Effects of dispersion technique and component ratio on densification and microstructure of multi-component Cu-based metal powder in direct laser sintering

Abstract

A multi-component Cu-based metal powder, which consisted of a mixture of Cu, CuSn, and CuP, was developed for direct metal laser sintering (DMLS). The effects of powder characteristics such as particle shape, particle size and its distribution, and dispersion uniformity on the sintering behavior were studied. It is found that using a homogeneous powder mixture produced by ball mixing coarse and fine powders with a broad particle size distribution could increase the original density of the loose powder and, thus, the densification and microstructural homogeneity of the laser sintered powder. The influence of the binder (CuSn) content on the densification and the resultant microstructures of the laser sintered samples were also investigated. It shows that with increasing the amount of the binder, the microstructure became denser. However, at a high content larger than 50 wt.%, the densification showed a decrease, because of the “balling” effect.

Introduction

As an important branch of rapid prototyping (RP), direct metal laser sintering (DMLS) possesses the capability of fabricating complex shaped three-dimensional (3D) parts directly from metal powder with minimal or no post-processing requirements such as furnace densification cycles or secondary infiltration steps [1]. The combination of high design flexibility, excellent process capabilities, and time- and cost-saving features makes this technique more attractive to industrial manufacturers [2]. Up to now, DMLS has been widely used to create fully dense, durable, and functional metal parts for both prototype and production applications and to build high quality tools with complex feature details for injection molding and die casting applications [3], [4].

Generally, the metal powder systems that have been investigated for DMLS can be classified into three main categories: single-component powder, pre-alloyed powder, and multi-component powder [5], [6]. Early experiments on laser sintering of the single-component powder such as Ni, Cu, Pb, Sn, and Zn [5], [7] have demonstrated that it was unsuitable for DMLS. Since DMLS is characterized by the extremely short time of laser-powder interaction, typically between 0.5 and 25 ms on any powder particle [5], it can only be realized by liquid phase sintering. For the single-component powder, the liquid phase presents owing to the surface melting of particles, and, accordingly, the powder is sintered by joining of non-melted cores of particles via liquid “bridge”. Thus, the requirements for adjusting laser processing parameters are extremely strict in order to ensure not complete but merely surface melting of particles [6]. However, “balling” phenomena are inevitable in this instance due to higher viscosity and surface tension effects, resulting in a large amount of porosity in the sintered structure [7].

To alleviate “balling” effects and improve sinterability, a multi-phase powder approach has been designed by using a pre-alloyed powder system in which melting occurs over a temperature range, or a powder mixture of various components with different melting points [5]. In the case of pre-alloyed powder, the liquid phase arises when the sintering temperature is between the solidus and liquidus temperatures, and the resulting mixture of solid and liquid phases leads to super-solidus sintering [8]. In the case of multi-component powder, melting one of the components with lower melting point (so-called the binder) forms the liquid phase, and, subsequently, this liquid phase bonds the component with higher melting point (so-called the structural metal) [7]. So far, considerable research efforts have been focused on direct laser sintering of the pre-alloyed powder and the multi-component metal powder. Pre-alloyed powder systems that have been investigated include Ti6Al4V [1], bronze [5], [9], and steels (stainless steel [10], [11], high speed steel [12], [13], low carbon steel [14], and tool steel [15], [16]). Multi-component metal powder systems such as Fe–Cu [6], iron–graphite [17], [18], Fe–C–Cu–Mo–Ni [19], Fe–Ni–Cu–P [20], Ni–bronze–CuP [4], [5], Ni–alloy–Cu [6], and Cu–Sn [7], Cu–SCuP [3], [8], [21] have also been studied. Most research work focuses on developing feasible materials and investigating fundamentals of the laser sintering process such as the microstructural evolution, the sintering mechanism, and the influence of process parameters [8], [18], [19]. It has already been understood that the densification mechanism and the attendant microstructural features of the laser processed material depend on both powder characteristics (particle shape, particle size and its distribution, loose packing density, etc.) and laser processing parameters (spot size, laser power, scan speed, scan line spacing, etc.). However, although a wide variety of metal powder systems are currently being investigated, very few materials exclusively for DMLS have been commercially available [3]. Furthermore, due to the complicated nature of DMLS, in which multiple modes of mass, heat and momentum transportation, and, in some cases, chemical reactions might occur, not much previous work has been reported on the basic principles of this process [18]. Common problems associated with DMLS such as “balling” effect, curling deformation, low sintered density, weak strength, and high surface roughness still exist.

Copper and copper alloys are widely used materials owing to their excellent thermal and electrical conductivities, outstanding mechanical properties, ease of material processing, and low cost [8], [21]. In this paper, experimental investigations on direct laser sintering of a self-developed multi-component Cu-based metal powder were carried out, and the effects of powder dispersion technique and component ratio on the densification and the resultant microstructural characteristics of the laser sintered powder were investigated.