Directed energy deposition and characterization of high-carbon high speed steels
Graphical abstract
Introduction
Metal-additive manufacturing has radically transformed the manufacturing industry by additively manufacturing components of desired dimensions as well as of desired material properties for a variety of applications [1], [2], [3]. Directed energy deposition (DED), is a laser based additive manufacturing process which uses metal powders to deposit layers of enhanced properties onto a given substrate with simultaneous laser irradiation [4]. During the DED, a high power laser beam provides thermal energy to simultaneously melt the substrate and the powder material injected by a single or multiple nozzles; creating a dynamic melt pool on the substrate to produce layer upon solidification [5]. The co-ordinated movement of the laser beam, powder material and the substrate produces a pre-defined layer track pattern and this process repeatedly continues until the fabrication is completed [6], [7]. The quality of an additively manufactured product is strongly influenced by the process parameters like laser power, laser scan speed, laser beam diameter, laser intensity profile, powder mass flow rate, powder size and morphology, pre-heating temperature, carrier and shielding gas flow rates [8]. In comparison to other additive manufacturing processes (such as powder bed systems), DED provides advantages of high throughput and larger build volume [9], [10]. The DED process is recognized in the literature with various terminologies such as laser cladding, direct metal deposition (DMD), direct laser deposition (DLD) and laser metal deposition (LMD) [11].
Repeatability of the DED process (and including all additive manufacturing processes) and the quality of additively manufactured components can be characterized by the microstructural homogeneity and the consistent mechanical properties. High thermal gradients, and repeated rapid heating and rapid cooling induce residual stresses and anisotropy [12]. Microstructural features such as grain size and morphology, and phase transformations are highly sensitive to the dynamic thermal history and directly influence the micro-hardness, tensile strength and fatigue properties [13]. Similarly the presence of residual stresses reduces the strength and fatigue life of the laser-fabricated components [14]. For these reasons, different destructive (Optical microscopy, tensile testing, hole drilling method) [15] and non-destructive (Ultrasonic inspection, X-ray computed tomography (CT), Synchrotron radiation and Neutron diffraction) analysis techniques are adopted to ensure and assess the quality (porosity and micro-cracks free), quantification of the mechanical characteristics and standardization of the additively manufactured parts [16], [17], [18], [19].
High speed steel (HSS) alloys are highly preferred materials in the hot metal forming applications such as hot stamping, extrusion and moulding [20]. The preference for HSS alloys is the result of their superior high temperature service performance (excellent wear resistance and micro-hardness) [21], [22], which is due to the presence of hard metal carbides (MC, M2C, M7C3, M6C and M23C6) and martensite matrix [23]. However, DED of HSS alloys is challenging as these alloys are highly susceptible to cracking due to the presence of brittle metal-carbides and higher micro-hardnesses (≥700 HV), and therefore additive manufacturing is often assisted by pre-heating [24], [25].
In the past, multilayer additive manufacturing of HSS alloys was performed by varying the chemical composition of the carbide forming elements (V, Mo, W and Cr) [26] and laser-metal processing conditions [27]. In addition, a new HSS alloy was additively manufactured by the addition of Co for high temperature applications [28]. The current paper describes continuation of the alloy development process by combining the fabrication advantages of DED. For that purpose, multilayer laser fabrication of two high-carbon high speed steel (HC-HSS) alloys; X1 (Febal-C-Cr-Mo-V) and X2 (Febal−x-C-Cr-Mo-V-Wx) was carried out followed by a comprehensive microstructural and comparative mechanical characterization. Mechanical characterization includes tensile testing and high temperature (500 °C) tribological characterization. Results from the Neutron diffraction of these alloys for residual strain scanning are also discussed.
Section snippets
Experimental setup and processing parameters
The additive manufacturing of the HC-HSS alloys was performed on 42CrMo4 steel cylindrical and flat plate substrates by using a 4 kW fiber equipped cw Nd:YAG laser source (Trumpf). The details of the DED facility is the same as described in our earlier work [27], [28]. To prevent excessive oxidation of the HC-HSS alloys during deposition and solidification, an additional shielding gas nozzle was used along with the powder injection nozzle to shield the melt pool. For deposition on the
Multilayer directed energy deposition
Multilayer additive manufacturing of X1 and X2 was performed to produce samples for microstructural and mechanical characterizations, see Fig. 4. The total maximum deposition thickness of multiple layers was about 5 mm. During the deposition with dragging powder injection, the melt pool was not perfectly stable, resulting in higher porosity and un-melt powder particles [28]. Multilayer additive manufacturing was performed at a pre-heating temperature of 150 ± 10 °C as deposition at room
Discussion
Metal additive manufacturing is extensively explored in the current manufacturing industry for the fabrication of new products. However, variation in the product quality due to inconsistent mechanical properties (micro-hardness, tensile strength and wear), presence of defects and high residual stresses can limit the product use for critical applications such as hot metal forming [36]. For such reasons additively manufactured samples of X1 and X2 HC-HSS alloys were thoroughly characterized in
Conclusions
Multilayer samples of two newly developed HC-HSS alloys, X1 (Febal-C-Cr-Mo-V) and X2 (Febal−x-C-Cr-Mo-V-Wx) have been additively manufactured by the DED process. The detailed microstructural and mechanical characterization of these alloys lead to the following conclusions.
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Micro-hardness measurements of both alloys showed no detrimental effect of repetitive laser thermal cycling. This was due to strengthening of the martensite matrix during secondary hardening. The micro-hardness of both alloys
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgment
This -project is funded by the European Research Fund for Coal and Steel (RFCS) under the grant agreement no. RFSR-CT-2015-00009. Authors are grateful to Thilo Pirling of ILL (Grenoble, France) for assisting in preparation of project proposal for the beam time. Authors are also grateful to Leo Tiemersma and Nick Helthuis from the University of Twente (Enschede, The Netherlands) for assisting in DED trials and chemical etching respectively.
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