Mass Loss and Surface Passivation Characteristics of Electropolished SLM 316L Steel

전해연마된 SLM 기반 316L Steel의 질량 손실 및 표면 피복 특성 연구

Abstract

Utilization of additively manufactured parts in intended applications is limited by surface roughness. Roughness reduction in internal surface area of AM parts is exponentially more challenging. Reported methodologies for roughness reduction, result in material loss and limits the operational life of these parts. Herein, we explored electropolishing to reduce surface roughness of SLM manufactured 316L steel. Furthermore, the mass loss incurred during electropolishing is deduced as a function of polishing time. The change in roughness, wettability and surface passivation were studied and discussed in detail.

1. Introduction

Manufacturing metallic parts have significantly improved since the advent of additive manufacturing (AM). It enabled production of ready to use highly complex engineering components. Selective Laser Melting (SLM) is a rapid prototyping technology and is preferred to fabricate customizable parts due to the advantages of high material utilization and geometric freedom. However, it is critical to improve surface finish for the components to be functional. The high surface roughness of AM parts are vulnerable to failures as a result of fatigue. Similarly, the high surface roughness are susceptible to corrosion and thus create reliability issues.

As a consequence of roughness, it is vital to consider microstructure of materials to enhance the applicability of SLM parts[1, 2]. The anisotropy and non-equilibrium of microstructures could arise from porosity, un-melted powders and gas entrapment during fabrication process[3, 4]. These, surface mechanical properties of SLM components demand precise and high efficiency post-processing methodologies.

Surface finish processing is highly challenging with intricate deign and geometries. Popular approaches such as machining, extrude honing and sand blasting could not be applied to such AM parts[5]. Alternatively, chemopolishing and electropolishing are being practiced for decades to improve the surface quality of metallic components[6-11]. The process of removing metal ions under the influence of applied potential demonstrate polishing. Due to the advantages of electrolyte intrusion in hidden surface areas, electropolishing is widely adapted for surface processing[9, 12]. However, the polishing efficacy is dependent on counter electrode separation proximity, temperature & concentration of electrolyte, applied potential and time[13, 14].

Electropolishing is being reported for roughness reduction and microstructure analysis of processed AM components[15-17]. However, material loss characteristics of SLM manufactured 316L and its associated surface passivation are least explored. In this study, we used electropolishing to improve surface aesthetics, estimate mass loss, control roughness and surface passivation of SLM manufactured 316L. Furthermore, the interfacial properties such as surface wettability (water contact angle - WCA), corrosion of polished 316L plates were studied and discussed in detail.

2. Materials and Methods

2.1 Materials

SLM manufactured rectangular plates (316L) of size 20*20 mm with thickness of 2mm were purchased. Phosphoric acid (85%) was purchased from Duksan reagents, Sodium Chloride (99%) and sulfuric acid (30%) was purchased from SAMCHUN Chemicals (Republic of Korea). The purchased chemicals were used without any further purification. The material composition of 316L plates are presented in Table 1.

Table 1 Material composition of 316L

2.2 Methods and Characterization

2.2.1 Method

Fig. 1 shows the schematic of electrochemical polishing experimental setup. The experiments were conducted in a beaker controlled with bath circulator maintained at a temperature of 20 °C. Keithley 2231A voltage source was used to apply 5V DC potential for time period ranging 10 to 40 minutes in an interval of 10 minutes. In the experiments, a two-electrode electrochemical system was utilized with SLM manufactured 316L and Platinum as working and counter electrodes respectively. Polishing was performed in a 1:1 mixture of sulfuric acid (30%) and phosphoric acid (85%). Before each experiment, the samples were ultrasonically cleaned in acetone and deionized (DI) water for 10 minutes.

Fig. 1 Experimental schematic of electrochemical polishing

2.2.2 Characterization

Field Emission Scanning Electron Microscope (FE-SEM) SU6600 (HITACHI, Japan) was used to obtain surface morphology. The industrially accepted arithmetic mean roughness (R_a) was measured via contact roughness tester. The change in thickness of polished surfaces were obtained using Vernier scale and mass loss was measured using analytical weighing balance (RADWAG AS220.R2 Plus). All the electrochemical measurements were obtained from Biologic (SP50e) workstation in 0.1M NaCl solution with Ag/AgCl as working electrode. Contact angle measurements were performed with SMARTDROP goniometer by placing 5 μL droplets at different locations and the averaged data is presented.

3. Results and Discussion

FE-SEM images of as-received SLM and polished SLM surfaces are presented in Fig. 2. Rough structures with air gaps are seen in as-received SLM surface [Fig. 2(a)]. The air gaps (marked as arrows) in this surface is due to the laser melting process. The rough protrusion of as-received steel surface is presented in the magnified SEM image [Fig. 2(b)]. The air gaps are referred to cracks and the aggravation is a result of large solidification temperature[18, 19]. While, the low magnification images shown in Fig. 2(c)-(f) does not show any evidence of surface cracks after polishing for 10, 20, 30 and 40 minutes. It is observed that the polished area appeared bright improving the aesthetics by removing oily contaminants from the surface. In high carbon steel, the grain boundaries possess cementite phase which is presumably etched leaving the surface with hexagonal regions. It is noteworthy to highlight that the electropolishing does not show any preferential etching, leaving the surface with smooth texture[20].

