Influence of Powder Surface Contamination in the Ni
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metals Article Influence of Powder Surface Contamination in the Ni-Based Superalloy Alloy718 Fabricated by Selective Laser Melting and Hot Isostatic Pressing Yen-Ling Kuo * and Koji Kakehi Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-Shi, Tokyo 192-0397, Japan; [email protected] * Correspondence: [email protected]; Tel./Fax: +81-42-677-2709 Received: 9 May 2017; Accepted: 7 September 2017; Published: 13 September 2017 Abstract: The aim of this study was to gain a deep understanding of the microstructure-mechanical relationship between solid-state sintering and full-melting processes. The IN718 superalloy was fabricated by hot isostatic pressing (HIP) and selective laser melting (SLM). Continuous precipitates were clearly localized along the prior particle boundary (PPB) in the HIP materials, while SLM materials showed a microstructure free of PPB. The mechanical properties of specimens that underwent SLM + solution treatment and aging were comparable to those of conventional wrought specimens both at room temperature and 650 ◦C. However, a drop was observed in the ductility of HIP material at 650 ◦C. The brittle particles along the PPB were found to affect the HIP materials’ creep life and ductility during solid-state sintering. Keywords: superalloy; surface contamination; prior particle boundary; hot isostatic pressing; selective laser melting 1. Introduction IN718 superalloy is widely used in gas turbine and related aerospace applications due to its excellent hot corrosion resistance, good weldability, and high mechanical properties—e.g., creep and fatigue strengths [1–3]. Macro segregation occurs during the conventional ingot metallurgy route due to melt-related problems such as segregation and porosity, which lead to degradation of the mechanical properties of the alloys [4,5]. One solution to this problem was to minimize segregation through rapid solidification of the metal or through the solid-state sintering process. A powder metallurgy technology—i.e., hot isostatic pressing (HIP)—was developed to eliminate segregation and macro-porosity, and to manufacture net shape components with homogeneous microstructure and improved mechanical properties [6]. Moreover, the temperature involved in the HIP process allows segregants to diffuse, thus enhancing the mechanical properties. After this type of solid-state sintering process, special care—i.e., rolling, extrusion and forging—is required to mitigate the detrimental effects of prior particle boundary (PPB) precipitation [7]. The so-called PPB problem results from surface contamination with pre-alloyed powder, and particles such as oxides, carbides, oxy-carbides and oxycarbonitrides are formed along the PPB [8–10]. These precipitates along the PPB are brittle and thus provide an easy fracture path, and so the undesirable effects of PPBs have been focused on because they limit the applications of such consolidated products in HIPed materials [11]. Although many efforts have been made to decrease the effects of PPBs to extend the life of such products in applications in the aerospace industry, no effective and reliable method has yet been determined. Hence, an additive manufacturing technology, selective laser melting (SLM), was used in the current research. The laser beam irradiates metal powders until they fully melt in the SLM process, which has the benefit of causing PPB breakdown. SLM is applicable to components of fully dense bulk material with complex Metals 2017, 7, 367; doi:10.3390/met7090367 www.mdpi.com/journal/metals Metals 2017, 7, 367 2 of 13 geometry [12]. In spite of the fact that the process is capable of making full density (~98–99%) parts, post-processes such as HIP were commonly used to eliminate the small amount of residual porosity for critical applications where high strength and fatigue resistance were necessary [13,14]. The obvious characteristics of the mechanical properties of SLM parts include anisotropy; the reasons for the special characteristics of IN718 built up by SLM—e.g., the high dislocation density, subgrain boundary and a rowMetals of interdendritic2017, 7, 367 δ-phase precipitates—have been explained in our previous2 studies of 13 from several perspectivesdense bulk material [15,16 with]. However,complex geometry research [12]. relatedIn spite of to the the fact surface that the contaminationprocess is capable onof powder in SLM materialsmaking full isdensity limited. (~98–99%) Therefore, parts, post-process the currentes such study as HIP focused were co onmmonly surface used contaminationto eliminate and on the consolidationthe small amount behaviour of residual between porosity SLMfor critical and conventionalapplications where HIP. high In strength order to and gain fatigue an in-depth understandingresistance of were the necessary surface contamination[13,14]. The obvious by char theacteristics powder of boththe mechanical in the SLM properties and of HIP SLM processes, parts