The effect of heat treatment on mechanical properties and microstructure of additively manufactured components

Components from maraging steel MS1 (1.2709) and aluminium alloy AlSi10Mg were fabricated by additive manufacturing (AM). As-built and heat-treated components were analysed in terms of mechanical properties and microstructure. The heat treatment of the material MS1 was performed by a solution annealing at 850°C for 1 hour, ageing temperature at 480°C for 3 hours and air cooling. The heat treatment of the AlSi10Mg was performed by stress relieve at 300°C for 2 hours. Ostbayerische Technische Hochschule Amberg-Weiden carried out tensile tests of as-built and heat-treated samples. Samples were afterwards analysed in COMTES FHT a.s. for further tests of porosity and microstructure using optical and scanning electron microscopy. The aim of the present work is to show the change of mechanical properties and the microstructure with a heat treatment after AM.


1
Introduction Additive manufacturing (AM) is technology, which builds up 3D objects for example of plastic, metal or concrete. Using AM technology, three dimensional parts are fabricated directly from CAD models and built in a layer-by-layer manner. AM technology allows freeform fabrication of geometrically complex parts without special fixtures as required in material removal processes. The term AM includes many technologies which cover: 3D Printing, Rapid Prototyping (RP), Direct Digital Manufacturing (DDM), layered manufacturing and additive fabrication. AM is being used to fabricate end-use products in aircraft, dental restorations, medical implants, automobiles, and even fashion products [1]. Currently, the most used alloys for additive manufacturing are stainless steels (e.g. GP1, 316L, 17-4PH), cobalt chrome (e.g. MP1, SP2), Inconel 625 and 718, titanium Ti6Al4V, maraging steel MS1 and aluminium AlSi10Mg [2].
This work presents the effect of heat treatment on mechanical properties and microstructure of maraging steel MS1 and aluminium AlSi10Mg parts additively manufactured from powders produced by EOS. The change of yield strength (YS), ultimate tensile strength (UTS), elongation (El) and hardness will be shown in this work. Light microscope and the scanning electron microscopy will be used for documenting fractography, microstructure and porosity change after heat treatment.
Maraging steel MS1 (1.2709) studied in this work is martensitic hardened steel, with excellent strength combined with high toughness, very good mechanical properties, and easily heat treatability Aluminium alloy AlSi10Mg is a typical casting alloy with good casting properties and is typically used for cast parts with thin walls and complex geometry. In the frame of aluminium alloys family, AlSi10Mg offers good strength, hardness and dynamic properties and is therefore also used for parts subjected to high loads. The nominal composition is shown in Table 2 [2]. 2 Experimental Procedure The maraging steel MS1 and AlSi10Mg had different processing history. Samples of tested materials were annealed (HT) or they stayed untreated (As-built). Table 3 summarized the heat treatment conditions. As-built and heat-treated components were analysed in terms of mechanical properties and microstructure.

Sample
Heat treatment condition

Mechanical properties
Ostbayerische Technische Hochschule Amberg-Weiden carried out tensile tests of as-built (AB) and heat-treated (HT) samples. They tested 3 AB samples and 3 HT samples of each material extracted in a horizontal direction. Average mechanical properties such as yield stress (YS), ultimate tensile strength (UTS), elongation (El) and hardness with HV5 load are summarized in Table 4 and shown in Figure 1.

Fractography
After the tensile test, a fractographic analysis was performed on AB and HT samples using the scanning electron microscopy (SEM). Although yield stress and ultimate tensile strength of the maraging steel MS1 increased after heat treatment, ductile fracture with dimple morphology was proved in both conditions. Also, AlSi10Mg as-built and heat treated samples have a similar ductile fracture in both conditions with high porosity content (see red arrows in Figure 2)  4 maraging steel MS1 exhibit very low porosity. Samples from AlSi10Mg exhibit porosity close to 5% for untreated samples and about 3.5% for samples after the heat treatment. Table 5 shows the values of porosity of all tested samples. For the microstructure analysis, the samples were cut by the scheme shown in Figure 3 in three directions. Planes XZ and YZ were taken along the building axis (axis Z) whereas XY has a normal parallel to the building axis. Samples from each plane underwent standard metallographic preparation consisted of grinding followed by polishing. The microstructure of the maraging steel was revealed by etching in the Vilella-Bain etchant and Picral 4%. Aluminium alloy AlSi10Mg was etched with Keller´s reagent. The metallographic observation was performed by means of a light microscope and scanning electron microscope.  (Figure 6 b). Figure 6 c) shows the maraging steel MS1 cellular dendritic structure with different grain shapes, which is due to the solidification rate of a local melted region. Figure 7 demonstrates the change of microstructure after the heat treatment when the melt pools boundaries disappeared and the carbides are precipitated along the grain boundaries as was proved by the electron microscopy [3,4,5].  (Figure 8 a).