Additive manufacturing of ZK60 magnesium alloy by selective laser melting: Parameter optimization, microstructure and biodegradability

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Highlights

  • Mg-Zn-Zr (ZK60) magnesium alloy was prepared by selective laser melting (SLM).

  • High-quality samples with refined grains were obtained at optimized parameters.

  • Comparative studies were carried out between cast ZK60 and SLM ZK60.

  • SLM ZK60 exhibited desirable biomechanical properties and biodegradability.

Abstract

Mg-Zn-Zr (ZK60) magnesium alloy samples were prepared by selective laser melting (SLM). The processability of ZK60 prepared by SLM was thoroughly investigated via varying the laser processing parameters. Experimental results show that laser power and scan velocity played an important role in determining the quality of SLM ZK60. SLM ZK60 with minimal defects and high dimensional accuracy could be obtained at a laser power of 50 W and scanning velocity of 500–800 mm/s. The formation mechanisms of pores were also identified and illustrated thoroughly in size and shape in the transition regimes. Significantly refined grains were observed in SLM ZK60, with an average grain size (Ga) of 7.3 μm for SLM ZK60 versus 56.4 μm for cast ZK60. Besides the α-Mg phase, the Mg7Zn3 eutectic phase was precipitated in the α-Mg matrix in SLM ZK60, and the MgZn phase in cast ZK60. The hardness of cast ZK60 and SLM ZK60 was 0.55 and 0.78 GPa, respectively, with similar elastic modulus. SLM ZK60 exhibited a higher corrosion resistance in Hanks’ solution compared with cast ZK60 as indicated by a decrease of about 30 % in hydrogen evolution rate and of about 50 % in the corrosion current density. The SLM ZK60 fabricated in the present study exhibits a great potential for biomedical applications due to desirable mechanical properties and a reasonably low degradable rate.

Introduction

Metallic materials, polymeric materials and ceramic materials are commonly used for orthopedic implants in clinical applications. Due to the low strength of polymeric materials and the high brittleness of ceramic materials, metallic biomaterials are more popular implant materials mainly on account of their combined strength and toughness [[1], [2], [3], [4]]. Metallic materials such as stainless steel and titanium alloys have been used as key materials for decades in the biomedical fields, especially for orthopedic applications owing to their high strength and high corrosion resistance in human body environment [5,6]. However, orthopedic implants made of stainless steel or Ti alloys will remain permanently inside the body after implantation, and permanent implants serve no purpose when healing is completed. On the contrary, its presence inside the body is a nuisance due to the shielding effect and release of toxic ions in vivo [7]. The shielding effect, which arises from the mismatch of the Young's modulus between the implant and natural bone, might eventually lead to osteopenia or even osteoporosis [8]. In view of this inherent drawback of conventional metallic materials, a new generation of metal-based biocompatible materials with degradable property, such as biodegradable Mg and its alloys, offers a solution to the problem of requiring a second surgery for implant removal after recovery [[9], [10], [11]]. In comparison with stainless steel (189−205 GPa) and titanium alloys (110−117 GPa), Mg and its alloys have the lowest Young's modulus (41−45 GPa), being closest to that of the natural bone among the metallic materials [[12], [13], [14]]. Meanwhile, Mg ions can induce the formation of bone and shorten the healing time [15,16]. However, the degradation rate of Mg and its alloys in body fluids is too high, which limits their biomedical applications [17]. Conventional manufacturing methods of Mg and its alloys are mainly based on deformation processing and casting [18,19]. In particular, better mechanical properties could be obtained by deformation processing [18]. However, the slip of dislocations during the deformation only occurs on the basal plane (0001) and the direction <120>, or the pyramidal plane (112) for the twining at room temperature owing to the nature of close-packed hexagonal structure, indicating a comparatively limited cold workability [20]. Orthopedic implants are difficult or impossible to fabricate by conventional manufacturing methods.

