Relation of microstructure, microhardness and underlying thermodynamics in molten pools of laser melting deposition processed TiC/Inconel 625 composites

Highlights

• Microstructure evolution in the molten pool of LMD-processed samples was disclosed.

• Varied microhardnesses in different zones of the molten pool were found.

• Mechanisms of microstructure evolution and microhardness variety were investigated.

• Relation of microstructure, hardness and underlying thermodynamics was explained.

Abstract

Laser melting deposition (LMD) was applied to deposit nano-TiC particles reinforced Inconel 625 composite parts. The mechanisms of microstructure evolution and microhardness distinction in the different zones of the individual molten pool which was produced in LMD-processed composites were investigated. The layer-wise microstructural features of the manufactured parts were generally observed with clear outline curves of the molten pool as a result of the layer-by-layer deposition manner of the LMD shaping process. It could be observed that the microstructures in the upper part of the molten pool were mainly cellular structures, whereas which in the bottom and edge region were predominantly columnar dendrites. The increasing ratio of the temperature gradient to the solidification velocity (G/R), which resulted in a gradual change from columnar dendrite growth to cellular grain growth in the solidification regime, accounted for this phenomenon. The different sizes of cellular grains and dendrite spacing were ascribed to the varied cooling rates of diverse regions in the molten pool as well as the heat affecting nearby the overlapping zone. The factors contributing to microhardness variety could be summed up in three aspects, which were sizes of grains, TiC reinforcing particles and solid solution strengthening.

Introduction

Inconel series alloys, as a precipitation or/and solid-solution strengthened Ni-based superalloy, are featured by superior combination properties, causing them to become attractive choices for diverse industrial applications. Due to its improved properties such as high temperature strength, high creep strength, excellent fatigue strength, good oxidation and corrosion resistance, and available processibility, Inconel is used broadly in aerospace, marine, chemical, and petrochemical industries [1]. However, the demands on higher performance nickel based superalloy are increasing with the development of aviation and aerospace in recent years. Previous investigations show that the metal matrix composites (MMC) produced by addition of ceramic particles give a great potential to improve not only hardness, but also high temperature mechanical properties compared with matrix alloys [2], [3]. The TiC particle, which is refractory compound, possesses a high thermal-stability during metallurgy process with high hardness, outstanding wear resistance, and hence being used as a promising reinforcement particle [4], [5]. Thus, the incorporation of TiC particles into the Inconel 625 can acquire even ameliorative comprehensive properties, especially high-temperature properties. The previous researches have proved that when the size of reinforcing particle to the nanometer range, the particles reinforced composites display even better mechanical [6]. Therefore, nano-sized TiC particles were incorporated into the Inconel 625 matrix as reinforcements in this paper. However, the precision machining of Inconel 625 based composite is still a challenge, which hinders the further applications in industry.

Laser melting deposition (LMD) is one of newly developed additive manufacturing (AM) processes which are particularly applicable to hard-to-machine metallic materials. LMD, based on coaxial powder-delivery as well as layer-by-layer deposition mechanisms, is an advanced computer-aided additive manufacture technology to fabricate the near-net shape products directly from a 3D CAD (computer aided design) model, by the combined use of the rapid prototyping and laser cladding [7]. In recent years, the researches for LMD processing technology have approved that it has promising application foreground in numerous sectors such as space, aeronautical, automotive,, mechanical engineering, and medical, as a result of its no tooling, high material utilization ratio, no restriction to component size and geometries, significantly reduced product time and cost with respect to traditional techniques. Moreover, the mechanical properties of the LMD parts are significantly improved with obvious characteristics of high strengths and anisotropy combined with slightly lower ductility due to the rapid solidification process, with respect to the components fabricated by conventional manufacturing methods such as casting and forging. The microstructures as well as mechanical properties of laser melting deposited Inconel 625 matrix composite components have been extensively investigated. Establishing a feasible microstructure control method for laser melting deposition additive manufactured TiC/Inconel 625 components is still a challenging question and needs more fundamental understandings on thermal and solidification behaviors during layer-by-layer laser manufacturing process.

Due to the layer by layer additive nature of LMD, the complex thermal histories are experienced repeatedly in different regions of a local molten pool. The thermal histories of LMD normally involve melting and numerous reheating cycles. Besides, a series of complex physical phenomena including heat transfer, fluid flow and so on are involved in the molten pool during LMD, which lead to a complicated heat behavior of the molten pool [8], [9]. Such complicated thermal histories and thermal behaviors during LMD result in the phenomenon of the different parts in a local molten pool possessing the diverse microstructural features, which can be explained through the theory of solidification. As a layered built-up manufacturing process, the solidification nucleation and growth behaviors of the local melt-pool have profound effect on grain structures of the final deposited components. To the best of the authors' knowledge, few previous researches paid attention to the microstructure and mechanical property evolutions of the different locations in a local molten pool produced in the LMD-processed TiC/Inconel 625 composite although some researchers have recognized the microstructure changes among the different layers of laser additive manufacturing parts.

In the present paper, in order to provide the foundation for analyzing solidification microstructure evolutions of the different locations in an individual molten pool which was produced in LMD-processed TiC/Inconel 625 composite, the solidification nucleation and growth mechanisms of the local molten pool were investigate. In addition, the mechanisms of microhardness distinction in the different zones of the individual molten pool and the relationship between microstructure and microhardness were researched in this paper.