PSI - Issue 23

Ivo Černý et al. / Procedia Structural Integrity 23 (2019) 493 – 498 Ivo Černy / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Laser hard overlaying is an advanced, very perspective technology with potentially wide industrial applications, one of them being overlaying to improve durability of forms and dies. In general, ASTM has defined additive manufacturing as “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. Synonyms: additive fabrication, additive processes, additive techniques, additive layer manufacturing, layer manufactur ing, and freeform fabrication”. The aim of the laser hard overlaying is to improve surface properties like wear resistance using special layers of powder sintered or remelted by laser beam. Other possible applications are repairs of worn dies or shape changes. By now, dies are manufactured by machining with following bulk heat treatment, which is quite an expensive process particularly due to use of expensive high-alloyed tool steels. The following heat treatment of massive and bulky parts, when the price per kilogram of treated pieces is calculated, is expensive. Another problem is to find suppliers with big size furnaces and limited capacity often results in long time dates and terms. At present, repairs are usually performed using arc or plasma welding. All these processes are made manually and depend on welders experience and skill. Another problem is a big amount of heat introduced to the part, which can cause dimension changes and material degradation. Weld deposits are not consistent, with high amount of material of the electrode and so further machining and treatment have to follow. These methods often fail also due to low weldability of the materials. An advantage of laser overlaying is minimization of the above mentioned difficulties. Overall heat introduced to the part is small, dimension changes, stresses are minimized. The process is robotised so requirements for additional machining are minimum. Laser, as a non-contact and high energy density source of heating, may be applied for material deposition or repairing of tools by melting incoming stream of powder, wire, rod, or strip and subsequently applying the molten materials on to the target layer by layer – Telasang et al. (2015), Steen and Watkins (2003). This laser-assisted manufacturing process is called laser cladding, as described in Steen and Watkins (2003) and Sun et al. (2014). This technology is being intensively investigated and quite a lot of information can be already found in the literature, e.g. Sun et al. (2014), Costa et al. (2009), Pleterski et al. (2011), Sun et al. (2001), Capello et al. (2005). The crucial property of the laser technology is that laser provides a localized and controlled heat input and reduces scope of distortion or cracking during material deposition. The paper contains a comprehensive evaluation of several types of hard overlayed powder of H13 tool steel on a S355 structural steel using laser beam. Properties like macro- and microstructure are evaluated, then basic fatigue resistance and fatigue damage mechanisms. Results are completed with basic measurement of residual stresses using destructive strain-gauge methods. Base material (BM) of the first experimental plate was a common structural steel S355. Several configurations of layers of a H13 tool steel, in the powder form, were welded on the surface of a plate of the thickness 20 mm. Laser additive welding was performed in all cases under argon protective atmosphere. Laser beam power was 4.5 kW, surface speed of the beam was 0.5 m/min. Three different configurations of the additive welded layers were prepared. The first one was a single layer – the upper layer in Fig. 1. The next configuration of additive laser weld consisted of five tracks, partially overlapping each other – Fig. 1, central part. The last configuration also contained five additive welded tracks with the same overlapping with the difference that another same layer was welded over the first weld – Fig. 1, bottom part. The experimental programme contained the following steps:  macrostructural analyses of the laser additive welds,  microstructural analyses of base material, additive welds and heat affected zones,  mechanical properties of the welds within the meaning of hardness and its course through the welds and  high-cycle fatigue test including evaluation of endurance limit. 2. Experimental Programme