CO₂ laser polishing of laser-powder bed fusion produced AlSi10Mg parts

Highlights

• A CO₂ laser was used to perform surface re-melting of AlSi10Mg L-PBF samples.

• The surface roughness (S_a) was reduced up to the 85%.

• Surface symmetry, analyzed by means of areal skewness (S_sk), was improved.

• Profile root mean square slope (P_dq) was also evaluated and reduced up to the 90%.

• Vickers microhardness increased up to the 30% in the polished area.

Abstract

Poor surface quality represents an important issue that needs improvement for metal Additively Manufactured parts. Both short or long wavelength lasers have been applied for surface polishing in order to improve the surface finish. In this work, the possible use of a CO₂ laser for the surface polishing of AlSi10Mg parts made by Laser-based Powder Bed Fusion (L-PBF) was explored. The high surface roughness in the as-built condition can lead to increased laser energy absorption. In order to assess the effects of the main laser-related process parameters, the experiments were carried out on L-PBF samples built vertically with respect to the build platform. Their effect on the surface were evaluated by means of the surface arithmetical mean height (S_a), surface skewness (S_sk) and primary profile root mean square slope (P_dq), obtained via confocal microscopy. Microstructure evolution was also investigated by means of SEM and EDS analysis. The results showed a large reduction in surface roughness, ranging from the 67% to the 85% of the starting value. Microstructure of the re-molten layer revealed an increased grain size and an increased Si content that led to an overall hardness increase from 85 to 121 HV.

Introduction

Over the last two decades, the interest in metal Additive Manufacturing (AM) processes has grown due to several benefits of these processes over traditional subtractive technologies. More specifically, the possibility to produce near-net-shape metal parts following a layer-wise fashion and with a higher freedom in design is allowing applications in many industrial contexts compared, for instance, to conventional CNC machining [1]. Moreover, the increasing availability of different metal and alloy feedstock suitable for AM provides a further boost to the already widespread use of this manufacturing process [2]. Powder-based metal AM processes such as Laser Powder Bed Fusion (L-PBF), Electron Beam Powder Bed Fusion (E-PBF) and Laser Metal Deposition (LMD) represent the leading technologies for the metal additive manufacturing of parts. These technologies provide at the same time a more efficient use of raw materials and a part building rate that is independent from the degree of part design complexity [1]. On the other hand, the current lack of a deep know-how combined with some intrinsic characteristics of these processes results in several drawbacks such as entrained porosity, high residual stresses and poor surface quality [3]. Concerning the latter, it is well established that metal AM produced parts have a high surface roughness (often identified as the profile arithmetic height distribution, R_a [4]), ranging from few to several tens of micron depending on the specific case [5]. This level of roughness makes such parts unacceptable for many end use applications. The fundamental mechanisms that contribute to this issue are also well described in the literature, namely the powder particle size distribution, the laser spot size, the staircase effect and the balling effect. These lead to a rough surface texture characterized by the presence of visible overlapped layers and sintered metal powders [3,6]. These aspects continue to be issues, despite the work on process parameter optimization and the adoption of what are called in-situ contour strategies [1,7]. This makes necessary in many cases therefore the adoption of a post-process surface finishing step in order to improve the surface quality of the manufactured parts and to meet the required final surface roughness.

According to the state of the art, surface roughness of metal AM parts can be reduced by means of different techniques that can be classified according to the interaction nature with the surface, i.e. mechanical, chemical/electrochemical and thermal. Given the large number of those techniques, some authors proposed a comprehensive review work in this context [8,9].

Mechanical interaction-based treatments mainly include CNC machining [10,11], shot peening [12,13], vibratory polishing, sand blasting [14,15] and so on. Moreover, other technologies typically used in other contexts were subjected to a feasibility study in the surface finishing context, given their higher capability of access into complex parts features in comparison with the previously mentioned techniques. Examples of these processes are the Fluidized Bed and Hydrodynamic cavitation technologies [[16], [17], [18], [19]]. However, in general, mechanical-based surface treatments present strong limitations related to the physical access of tools and abrasive media into complex parts shapes such as the ones achievable with AM.

