A survey on mechanisms and critical parameters on solidification of selective laser melting during fabrication of Ti-6Al-4V prosthetic acetabular cup

Highlights

• The main factors to interpret solidification on SLM are capillarity, the geometry of intended part, cooling rate, undercooling and process parameters.

• Equilibrium temperature, Monge patch, Interfacial and Gibbs energy describe the solidification mechanism.

• The main key points to explain intricate solidification behaviour are, trapping gas under the powder, unstable melting pool due to scanning speed, high cooling rate, splashing of melted powders on melting pool, contamination of raw powder.

Abstract

Additive Manufacturing (AM) includes a range of approaches that correlate with computer aided design (CAD) and manufacturing by fabrication via precise layers and is a promising method for the production of medical tools. In this study, different aspects and mechanisms of solidification for curved surfaces based on equilibrium at curved interfaces, Monge patch, interfacial and Gibbs energy will be discussed. Also, the effect of capillarity, geometry, substrate temperature, cooling rate and scanning parameters in the solidification of a prosthetic acetabular cup (PAC) using selective laser melting (SLM) is analysed. The contributions of this work are analysing solidification and effective factors in this process to produce parts with a higher quality and mechanical properties such as strength, strain, porosity, relative density and hardness. Results indicate that due to the surface to volume (S/V) ratio, and the increasing effect of the radius on Monge patch, thermal stresses and surface forces are more prevalent on outer surfaces. Moreover, solidification and mechanical properties are related to capillarity, geometry, substrate temperature, cooling rate, scanning power and speed. The results also indicate the interaction of solute diffusion and heat transfer with interatomic forces in large S/V ratio and at small scales tend to improve solidification.

Introduction

Additive manufacturing (AM) is the engineering name for what is commonly termed as 3D printing. The term “3D printing” is used in various applications and industries to explain a rapid fabrication process which is moving directly from computer-aided design (CAD) to computer aided manufacturing (CAM) [1]. In AM, artefacts are fabricated by adding thin layers of material and in this way complex and intricate shapes can be produced in one production process. AM has high applications in a variety of industries such as aerospace, automotive, manufacturing, architectural, micro-prototyping and so on [2]. With polymers, long-chain molecules are formed primarily by carbon-to-carbon bonds and these are mostly using to fabricate parts in AM. However, the number of parts produced using metal AM techniques is increasing dramatically due to recent improvements in mechanical properties [3], [4].

AM is a highly suitable method of producing medical parts such as implants, complex scaffolds and even forensics and, as a result, the overall quality of medical services have improved [5], [6]. For a better understanding of medical issues surgeons and doctors might prefer to have a physical model instead of looking images on the screens. AM has the potential to produce parts directly from CT and MRI images. Likewise, another significant effect of AM is in speeding up the production procedure and thus decreasing waiting and surgery time per procedure and during care regimes [7], [8]. The prosthetic hip roundness and roughness of the cup and femoral head must lie within the range of 0.9–7.3 μm and 0.02–0.036 μm respectively. The wear rate in a hip joint is highly related to clearance, surface roughness and sphericity of the femoral head and acetabular shell [10].

SLM is one of the emerging and promising methods of AM that has found usages in widespread industries, such as medical fields, biofabrication, implants, fabrication of skull and soft tissue, prosthetic knee and hip in recent years [11], [12], [13]. Producing parts using SLM results in both a variable microstructure and mechanical properties [14], [15], [16], [17]. Studies on the microstructure and mechanical properties of Ti alloys produced by electron beam melting and SLM showed increases in hardness and tensile strength compared to conventional wrought and cast products. The reason was associated with epitaxial growth and production of (α′) HCP martensite phase in the process. In the SLM operation, the direction of the grains has a direct correlation with the process parameters. SLM produces homogeneous parts, and it is reported that to increase the homogeneity, increasing laser power would be beneficial [18], [19], [20], [21]. Also, adding 10 wt% Mo to Ti-6Al-4V resulted in a novel microstructure consisting of a titanium matrix with Mo participates. Heat treatment for the mentioned alloys and their substrate at transus temperature showed that evaluation of tensile properties compares to wrought production and better elongation properties [22], [23].

The geometry of crystal-melt interfaces affects the SLM solidification process and crystal growth. Furthermore, the high melting (2436 °C) and solidification temperatures (1616 °C) and heating/cooling rate make it difficult to predict the behaviour of material during and after solidification especially for complex parts [24]. Solidification of the melt pool, residual properties and layer distribution state are dependent on the part geometry, part orientation and SLM settings. In SLM, thermal shrinkage occurs immediately after melting powders and the value of shrinkage increases as a function of increasing power and decreasing scanning speed [25]. Enriching hydrogen in the melt pool during solidification of SLM parts increases the chance of generation of pores if the local hydrogen value is higher than the maximum solubility in the liquid phase. Losing density and reduction of tensile stress are the results of pores in solidification of SLM [26]. Li et al. [27] studied the effect of substrate temperature on final solidification and microstructure of Al–Ni–Y–Co–La and found that the interface bond between metallic glass directly related to substrate temperature. This phenomenon can be related to different cooling rate (on account of substrate temperature) in the solidification process. Investigation on melt flow and solidification on SLM Ti–6Al–4V was performed by Qiu et al. [28] showing that increasing process power and decreasing layer thickness and scanning speed resulted in surfaces with the lower pores. Pores are ascribed to unstable melt flow and splashing of melted powders and described as one of the reasons for rough surfaces in as-built prototypes. Also, scanning speed was reported as one of the most important parameters in solidification of Ti alloys in SLM and mechanical properties are related to the value of this parameter [29], [30]. High cooling rate and entrapping gas in powder material during solidification of SLM leads to residual stress and the formation of cracks and pores and is a well-known problem in SLM [31]. Attar et al. [30] investigated solidification of TiB in SLM. Their research showed that solidification enrichment by elements increased constitutional supercooling leading to unstable solid-liquid phase and increased driving forces for nucleation of fine grains. Finer grains are also ascribed to the higher cooling rate in SLM (10³–10⁸ K s^− 1) and rejecting B from solid-liquid that resulted in Ti nucleation restriction. The significant key differences in printing near net shape and simple parts can be related to the geometry and subsequently thermal stresses. Another important factor is unstable surfaces that due to a combination of weight and dynamic shear force between particles in solid-melt phase, surfaces with lower than 45° (between building surface and horizontal plane) need support to increase consolidation. Micromotion of particles due to gravity force in lateral surfaces in complex parts has the effect on cooling procedure, so the number of particles remaining on these surfaces is higher than the flat (simple prototypes) surface [32], [33].

As a technology under development, the intricate solidification processes for producing SLM prototypes with complex shapes are not yet fully understood. In this paper, to characterize the solidification of PAC produced by SLM, an analytical solution based on equilibrium at curved interfaces, Monge patch, interfacial and Gibbs energy has been developed. Then the effective parameters such as capillarity, geometry, substrate temperature, scanning parameters and cooling rate on solidification process will be analysed.