Fig. 2 FE-SEM images of as-received (a & b) and electropolished specimens (c – f)

As presented in FE-SEM images, electropolishing greatly removed the rough textures on the surface and hence an evident change in surface wettability (WCA) is expected. To deduce the changes in surface-water interface, WCA measurements were obtained with 5 μL droplets. The surface wettability obtained from WCA measurements support the surface characteristics observed in FE-SEM images. The as-received SLM specimen shows hydrophobic wetting with contact angle of ~ 120°. However, the polished specimens show contact angle ranging between 65° and 70° depicting hydrophilic wettability. The change in contact angle with polishing time is presented in Fig. 3 (black-legend). Meanwhile, the arithmetic mean roughness (R_a) of polished surfaces show decreasing trend with increase in time as shown in Fig. 3 (red-legend). As-received specimen having roughness of ~ 2.5 μm is reduced to 0.75 μm for a polishing period of 40 minutes. Alternatively, this polishing process can be boosted up by increasing the electrolyte temperature that can trigger high material loss. To ensure minimal loss in material and to keep the thickness of specimen intact, time is varied by holding other experimental conditions unchanged. Material loss and smoothening efficiency was calculated by measuring the mass and roughness of specimen before and after polishing. A higher surface smoothening efficiency of 72% was achieved with a maximum mass loss of 8% for the specimen polished over 40 minutes. A graph depicting change in water contact angle, roughness and mass loss (inset of Fig. 3) against polishing time is shown in Fig. 3.

Fig. 3 Change in contact angle and surface roughness of as-received SLM surface (0 minutes) and electropolished surfaces (10 – 40 minutes). Mass loss of electropolished surfaces (Inset)

Surface passivation of test specimen was determined by potentiodynamic polarization performed between -0.9 V to 1.5 V at a scan rate of 10 mV/s and is depicted in Fig. 4. The change in current density and corrosion potential was calculated from Tafel curves and is shown in Fig. 4. It is observed that the as-received SLM surface shows an evident passivation region. Specimen polished for 10 minutes shows a mere passivation region with change in corrosion potential from -0.488 V to -0.442 V. Additionally, a significant change in corrosion current density [Log I (mA/cm²)] is observed depicting surface passivation through electropolishing. A study by Hryniewicz et al.[21] compares the ratio of Cr/Fe with mechanical and electropolishing methodology. Interestingly, electropolishing shows high Cr/Fe ratio than mechanically polished surface, depicting minimal dissolution of corrosion protection ions. Furthermore, it is highlighted that the ratio of Cr/Fe rely on thickness of passivation layer and anodic current density maintained during polishing[22]. In our study, the potential was maintained constant with control over polishing time to ensure high surface passivation with better Cr/Fe ratios.

Fig. 4 Potentiodynamic polarization curves of as-received and electropolished surfaces

With progress in electropolishing time to 20, 30 and 40 minutes, the perturbation in anodic curves are increasing. This perturbation in current density over anodic slope is attributed to the roughness of surface cracks thus increasing the metastable state of pitting associated with metal dissolution[23-25]. The surface cracks are a consequence of large solidification temperature in the SLM fabrication process and is exposed after polishing. It is reported that the surface cracks obtained during various conditions can significantly affect mechanical strength and roughness of AM parts. Furthermore, research is being devoted to standardize the fabrication conditions to achieve stress-free SLM manufactured parts[26, 27]. Notably, surface polished at 20, 30 and 40 minutes does not show evident change in corrosion potential while showing increased spikes in the passive region as result of change in surface roughness that increase the sluggish electron transfer. Digital micrographic images of surface cracks of SLM manufactured specimens before and after electropolishing are presented in Fig. 5.

Fig. 5 Digital micrographs of as-received SLM specimen (a); exposed cracks of electropolished specimen (b)

The corrosion current density of 10 minutes polished surface reads 16.6 μA/cm² and is roughly a 3-fold decrease compared to as-received SLM specimen (48 μA/cm²). Surface polished for 40 minutes showed corrosion current density as low as 8.3 μA/cm². The change in current density between 8 – 16 μA/cm² could be attributed to a maintained Cr/Fe ratio. The corrosion potential, current density and corrosion rate (CR) obtained by extrapolating cathodic and anodic curves of test specimens are tabulated in Table 2.

Table 2 Corrosion potential (E_corr), corrosion current density (i_corr), and corrosion rate (mm per year) of

4. Conclusions

Electropolishing was explored to suppress roughness and to achieve lustrous SLM manufactured surface. Control over polishing time and its effect over roughness change, material loss and surface passivation were evaluated. The synergy of surface features and liquid-solid interfacial properties were explored with contact angle measurements and found no evident change in wettability with decrease in roughness from 1.8 microns to 0.75 microns. With a mass loss of 8%, electropolishing decreased the roughness by 72% and improved surface passivation to 6-fold. The experimental observations in this study, reports the significance of polishing time in roughness control and material loss for the given temperature of 20 °C.