Chemical and electrochemical-based surface treatments, on the other hand, have the advantage of the higher mobility of the chemically aggressive solutions employed in comparison to abrasive media and machining tools, making therefore these treatments very suitable for high complex parts. Moreover, the presence of an electric potential improves the not close control of the material removal rate [20], probably the main drawback of pure chemical interaction-based finishing processes. In literature, those treatments are often called Chemical polishing and Electropolishing, according to the work of several authors conducted on parts produced by L-PBF and E-PBF technologies [[21], [22], [23]]. It is worth to mention that and both Chemical polishing and Electropolishing are capable to perform a selective material dissolution with respect to the surface asperities. In Chemical polishing, higher etching rates can be achieved on the surface peaks against the valleys if the chemical solutions are properly conceived. This effect is also quite useful for the removal of the molten pool edges underlying the sintered powders-rich texture of powder-based metal AM parts. On the other hand, the surface symmetry improvement can be enhanced in Electropolishing due to greater etching rates for the surface peaks imposed by the electric potential.

Thermal interaction-based finishing processes consist in providing a certain heat input on the surface, in order to promote its smoothing through different surface modification mechanisms such as re-melting and ablation [24]. Laser polishing represents in this scenario one of the main investigated finishing processes. Despite similar considerations to mechanical finishing processes are also valid in this case such as the limited physical access into complex part features, laser polishing offers several advantages such as very precise control of the process parameters, high level of process automation and a very high surface quality. Moreover, the thermal cycles imposed by laser polishing determines a superficial microstructure refinement, enhancing properties such as corrosion and wear resistance. In this scenario, several authors have investigated the effects of laser polishing on the surface quality of metal AM parts. This work has mostly been performed for the L-PBF process and both in-situ and ex-situ approaches have been investigated for several alloys such as aluminum [25,26], stainless steel [27,28] and titanium alloys [29]. The majority of the provided contributions have been related to the use of fibre laser, such as Neodymium-doped Yttrium Aluminum Garnet (NdYAG) and Ytterbium (Yb) sources. The latter laser type also represents the most utilized laser in L-PBF machines. The relatively short wavelength (1.07 μm) of the fibre lasers compared to that of the CO₂ laser (10.6 μm) enables better energy absorption. The lower wavelength is more typically highly absorbed by most of the metals and alloys, for which the CO₂ laser wavelength is relatively poorly absorbed [30], especially for high reflective metals such as aluminum and copper. This aspect was further demonstrated by the recent yet poorly explored research trend in the context of laser polishing, related to the use of pulsed lasers with an ultrashort pulse duration that guarantees a very high radiation absorption as well as a high retaining of the microstructure features [31]. On the other hand, it is also well known that the laser radiation absorption mechanisms do not depend solely from the material itself, but also from the surface roughness. If the latter is greater than the laser wavelength, the absorption mechanisms are also dictated from optical considerations such as multiple reflections of the laser beam within the surface asperities [24]. Taking in consideration that the typical surface roughness range of the as-built L-PBF parts is 6–25 μm [5], the aforementioned roughness-induced laser absorption enhancement could extend the use of the more widely available and employed CO₂ laser [28,[32], [33], [34]] also for the polishing of the aforementioned high reflective materials. With this premise, this work investigated the possibility to employ a CO₂ laser to perform ex-situ surface polishing of L-PBF parts made with AlSi10Mg alloy, the latter widely used in aerospace and automotive parts applications. More specifically, the following questions were arisen and answered, assumed that CO₂ laser is less absorbed than fibre laser but it provides higher output power and it is still the most used laser type for a large number of applications: (i) Is it possible to perform a satisfactory polishing process by taking advantage of the undesired rough surface of the as-built AM parts for an enhanced laser absorption? (ii) If so, which is the effect of the laser power, scanning speed, overlap between the tracks and focus position on the surface texture and microstructure evolution of the considered samples?