Effects of processing parameters on consolidation and microstructure of W–Cu components by DMLS

https://doi.org/10.1016/j.jallcom.2008.05.065Get rights and content

Abstract

The densification behavior and attendant microstructural characteristics of direct laser sintered submicron W–Cu/micron Cu powder system under different processing conditions were investigated in this work. The methods for improving the controllability of laser processing were elucidated. A “linear energy density (LED)”, which was defined by the ratio of laser power to scan speed, was used to tailor the powder melting mechanisms. It showed that using a suitable LED between ∼13 and ∼19 kJ/m combined with a scan speed less than 0.06 m/s led to a continuous melting of the Cu component, yielding a sound densification larger than 92% theoretical density without any balling phenomena. With a favorable sintering mechanism prevailed, a proper increase in the LED above ∼13 kJ/m, which was realized by increasing laser power or lowering scan speed, produced a homogeneous microstructure consisting of a novel W-rim/Cu-core structure. Narrowing the scan line spacing to 0.15 mm was able to enhance the inter-track bonding, and to reduce the roughness of laser sintered surface. Decreasing the powder layer thickness to 0.15 mm was a promising approach for improving the inter-layer bonding coherence. A “volumetric energy density (VED)” was defined to facilitate the integrated process control by considering the combined effect of various processing parameters. It was found that setting the VED within ∼0.6 and ∼0.8 kJ/mm3 favored a better yield of high-density sintered parts.

Introduction

Due to a unique combination of high electrical and thermal conductivity of copper and low coefficient of thermal expansion (CTE) and high hardness of tungsten, tungsten–copper (W–Cu) composites have been widely used for thermal and electrical applications such as heavy-duty electronic contacts and heat sink materials for high-power microelectronic devices [1], [2], [3], [4], [5].

W–Cu composites are commonly produced by infiltrating porous sintered W with liquid Cu or powder metallurgy (PM) techniques. However, for the infiltration method, there exist a series of process defects such as porosity, copper lakes, and tungsten agglomerates [6]. Additionally, there is a limit to the copper content of 30 wt.% if using this method [7]. In the case of PM method, a significant technological problem is the resultant porosity, due to the mutual insolubility between W and Cu. Furthermore, almost all PM-processed pieces are required to be pre-worked by some dedicated tools (e.g., moulds or dies) to obtain a desired shape. However, because of the limitation of currently available tools, some complex-shaped W–Cu objects are always difficult to be produced.

To overcome these shortcomings, a new fabrication technique, i.e., direct metal laser sintering (DMLS), has been introduced for the production of W–Cu components [8]. DMLS is a laser-based rapid prototyping (RP) technology that builds objects in a layer-by-layer fashion using a bed of loose powder and a computer controlled laser beam [9], [10], [11], [12], [13], [14], [15], [16]. DMLS, due to its flexibility in materials and shapes, allows complex three-dimensional (3D) W–Cu objects to be produced without any tools.

However, a review of existing literature reveals that not much previous work has focused on the basic principles of the fabrication of W–Cu components using DMLS. Actually, because of the complex nature of DMLS, which involves multiple modes of heat, mass, and momentum transfer [11], [12], process defects associated with DMLS such as balling, curling, and delamination are still difficult to be eliminated completely. It has been found that both powder characteristics (e.g., particle shape, size and its distribution, and component ratio) and processing parameters (e.g., laser power, scan speed, scan line spacing, and powder layer thickness) influence the densification level and the attendant microstructures of DMLS-processed materials [17]. In our previous work [8], we have reported the influence of Cu elemental content on the microstructural evolution and densification response of direct laser sintered W–Cu composites. A sound sintered density accompanied by an interesting W-rim/Cu-core structure has been obtained at a suitable Cu content. Besides the optimization of material characteristics, the possibility of controlling processing parameters in improving the properties of laser sintered W–Cu components is becoming another important consideration. This article presents the sintering mechanisms, surface morphologies, and microstructural features of DMLS-processed W–Cu composites using a wide range of processing parameters, with an aim to elucidate the optimal processing conditions and the relevant control methods.

Section snippets

Powder preparation

Electrolytic 99% purity Cu powder with an irregular shape and a mean particle size of 15 μm and submicron W–20Cu (wt.%) composite powder with an irregular morphology and an average particle size of 0.24 μm were used in the current study. The W–20Cu powder was synthesized via combining spray drying and hydrogen reduction [18]. The two powder components were mixed in a Fritsch Pulverisette 6 planetary mono-mill at a rotation speed of 350 rpm for 60 min, with a ball-to-powder weight ratio of 10:1. A

Mechanisms of powder melting

Fig. 2 depicts the process map regarding the change of mechanisms of single layer melting. Over the entire range of laser powers and scan speeds, the following four process windows can be defined:

  • (I)

    Slight melting. The incident laser energy was great enough to exert an effect on the powder bed, but insufficient to induce any significant melting of the powder.

  • (II)

    Continuous melting. The delivered energy produced continuous molten tracks by means of the complete melting of the Cu component, leading to

Control of melting mechanisms by “linear energy density”

During DMLS, laser scanning is performed line-by-line over the powder bed at a constant speed. The dwelling time of the laser spot at any irradiating region is extremely short, typically less than 4 ms [10]. Due to such a localized nature of the rapid DMLS process, particle bonding must occur speedily, in the order of seconds [21]. Therefore, a sintering mechanism involving a liquid Cu phase is more reasonable for DMLS. Laser sintering of W–Cu system starts by means of the selective melting of

Conclusions

The processing conditions and control methods for direct laser sintering of submicron W–Cu/ micron Cu powder system are investigated, and the following conclusions can be drawn.

  • (1)

    A linear energy density between ∼13 and ∼19 kJ/m combined with a suitable scan speed (<0.06 m/s) leads to a continuous melting of the Cu component, yielding a sound densification level (>92% theoretical density) free of any balling phenomena.

  • (2)

    A proper increase in the linear energy density above ∼13 kJ/m, which is realized by

Acknowledgements

The authors thank the financial support from the National Natural Science Foundation of China (Grant No. 50775113). One of the authors (Dongdong Gu) gratefully appreciates the financial support from the Scientific Research Foundation for Newly Employed Talents in Nanjing University of Aeronautics and Astronautics.

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