**1. Introduction**

As a rapid prototyping (RP) technique, selective laser melting (SLM) is a new technology and widely used in aerospace, automobile manufacturing, medical applications, industrial product design, architectural design, entertainment products, biotechnology, and other industrial fields [1,2]. SLM is an additive manufacturing technology and the principle is the discrete stacking which uses the high-energy laser beam to melt metal powder. The parts can be quickly formed without tools, fixtures, and molds with the advantages of short production cycle and high material utilization rate by using SLM [3–5].

Nonhorizontal suspension structure is the most basic and common structure encountered in the forming process, and is also the biggest geometric problem of forming in SLM experiments. Due to the inherent defects such as warpage, suspension, and sticky powder during processing, SLM cannot form parts with high quality and high precision. Therefore, if SLM can form overhanging structures

with good quality, the technology will be improved and even promote the large-scale popularization and application of SLM. The support of forming is necessary and ensures the stability of the structure during the forming of suspension structures. At the same time, the excess heat will be transmitted by the support to prevent the structure from warping and deforming. However, the additional support will increase the time of processing, so it is necessary to investigate the strategy of forming suspension structures without support.

Kruth et al. [6] found that the forming level or near-level hanging surface can only be accurately formed by adding support. He [6,7] also proposed that adding monitoring and feedback devices in the optical system can flexibly change the laser power and improve the forming quality of the hanging surface. Yasa et al. [8] increased the feedback control of the forming process, laser surface remelting, laser etching, and other postprocessing methods to improve the forming effect of the suspension structure. From the microlevel view, the forming layer of the underside overhanging surface is a powder rather than metal entity, resulting in heat conduction, micromelting, adhesion, and other behaviors which are different from the solid structure. The behavior change of the molten pool can directly reflect the basic principle of SLM forming on the hanging surface. Therefore, many experts devote themselves to the forming mechanism of the microlevel suspension structure, and have obtained many achievements. Lott et al. [9] preliminarily expounded the dynamic behavior of the molten pool. Khan et al. [10] numerically simulated the molten pool instability in SLM forming, and concluded that the area of molten pool depended on the boundary conditions. Alkahari et al. [11] studied the molten pool behavior in the first-layer forming process under different laser powers, scanning speeds, and layer thicknesses by single-layer scanning experience.

The metal powder undergoes rapid melting, rapid cooling, and solidification during the SLM forming process due to the laser directly acting on the surface of the metal powder. The defects such as spheroidization, pores, cracks, dross, over-burning, warping, and so on are also the result of the varying of fast heat and rapid cooling. Some postprocessing methods (such as heat treatment, sanding, etc.) can improve the surface quality of the formed parts, but they are time-consuming and laborious and have limited effect, and for some complex structures (such as overhanging structures, hollow porous structures, etc.), it cannot be fundamentally solved by postprocessing methods. To eliminate the defect, it is necessary to decrease or even avoid the defect by adjusting the process parameters and other prior processes. Therefore, when forming parts such as a suspended structure or a complicated curved surface, a good forming effect should be obtained from the aspects of process parameters, scanning strategy, support added or not, and so on. In this study, the energy of the laser input was strictly controlled, and the process parameters, scanning strategy, support addition method, and auxiliary optimization process were changed. At the same time, the substrate was preheated by 100 ◦C to reduce the influence of temperature difference on the forming, and the optimal suspension structure was obtained with a good surface quality and high precision.

## **2. Materials and Methods**

#### *2.1. Materials and Experimental Setup*

The experiment was carried out on the Renishaw AM400 (Renishaw plc, London, UK). The AM400 uses a continuous laser mode with a maximum power of 400 W and a 1075 nm Nd: YAG laser with a laser beam diameter of 70 μm. The working area (shown in Figure 1) can be machined to a maximum volume of 250 mm × 250 mm × 300 mm and provides a closed environment filled with argon as a shielding gas to maintain oxygen concentrations below 200 ppm. The experimental scanning strategy is based on the meander scan strategy, a schematic of which is shown in Figure 2; it can be seen that the angle between the Nth layer and the N+1th layer is 67◦,where *d* is the point distance, *δ* is the hatch space, and *Φ* is the spot diameter.

There were two materials used in the experiment: one was the 316L stainless steel and the other was the chrome–nickel alloy steel. The 316L stainless steel had a particle diameter ranging from 5 μm

to 41 μm and the average particle diameter was 17 μm. The chemical composition of the 316L stainless steel powder was Fe (Balance), Cr (16% to 18%), Ni (10% to 14%), Mo (2% to 3%), Mn (2% max), Si (1% max), N (0.1% max), O (0.1% max), P (0.045% max), C (0.03% max), S (0.03% max). The chrome–nickel alloy steel powder had an extremely high sphericity and a particle diameter ranging from 17 μm to 58 μm, and the average particle diameter was 31 μm. The chromium–nickel alloy steel included Fe (Balance), Cr (1% max), Mn (1% max), Ni (1% max), Mo (0.15% max), C (0.228% max), N (0.228% max). Electron microscopy (SEM) photographs and compositional contents are shown in Figure 3.

**Figure 1.** The equipment workspace of Renishaw AM400.

**Figure 2.** The schematic of forming experiment piece.

**Figure 3.** Powder characteristics and composition content: (**a**) morphology of 316L stainless steel powder; (**b**) composition content of 316L power; (**c**) morphology of chrome–nickel alloy steel powder; (**d**) composition content of chrome–nickel alloy steel.