In view of these considerations, selective laser melting (SLM) offers a very attractive solution to the problem, which can directly build near net shaped component parts with complex geometry, requiring only little or even no post-machining [[21], [22], [23]]. However, studies on SLM of Mg alloys are scarcely reported in the literature due to the high vapor pressure of Mg, which makes SLM fabrication difficult. Ng et al. [14] reported fabrication of single-track pure Mg samples using SLM. Zhang et al. [24] sintered a powder mixture of Mg-9%Al by SLM, and only a maximum relative density of 82 % could be achieved. Wei et al. [25] successfully fabricated AZ91D magnesium alloys with nearly full-dense parts of approximate 99.52 % relative density by SLM, and this particular SLM AZ91D magnesium alloy exhibited superior microhardness and tensile strengths at room temperature compared with the die-cast AZ91D. In addition to the capability of tailor-making, SLM is characterized by high melting/solidification rate with a cooling rate of above 105K/s, which could extend the solubility of alloying elements, refine the grain size and inhibit segregation. It has been reported that Mg alloys with small grain size and homogeneous microstructure have a lower degradation rate due to enhanced passivation and reduced microgalvanic attack [26,27]. Argade et al. [26] demonstrated that the corrosion rate of the Mg-Y-RE alloys with ultrafine grain size was one order magnitude lower than that of the alloys with coarse grain size. Lu et al. [27] investigated the effects of intermetallic phases on the degradation behavior of Mg-3Zn alloys. This particular research revealed that Mg alloys exhibited a low degradation resistance when a large volume fraction of intermetallic phases were precipitated out. These studies suggest that SLM would be a promising method to manufacture Mg implants with favorable mechanical properties and improved corrosion resistance via optimal selection of laser processing parameters.

Previous studies have shown that Mg-Al binary and Mg-xAl-Zn ternary magnesium alloys exhibit excellent degradation resistance and good cytocompatibility, which are important for degradable orthopedic implants [28,29]. It should be, however, noticed that the Al elements in Mg alloys may impair bone mineralization and have an adverse effect on human neurons, and Al-containing Mg alloys are now gradually eliminated in implant applications [30]. On the other hand, Mg-Zn binary alloys show great potential in biodegradable implant materials due to their lower cytotoxicity compared with Al- or RE-containing Mg alloys [31]. In addition, Zn element is an essential element in human body and is crucial for biological function. However, the degradation rate of Mg-Zn alloys is undesirable in physiological environments, which can result in local intense alkalization and the formation of subcutaneous hydrogen bubbles. Therefore, the high degradation rate of Mg-Zn alloys is currently the biggest hurdle to be overcome. In addition, an implant is supposed to have patient-specific 3D profile and shape for best surgical outcomes. Additive manufacturing via SLM is an idea process for these purposes. Until now, studies on SLM of Mg alloys are scarcely reported in the literature due to major issues such as insufficient fusion of powder, the high vapor pressure of Mg and the presence of large pores, which can be attributed to poor process control during laser-powder interaction and melt pool solidification. In this paper, Mg-Zn-Zr (ZK60) magnesium alloy samples were prepared by SLM. The current study aims at (a) studying the processability of ZK60 prepared by SLM via varying the laser processing parameters; (b) exploring the formation mechanisms of pores and illustrating in size and shape in the transition regimes and (c) carrying out a comparative study on the microstructure, microhardness and biodegradation between SLM ZK60 and cast ZK60. The research target is to fabricate more corrosion resistant SLM samples in simulated body fluids.

Section snippets

Materials and specimen preparation

Mg-Zn-Zr alloying powders, namely ZK60, were used as the SLM materials and the chemical composition is shown in Table 1. Particle size distribution of the powders was measured using a laser diffraction particle size instrument (Malvern Mastersizer 2000, UK). The morphology and particle size distribution of the powders are shown in Fig. 1. These powders were spherical in shape as shown in Fig. 1(a) and displayed an approximate Gaussian size distribution with median diameter of ∼51 μm as shown in

Optimization of processing parameters

Fig. 3 shows the macro morphology of the SLM ZK60 samples in the x–y plane and the corresponding laser processing parameters used.

As can be seen from Fig. 3(a), the processability of ZK60 during SLM depended on the laser input energy. Based on the morphology of the SLM samples, five groups of combinations of laser power and scanning velocity could be identified as shown in Fig. 3(b). When the laser processing parameters were located in Region I, smoke were observed during SLM and the sample

Conclusions

In this work, different combinations of laser processing parameters were employed to prepared SLM ZK60 magnesium alloy samples, aiming at fabricating samples with minimal defects and improved dimensional accuracy. Furthermore, the formation mechanisms of pores were identified in different processing regimes. The corrosion behavior of SLM ZK 60 samples in Hanks’ solution was compared with the cast samples. The main conclusions as are follows:

  • (1)

    The quality of SLM ZK60 directly depended on laser

CRediT authorship contribution statement

C.L. Wu: Methodology, Investigation, Data curation, Writing - original draft. Wei Zai: Methodology, Investigation. H.C. Man: Conceptualization, Funding acquisition, Methodology, Formal analysis, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region (152131/18E). Support from the infrastructure of The Hong Kong Polytechnic University is also acknowledged. The authors are thankful to the help of C.F. Yuen and C.H. Ngai from U3DP of The Hong Kong Polytechnic University.

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