1. Introduction

As a kind of advanced manufacturing technology, additive manufacturing (AM) provides an effective and 'bottom up' manufacturing where a complex structure can be built into its designed shape by a 'layer-by-layer' approach, which can directly create geometric metal parts. AM is versatile, flexible, highly customizable and, as such, can suite most sectors of industrial production [1]. Even though metal additive manufacturing involves creating parts layer-by-layer, there are many different types, including material extrusion, material jetting, material droplet printing, binder jetting, sheet lamination, powder bed fusion, and directed energy deposition [2]. Most current metal AM systems are of the powder bed fusion type [3]. Due to the complexity of the physical process in the process of metal AM, it is

very difficult to ascertain metal parts with high dimensional accuracy, no defects, small residual stress/deformation, compact microstructure and high mechanical properties [4–8]. At present, defect control and microstructure/composition control are key bottleneck problems that restrict the further development of metal AM technology. Both of these problems are closely related to the energy and mass transport process in deposition process, especially at the solid-liquid interface of molten pool [9–12]. How to understand and control the complex heat and mass transport in the molten pool is the key trend of the current research, and it is also the basis and prerequisite to break through the current technical bottlenecks and further improve the mechanical properties of the parts, such as strength, stiffness and fatigue. Therefore, this paper focuses on the scientific issues of transport phenomena and solidification behavior of molten pool during metal additive manufacturing.