include anisotropy; the reasons for the special characteristics of IN718 built up by SLM—e.g., the microstructure-mechanicalthe high dislocation density, relationship subgrain boundary of IN718 and a nickel-based row of interdendritic alloy fabricatedδ-phase precipitates— by HIP and SLM processeshave were been compared explained in in theour currentprevious study.studies from several perspectives [15,16]. However, research related to the surface contamination on powder in SLM materials is limited. Therefore, the current 2. Materialsstudy and focused Experimental on surface contamination Procedure and on the consolidation behaviour between SLM and conventional HIP. In order to gain an in-depth understanding of the surface contamination by the Thepowder prealloyed both IN718in the SLM superalloy and HIP processes, powder wasthe microstructure-mechanical prepared by vacuum meltingrelationship and of atomizedIN718 into powdersnickel-based using the gas-atomizingalloy fabricated by method. HIP and SLM The pr gas-atomizedocesses were compared IN718 powdersin the current used study. for both the HIP process and the SLM process were typically spherical particles that were finer and had a broad 2. Materials and Experimental Procedure particle size distribution below 60 µm (Figure1), which may provide high packing density and reduce the percentageThe of prealloyed voids [ 17IN718,18]. superalloy The chemical powder compositionwas prepared by of vacuum IN718 melting powder and usedatomized in theinto HIP and powders using the gas-atomizing method. The gas-atomized IN718 powders used for both the HIP SLM processes is shown in Table1. The oxygen contents in the powders used for HIP and SLM were process and the SLM process were typically spherical particles that were finer and had a broad comparable.particle For size the distribution HIP process, below the 60 powders μm (Figure were 1), entirely which may canned provide into high SUS304 packing stainless density steel and capsules −3 which wasreduce heated the andpercentage evacuated of voids to [17,18]. 1.0 × 10The chemicPa byal an composition oil-diffusion of IN718 pump. powder An used optimal in the HIP HIP condition which consistingand SLM ofprocesses a soaking is shown temperature in Table 1. ofThe 1180 oxyg◦enC contents and a pressure in the powders of 175 used MPa for forHIP 4 and h was SLM used [19]. On the otherwere hand,comparable. a set For of the SLM HIP process process, parametersthe powders were (laser entirely power: canned 400 into W; SUS304 scanning stainless speed: steel 7.0 m/s; capsules which was heated and evacuated to 1.0 × 10−3 Pa by an oil-diffusion pump. An optimal HIP µ µ µ layer pitch:condition 20 m; which layer consisting thickness: of a 40 soakingm;beam temperature diameter: of 1180 100 °C andm; atmosphere:a pressure of 175 pure MPa 99.9999% for 4 h argon) 3 was utilizedwas toused fabricate [19]. On anthe INother 718 hand, cube a set with of SLM the dimensionsprocess parameters35 × (laser35 × power:35 mm 400. W; The scanning SLM cube was built usingspeed: the 7.0 alternating m/s; layer pitch: scan 20 vector μm; layer with thickness: a 66.7◦ 40rotation μm; beam of diameter: the scanning 100 μm; direction.atmosphere: The pure scanning strategy of99.9999% SLM process argon) was that utilized were to used fabricate are illustratedan IN 718 cube in with Figure the2 dimensionsa. After the 35 HIP× 35 × and 35 mm SLM3. The processes, the specimensSLM cube were was subjected built using tothe solution alternating treatment scan vector and with aginga 66.7° (STA):rotation aof solution the scanning treatment direction. at 980 ◦C The scanning strategy of SLM process that were used are illustrated in Figure 2a. After the HIP and ◦ for 1 h/airSLM cooling processes, (AC) the to specimens room temperature were subjected and to a solution two-step treatment aging treatmentand aging (STA): consisting a solution of 718 C for ◦ ◦ 8 h/furnacetreatment cooling at 980 to621 °C forC 1 and h/air holding cooling (AC) at 621 to roomC for temperature 10 h before and AC a totwo-step room aging temperature. treatment The heat histories ofconsisting the specimens of 718 °C arefor shown8 h/furnace in Figurecooling3 toa,b. 621 Tensile °C and testholding and at creep 621 °C test for specimens 10 h before wereAC to cut from the HIPedroom ingots temperature. and cubed The using heat histories a spark of cutter, the spec andimens the gageare shown length in Figure of each 3a,b. was Tensile 19.6 ×test2.8 and× 3.0 mm3 creep test specimens were cut from the HIPed ingots and cubed using a spark cutter, and the gage (Figure2b). The tensile tests were conducted at room temperature (25 ◦C) and at 650 ◦C under a strain length of each was 19.6 × 2.8 × 3.0 mm3 (Figure 2b). The tensile tests were conducted at room −4 −1 ◦ rate of 4.25temperature× 10 s(25 °C). The and creep at 650 °C test under was a conductedstrain rate of at4.25 650 × 10−C4 s− and1. The 550 creep MPa. test was The conducted microstructures were observedat 650 by°C aand scanning 550 MPa.