Microstructure and mechanical properties of maraging steel 300 after Selective Laser Melting

Selective laser melting (SLM) is an additive manufacturing process for the direct fabrication of prototypes, tools and functional parts. The process uses a high intensity laser beam to selectively fuse fine metal powder particles together in a layer-wise manner by scanning cross-sections generated from a three-dimensional CAD model. The SLM process is capable of producing near fully dense functional products without almost any geometrical limitation and having mechanical properties comparable to those produced by conventional manufacturing techniques. There is a wide range of materials that are suitable to be processed by SLM including various steels, Ti, Al and CoCr alloys. Being one of these materials, maraging steel 300 (18Ni-300) is an iron-nickel steel alloy which is often used in applications where high fracture toughness and strength are required or where dimensional changes have to remain at a minimal level, e.g. aircraft and aerospace industries for rocket motor castings and landing gear or tooling applications. To achieve its superior strength and hardness, maraging steel, of which the name is derived from ‘martensite aging’, should be treated with an aging heat treatment. In this study, the effect of the SLM parameters (scan speed and layer thickness) on the obtained density, surface quality and hardness of maraging steel 300 parts is investigated. Moreover, various aging heat treatments (different combinations of duration and maximum temperature) are applied on the SLM parts to achieve high hardness values. The mechanical testing of maraging steel 300 specimens produced by SLM and treated with an appropriate aging treatment is accomplished by impact toughness and tensile tests and compared to the results obtained using conventional production techniques. Additionally, the microstructures of as-built and heat treated parts are investigated. Introduction Maraging steels are well known for their high strength, high fracture toughness, good weldability and dimensional stability during aging. Due to this unique combination of several attractive features, maraging steels find extensive use in high performance industrial and engineering parts such as aerospace and motor racing applications. Some of the applications can be listed as rocket motor castings, drill chucks, tools for punching, extrusion, plastics injection moulds and metal casting dies [1]. Maraging steels offer an attractive alternative to the medium to high carbon tool steels since they do not suffer some problems like high carbon content promoting corrosion and quench cracking which may only become evident during service and result in unexpected failure. The low carbon content of maraging steels reduces the risk for quench cracking, while the high nickel content and absence of carbides provides a good corrosion resistance [2]. Selective Laser Melting (SLM) is a promising technology due to almost unlimited geometrical freedom especially for tooling applications. It is possible to produce complex geometries with internal cavities by SLM such as conformal cooling channels. Conformal cooling channels are generally considered to be almost impossible to be built by conventional machining techniques but allow major reduction in injection moulding cycle time [2]. Therefore, SLM of maraging steels can offer new opportunities in tooling applications by producing parts with almost full density and high freedom in geometrical complexity. In this study, SLM of maraging steel 300 is taken under investigation regarding many aspects. In the scope of this study, the change of part density and surface quality was determined while varying the SLM process parameters (scan speed, layer thickness) from the recommended values by the SLM machine vendor. Micro and macro hardness measurements were conducted on the produced specimens to test the effect of those parameters on hardness of as-built parts. Since maraging steels obtain their superior mechanical properties, i.e. high strength and toughness, after an aging heat treatment, different aging conditions were tested to find the relationship between aging conditions (duration and maximum re-heating temperature) and obtained hardness. Before testing this material mechanically (by tensile and Charpy impact testing), the influence of laser re-melting after every layer is studied to reduce the porosity formed during SLM. Experimentation During the experiments, maraging steel 300 from Concept Laser GmbH (commercially named as CL50WS) was used. The test specimens were built from gas atomized powder of specification and size fraction listed in Table 1. Figure 1 provides typical morphology of the tested powders before processing showing that the majority of powder particles bear spherical and near-spherical morphology. This morphology without sharp edges and corners ensures free flow of the powder during layer deposition thereby increasing process efficiency [3]. Table 1: Specification and size ranges of gas atomized maraging steel powder Material DIN No. Material name Size ranges (μm) d50 (μm) bulk density (g/cm 3 ) CL50WS 1.2709 X3NiCoMoTi 18-9-5 25-63 43.7 8.1 Table 2: Experimental processing parameters Material Scan speed (mm/s) Scan spacing (μm) Layer thickness (μm) CL50WS 120-600 125 (a1 = 70% of the spot size % 99 φ ) 30, 40, 50, 60 CL50WS powder was processed on a Concept Laser M3 Linear SLM machine [4] which is equipped with a 100 W Nd:YAG laser and has a laser beam diameter ( % 99 φ ) of about 180 μm at the powder bed surface. Experiments were carried out using the maximum available laser power (~105 W). A wide range of scan speed at various layer thicknesses was examined as given in Table 2. In order to minimize the effect of thermal gradients and thermal induced stresses in the component, island scanning (a patented scan pattern from Concept Laser) was used [5]. In this scanning strategy, part surface at each slice is divided to small square islands of 5 mm × 5 mm. The islands are scanned in a random way while scanning direction is altering a right angle with respect to the neighboring islands. The surface roughness of the as-processed samples was measured on horizontal top surfaces using a contact surface profilometer, Talysurf 120L from Taylor Hobson Ltd. Density was measured according to the Archimedes’ method by weighing the samples in air and subsequently in ethanol. The Archimedes’ method is a simple and fast method to evaluate different processing conditions. However, it is not very suitable to compare the densities of SLM parts having very close results due to pores being sometimes filled with un-melted powder particles. A coating with lacquer was also applied to avoid ethanol absorption in open pores at lower densities (<90%). Density results are presented as the arithmetic means of three measurements at each processing condition and are expressed as relative density by taking materials’ bulk density as 8.1 g/cm 3