Consideration of SLM additive manufacturing supports on the stability of flexible structures in finish milling

TL;DR: In this paper, the authors highlight the importance of additively manufactured support structures on the stability of Ti-6Al-4 V parts milling by using supports as a machining fixture.

About: This article is published in Journal of Manufacturing Processes.The article was published on 2021-02-01 and is currently open access. It has received 15 citations till now. The article focuses on the topics: Machining & Selective laser melting.

Summary

1. Introduction

• The growing interest in Additive Manufacturing (AM) processes and particularly in the Selective Laser Melting (SLM) technology resulted in the development of the associated techniques: "design for additive manufacturing".
• The process to obtain powder for SLM technology is expensive and thus the achievable alloy compositions are limited.
• During the manufacturing processes, manufacturing supports are always used to limit strains by strengthening the overall structure and favoring a better heat diffusion during manufacturing [11] .
• These factors are unfavorable for the milling, due to the bending of the flexible part, and thus vibrations can appear between the tool and the part [17] .
• A novel approach is to consider lattices structures as supports with the ability to control the mechanical properties of the overall additive manufacturing part, with the objective of post-processing the surfaces by milling.

2.1. Geometry and SLM additive manufacturing of the samples

• Differences of stability in milling and potential vibration problems are highlighted by the design of adequate SLM samples.
• In the previous studies about chatters when machining, milling instability directly resulted from the thin thickness of thin-walled plates.
• The lower part of the samples corresponds to the manufacturing support.
• The Block support consists of intersecting walls with diamond-shaped perforations.
• By adjusting the topology and the beam diameter, the relative stiffness and the relative density of the global structure can be controlled and numerically implemented.

2.2. Analysis of the first modal frequency

• The sample is composed of two parts: the upper plate and the lower support that has the equivalent behavior of a bulk plate structure.
• Since the natural frequencies of the plate increase with its rigidity, the stiffness of each sample is analyzed through the natural frequency of the first mode f1.
• The displacements are measured by a laser vibrometer.
• Owing to the resolution, the reproducibility of the process is validated, particularly for the Magics® supports.

2.3. Experimental tests of finish milling

• The peripheral finish milling of the plate (thin wall) is proceeded in a Roeders RXP200DS (3000 -60000 rpm) 5 axis machining center, with an Erowa ITS50 clamping system.
• It allows measuring the cutting forces in three directions in the three-dimensional space.
• The displacements of the thin wall when machining are also measured with a laser vibrometer in the middle of the free end of the plate; the laser beam is represented Figure 3 (a) .
• The first pass can't be considered as constant, due to the initial surface state, including roughness and particles partially melted.
• Surface topography is obtained after milling tests by confocal microscopy with a Leica DCM3D system.

3. Results

• Table 2 summarizes the overall results for all structures.
• This takes into account the average amplitude of displacement and force on the two machining passes, the height between the machined surface and the rough surface (in confocal microscopy) and the roughness Ra.
• Finally, the surface roughness is much smoother for more rigid structures.
• In order to study more precisely the impact of the stiffness of the supports on the machining operation, three structures are studied more closely.

3.1. Displacement

• The displacement at the global scale is firstly considered, see Figures 4 (a), (d ) and (g).
• It shows that whatever the stiffness of the sample, the signal has the same aspect with a stable central area, composing the majority of the signal.
• At the beginning and at the end of the signal, edge effects are visible with different amplitudes compared with that at the stable central area.
• When the three plates are compared to each other, it can be seen that the more the plate is flexible, the higher the amplitude of the signal is.
• The same trend is observed for the edge effect.

3 mm diameter structure (g) (h) (i) -Global scale

• Furthermore, the displacement imposed by each tooth is not equivalent and is decreasing with the four successive teeth, typical to a tool run out.
• The signal of the higher stiffness plate presents a 0.2 ms phase without movement, equivalent to a 0.1 mm displacement, see Figure 5 (a).
• The Octet-truss structure with 0.375 mm diameter is more flexible, and the signal amplitude is higher and oscillates continuously from -0.35 to 0.35 mm, as seen in Figure 5 (d) .
• At last the Octet-truss structure with 0.3 mm diameter is less rigid with a displacement oscillating between -0.6 and 0.35 mm, see Figure 5 (g).

