Intensive processing optimization of Zn-Cu fabricated by laser powder-bed fusion

Abstract

Laser powder-bed fusion (LPBF) of Zn-0.8Cu (wt.% ) alloys exhibits significant advantages in the customization of biodegradable bone implants. However, the formability of LPBFed Zn alloy is not sufficient due to the spheroidization during the interaction of powder and laser beam, of which the mechanism is still not well understood. In this study, the evolution of morphology and grain structure of the LPBFed Zn-Cu alloy was investigated based on single-track deposition experiments. As the scanning speed increases, the grain structure of a single track of Zn-Cu alloy gradually refines, but the formability deteriorates, leading to the defect’s formation in the subsequent fabrication. The Zn-Cu alloys fabricated by optimum processing parameters exhibit a tensile strength of 157.13 MPa, yield strength of 106.48 MPa and elongation of 14.7%. This work provides a comprehensive understanding of the processing optimization of Zn-Cu alloy, achieving LPBFed Zn-Cu alloy with high density and excellent mechanical properties.

1. Introduction

Biodegradable metals have promising prospects as the major constituent of medical implants. Biodegradable bone implants are expected to be naturally degraded in the human body without subsequent removal surgery, therefore, alleviating the risk of infection. Among the biodegradable metals, Zn has a moderate degradation rate with its standard electrode potential (−0.76 V) lying between Mg (−2.37 V) and Fe (−0.44 V), which is congruent with bone growth [1]. Besides, the degradation by-product of Zn does not contain hydrogen, leading to a more controllable degradation process. Zn ranks sixth among all the life metal elements in the human body [2]. During degradation, the released Zn ions can be absorbed and metabolized by the human body, showing excellent biocompatibility. Several biodegradable Zn alloys have been developed to overcome the lack of mechanical strength of pure Zn, including Mg, Cu, Ca, Li, Sr and Fe [3, 4]. Among them, Cu is the element that has received much attention [5]. Cu is a relatively abundant trace element in the human body, participating in the functioning of the hematological, osteological, and immune systems [6, 7]. Besides, Cu has good antibacterial properties that could irreversibly damage the microbial membrane [8 −10]. Also, Cu can form a dispersed second phase of CuZn5 with Zn, which can simultaneously improve the strength and ductility of the alloy [1].

Nowadays, advanced fabricating techniques are required to fabricate bionic implants with precise structures and customized shapes [11]. Laser powder-bed fusion (LPBF) has shown strong potential in fabricating orthopedic implants because it can precisely customize complex structures in terms of macro and microgeometry [12, 13]. However, due to the low melting and boiling points of Zn, evaporation is inevitable during the high laser energy input. CUNNINGHAM et al [14] found that strong evaporation occurs in the LPBF process, evidenced by the formation of keyholes. The keyhole, also called vapour depression, is essentially a ca