Although the concept of additive manufacturing has been proposed for several decades, momentum in the area of selective laser melting (SLM) is finally starting to build. In SLM, density and surface roughness, as the important quality indexes of SLMed parts, are dependent on the processing parameters. However, there are few studies on their collaborative optimization during SLM to obtain high relative density and low surface roughness simultaneously in the literature. In this work, the response surface method was adopted to study the influences of different processing parameters (laser power, scanning speed and hatch space) on density and surface roughness of 316L stainless steel parts fabricated by SLM. A statistical relationship model between processing parameters and manufacturing quality is established. A multi-objective collaborative optimization strategy considering both density and surface roughness is proposed. The experimental results show that the main effects of processing parameters on the density and surface roughness are similar. We observed that the laser power and scanning speed significantly affected the above objective quality, but the influence of the hatch spacing was comparatively low. Based on the above optimization, 316L stainless steel parts with excellent surface roughness and relative density can be obtained by SLM with optimized processing parameters.

As novel engineered materials, metamaterials have properties that are not found in naturally occurring materials. These unique mechanical and/or electromagnetic properties are different from the properties of naturally occurring singlecrystalline materials such as 2D transition metal compounds (e.g., metal selenides and halides [1] ) and can be realized using assemblies of multiple composite materials such as metals and plastics. Some examples of these materials are hypertonicity materials with directional propagation of light within and on the material surface, [2] electromagnetic bandgap materials, [3,4] bi-isotropic and bianisotropic electromagnetic materials, [5,6] and mechanical cellular materials [7] with low density, high mechanical strength, [8] significant damping, damage tolerance, [9] and negative Poisson's ratio (PR). [10] Due to their advanced engineered properties, metamaterials can potentially enable many improved engineering products and applications such as high-performance antennas, acoustic control, and structural stability enhancement [11,12] that cannot be realized using naturally existing materials. In particular, auxetic materials display negative PR during tension/compression and can be used to increase the shear modulus, [13] damping effect, sound bandwidth control, [14] structural flexibility, [15,16] and energy absorption effect. [17,18] Their special enhancement effects make them useful for structural protection, [19] wearing comfort, sensing, fastening, and sealing applications. [11] It is clear that the positive/negative thermal expansion coefficient [20] and positive/ negative PR [21] in the same origami pattern are related during the material deformation under temperature variation. Solid materials with a tunable PR are desirable for engineering applications including tunable acoustics, vibration control, and soft robotics. [22] Previous studies have developed different forms of 2D and 3D metamaterials such as dichalcogenides, [23] foams, [24] and origami structures, [21,25] with a large tuning range of negative PR [23] and high average recoverability from compression by more than 30%. [26] For biomedical applications, tissue engineering scaffolds with positive/negative PR can be tailored to match the attributes of the target tissue [27,28] and give rise to deformations that are synchronized with the beating of the heart. [29] Recently, a design framework integrating topology optimization, parametric design, and compression was developed for the gradually stiffer mechanical metamaterials. [30] Many designs have been proposed to realize auxetic cellular materials, [11] such as 2D re-entrant honeycombs composite, [1,31,32]

A fan-shaped laser beam scanning measuring system based on the triangulation principle, was newly developed in this study to measure simultaneously the flatness and the surface roughness of a surface plate with good reflectivity after a suitable calibration. The developed system mainly consists of a micro-focus fan-shaped line laser, an image processing system, a PC-based 3-axis machine, and a PC. The height value of the object can be calculated, based on the offset between the position projected onto the object and the position projected onto the platform, after carrying out the height calibration with gauge blocks. According to the scattering theorem, a correlation between the surface roughness of the test object measured with a precision stylus instrument and the average fringe width projected onto the test object surface, has been constructed.

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Although the concept of additive manufacturing has been proposed for several decades, momentum of selective laser melting (SLM) is finally starting to build. In SLM, density and surface roughness, as the important quality indexes of SLMed parts, are dependent on the processing parameters. However, there are few studies on their collaborative optimization in SLM to obtain high relative density and low surface roughness simultaneously in the previous literature. In this work, the response surface method was adopted to study the influences of different processing parameters (laser power, scanning speed and hatch space) on density and surface roughness of 316L stainless steel parts fabricated by SLM. The statistical relationship model between processing parameters and manufacturing quality is established. A multi-objective collaborative optimization strategy considering both density and surface roughness is proposed. The experimental results show that the main effects of processing parameters on the density and surface roughness are similar. It is noted that the effects of the laser power and scanning speed on the above objective quality show highly significant, while hatch space behaves an insignificant impact. Based on the above optimization, 316L stainless steel parts with excellent surface roughness and relative density can be obtained by SLM with optimized processing parameters.