{z}\) as well as the respective median absolute deviation (MAD) during machining of samples without a prebuilt bore can be seen in Table  2.

Listed are the conventionally manufactured steel samples, AM samples built with a fully dense material and the two investigated weakened structures. The difference between the additively produced solid samples and the reference steel samples is very small with 2.9% in force \(F_{z}\) and 1% in moment \(M_{z}\). The weakened structure samples on the other hand, deviate much more clearly. The measured force is reduced by 16% and 31% for W2 and W3 samples respectively compared to the AM solid samples. The reduction in drilling moment is measured to be 11% and 19% for W2 and W3 samples, respectively. In general, all machining forces of samples without a prebuilt bore are exceedingly high, leading to challenges in machining of finely detailed highly optimized parts.

The reduction in machining forces encountered with the weakened structure are even more prevalent when inspecting the results of samples with a prebuilt bore and different support structures. The “Lines” and “Rhombus” samples show widely fluctuating machining forces of over ±  50%, due to their geometry. The “Ring” samples on the other hand only show fluctuations around ±  10%. Figure  8 shows the axial drilling force \(F_{z}\) and the drilling moment \(M_{z}\) plotted over time when machining the different support structures. To better visualize the traces, they are uniformly smoothed using a Savitzky-Golay filter.

The varying geometry encountered with the rhombus or line supports is the cause of vibrations in the tool and workpiece with negative effects on tool wear, surface quality and part clamping.

The weakened samples with cylindrical bore geometry on the other hand provides a more uniform cutting cross-section with less vibrations impacting the process. Furthermore, the magnitude of the machining forces can be significantly reduced using the weakened structure. The force \(F_{z}\) of samples with the same geometry, but different variants with fully dense, W2 and W3 structures is measured at 730  N, 440  N and 350  N respectively. The moment \(M_{z}\) is reduced from 1060  Ncm for fully dense samples, down to 900  Ncm for W2 and 830  Ncm for W3 samples.

Due to the high toughness of 1.2709, the chips during machining with a low degree of deformation are typical long and snarled ribbon chips [26, 27]. This characteristic is negatively affecting the chip removal while drilling out pre-build bores, and detrimental to the resulting surface quality [27]. The internal cooling of the utilized drilling tools improves the chip removal but can not fully mitigate this issue. Ribbon chips are also wrapped around the drill bit and have to be removed by hand after cutting is finished. Therefore, a secondary focus of this investigation for the application specimen is on the optimization of chip formation towards shorter break chips, in addition to reduced cutting forces.

These typical snarled ribbon chips can be clearly seen after processing the line support samples. The thin support structure bars are broken off and pushed in front of the drill and the remaining machining allowance is cut and produces ribbon chips. The broken off support bars also influence the chip flow and cutting behavior, leading to surface scoring with deep grooves as can be seen in Fig.  9. The median roughness depth across all line support samples is measured at \(R_{z}\)  =  36.82  \(\upmu {\text {m}}\).

The rhombus geometry can still be identified in chips resulting from the rhombus samples (see Fig.  9). The varying geometry leads to highly fluctuating process forces, changes chip formation and results in helical chips instead of snarled ribbon chips. This reduces the volume occupied by the chips and improves chip removal from the bore. Compared to the line support samples, there are no deep groves visible on the surface and the median roughness depth is improved to \(R_{z}\)  =  13.75  \(\upmu {\text {m}}\). The long helical chips are still detrimental to the process however. Furthermore, it can be seen that the fluctuating process forces are causing process instability, resulting in visible marks on the surface (see Fig.  9).

In comparison to the reference support structures, the samples with the developed weakened structure only form short spiral chips (see Fig.  9). Due to the unique sample design, only a very small volume of solid material is being cut, with the rest made out of weakened material. This leads to a segmentation of the chip, and consequently to chip breakage. Chip removal out of the bore and away from the tool and workpiece is significantly improved, which is reflected in the resulting surface quality as well as the overall process stability. Median roughness depth is measured at \(R_{z-W2}\)  =  9.78  \(\upmu {\text {m}}\) and \(R_{z-W3}\)  =  11.45  \(\upmu {\text {m}}\) for the weakened structures W2 and W3 respectively and no grooves or chatter marks are visible.

Due to the large amount of material to be cut, there is enough energy present to break the chips of the fully dense AM samples without a prebuilt bore, resulting in short spiral chips (see Fig.  9). This also leads to an acceptable surface finish with a median roughness depth of \(R_{z}\)  =  11.08  \(\upmu {\text {m}}\). The conventionally manufactured reference samples show the same results.

The cylindricity measurements also show that machining conventional support structures can negatively affect the resulting surface. Due to fluctuating cutting conditions, the measured cylindricity of line and rhombus samples is higher than that of W2, W3 and fully dense samples (see Fig.  9).