3.2. Cutting forces

• The displacement acquired with the laser interferometer in the normal direction of the plate is correlated with the normal cutting force in the same direction Fx. Moreover, as seen from the displacement, the edge effect is more marked for the less rigid structure with the more perturbed signal at the extremities.
• Some irregularities and punctual instabilities, not observed on the displacement signal, are observed all along the Octet-truss with 0.3 mm diameter, as shown in Figure 4 (h).
• At the local scale, the impact of each tooth is clearly identified for the Block structure, as seen in Figure 5 (b).
• It confirms the correlation between the two signals.
• In terms of the amplitude, the force signal decreases with the loss of the stiffness, both locally and globally.

3.3. Topography of surface

• Figure 4 also presents the surface topography on all the cutting area, obtained by confocal microscopy.
• This variation is due to the side effect, especially at the output of milling.
• It corresponds to the feed rate per revolution.
• The surface is considerably more degraded for the flexible Octettruss structure with 0.3 mm diameter, where the irregularity of the cutting is evident, as seen in Figure 5 (i).

4. Discussion

• SLM process allows a near-net-shape product but some functional surfaces still need to be postprocessed.
• A correlation can be established between the stiffness of the workpiece and the plate displacement in flexion.
• This trend is confirmed by the evolution of the average displacement (in the stable area) according to its first modal frequency, as seen in Figure 6 .
• On the global displacement signals, edge effects are visible when the tool is cutting the extremities of the plate.
• Indeed, contrary to traditional supports designed to build overhang surface during the additive process, lattices structures stiffness can be controlled by a numerical method.

5. Conclusions

• The study proposes a novel approach considering supports as a custom-made machining fixture for machining operation.
• To demonstrate the concept, experimental tests of finish milling have been done on plate structures on their supports, manufactured by SLM additive process.
• The equivalent stiffness of the overall workpiece varies with the geometry and architecture of the manufacturing supports.
• The correlation between the displacement and the cutting forces confirms a more and more irregular and ineffective cutting with the loss of stiffness of the supports.
• The actual associated digital chain does not allow optimal control of the mechanical behavior of the supports.

Figures

Table 1 - Average of the first modal frequency of each structure and dispersion

Figure 6 - Evolution of the average displacement amplitude and cutting force according to the first modal frequency

Table 2 - Summary of machining test results: displacement amplitude, cutting force, height difference between machined and rough surface and roughness Ra

Figure 3 - (a) Experimental procedure implemented, (b) Cutting schema of finishing pass and associated parameters and (c) Manufactured sample presenting the milling pass.

Figure 5 - Normal displacements, normal cutting force and surface topography for the Block structure (a) (b) (c), the Octettruss with a 0.375 diameter (d) (e) (f) and the Octet-truss with a 0.3 mm diameter structure (g) (h) (i) - Local scale

Figure 1 – Machining operation of an additive manufactured part with consideration of supports as a machining fixture.

Figure 4 - Normal displacements, normal cutting force and surface topography for the Block structure (a) (b) (c), the Octettruss with a 0.375 diameter (d) (e) (f) and the Octet-truss with a 0.3 mm diameter structure (g) (h) (i) - Global scale

Figure 7 - Profile between rough and cut surfaces for (a) Block structure and (b) Octet-truss 0.3 mm diameter structure, acquired by confocal microscopy

Figure 2 - Geometry of the two types of sample: lattice structures and AM software structures

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Consideration of slm additive manufacturing supports on the stability of flexible structures in finish milling" ?

This study highlights the importance of additively manufactured support structures on the stability of Ti-6Al-4V parts milling by using supports as a machining fixture. The objective of this study is to show that the stiffness of the manufacturing supports is crucial for the machining operation. The study reveals that milling can induces chatters.