Wrought Counterparts Research Articles

Wire-fed electron beam directed energy deposition of Ti–6Al–2Zr–1Mo–1V alloy and the effect of annealing on the microstructure, texture, and anisotropy of tensile properties

Wire-fed electron beam directed energy deposition (EB-DED) is gaining increasing attention owing to its significant advantage of producing high-quality, large-scale metallic components. In this study, EB-DED of a near-α titanium alloy (Ti–6Al–2Zr–1Mo–1V) was conducted, and the effect of post-deposition single annealing treatment on the microstructure, texture, and anisotropy of the tensile properties was investigated. Optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy were employed to study the microstructure and texture characteristics. The tensile properties along the vertical (Z axis) and horizontal (Y axis) directions were evaluated at both room temperature and 500 °C. The results indicated that the prior β grains had a strong < 001 >β texture along the grain growth direction. Although the α variant selection occurred in some micro-regions, the overall α phase texture was weak. In terms of tensile properties, the vertical specimens exhibited lower strength but higher ductility than the horizontal specimens at both room and elevated temperatures. The anisotropic elongation results from the directional columnar prior β grains and continuous grain boundary α phase, which facilitated the intergranular cracking. The tensile strength and ductility were simultaneously enhanced by annealing at 950 °C for 2 h to meet the standard requirements for wrought counterparts. Moreover, the anisotropy of the tensile properties was decreased significantly. The enhanced and isotropic mechanical properties can be attributed to the combined effect of the bi-lamellar microstructure, discontinuous grain boundary α phase, and the weak α texture.

Structural and Tribological Assessment of Biomedical 316 Stainless Steel Subjected to Pulsed-Plasma Surface Modification: Comparison of LPBF 3D Printing and Conventional Fabrication.

The structural features and nanoindentation/tribological properties of 316 stainless steel fabricated by conventional rolling and laser-based powder bed fusion (LPBF) were comparatively investigated regarding the effect of surface-pulsed plasma treatment (PPT). PPT was performed using an electrothermal axial plasma accelerator under a discharge voltage of 4.5 kV and a pulse duration of 1 ms. Optical microscopy, scanning electron microscopy, X-ray diffraction, nanoindentation measurements and tribological tests were applied to characterize the alloys. The LPBF steel presented almost the same modulus of elasticity and double the hardness of rolled steel. However, the LPBF steel manifested lower dry-sliding wear resistance compared with its wrought counterpart due to its porous structure and non-metallic inclusions. Conversely, LPBF steel showed three times higher wear resistance under sliding in simulated body fluid (SBF), as compared with wrought steel. PPT led to steel modification through surface melting to a depth of 22–26 μm, which resulted in a fine cellular structure. PPT moderately improved the dry-sliding wear resistance of LPBF steel by fusion of pores on its surface. On the other hand, PPT had almost no effect on the SBF-sliding wear response of the steel. The modification features were analyzed using a computer simulation of plasma-induced heating.

3d printing of stainless steel 316L and its weldability for corrosive environments

Critical to its successful deployment, metal 3D printed parts must demonstrate not only corrosion resistance to operate in harsh environments but also good weldability to existing assets for installation and long-term durability. To address this issue, this paper evaluates the corrosion resistance of additively manufactured Stainless Steel (SS) 316 L via Laser powder bed fusion (L-PBF) and its weldability to its wrought counterpart under corrosion distress. First, 3D printed pellets are separately characterized by accelerated corrosion testing (i.e. chronoamperometry, cyclic potentiodynamic polarization and electrochemical impedance spectroscopy) following ASTM standards, and its improved corrosion resistance is demonstrated and compared to its wrought counterpart. Next, to evaluate the weldability, 3D printed SS 316 L plates are welded to wrought counterparts, dissimilar welded joints dog-bone specimens are machined from it and submitted for uniaxial mechanical tensile testing. When compared against similar welded joints wrought dog-bone specimens (i.e. wrought on both halves), dissimilar welded joints dog-bone specimens exhibit 36.02% less ducility and 12.59% lower yield strength, and those values reduce to 74.65% and 27.29%, respectively, when the former undergoes accelerated corrosion. These results evaluate 3D printed SS 316 L parts against wrought for applications as replacement parts in harsh environments that require welding for installation.

Effects of crystallographic orientation on the corrosion behavior of stainless steel 316L manufactured by laser powder bed fusion

The corrosion behavior of the {100}, {110}, {111} planes of a single-crystalline-like stainless steel 316L (SS316L) produced by laser powder bed fusion (LPBF) was investigated in 3.5 wt% aqueous NaCl and compared with their LPBF-polycrystalline and wrought counterparts. The general corrosion resistance followed: wrought ≈ {110} < polycrystalline < {100} < {111} trend, which was rationalized by the surface atom density and corrosion activation energy. All LPBF samples exhibited a significantly higher pitting corrosion potential than the wrought SS316L due to the lower concentration of oxygen vacancies in their passive oxide films. These findings provide insights into improving the corrosion performance by tailoring the crystallographic texture of LPBF-SS316L.

Dependence of microstructures on fatigue performance of polycrystals: A comparative study of conventional and additively manufactured 316L stainless steel

The fatigue properties and microstructural evolution of 316 L stainless steel (316LSS) manufactured by laser powder bed fusion (L-PBF) were systematically studied and compared with its wrought counterpart. The as-built L-PBF 316LSS shows a pronounced heterogeneity, not only structurally but also chemically, with a unique microstructure of highly serrated grain boundaries, bimodal grain structure, nano-precipitates, solidification cell structures, and chemical segregations. The microindentation test showed that the hardness of the as-built L-PBF 316LSS reached 2.589 GPa, which was about 1.6 times higher than that of the wrought solution annealed counterpart, and the sparser slip steps around indentations revealed its greater dislocation storage capability. The S-N curves indicated that the fatigue resistance of the as-built L-PBF 316LSS was significantly better than that of the wrought solution annealed samples, and this was ascribed to its unique microstructural characteristics, especially the pre-existing high-density dislocations and chemical microsegregation within cellular solidification features. Furthermore, the enhanced planar slip in L-PBF 316LSS by its unique microstructure, especially the formation of deformation twins, delays the strain localization and restrains slip band generation, thereby significantly inhibiting crack initiation, and contributing greatly to the fatigue performance. The unique cell structure appears to be more effective in improving the low-cycle fatigue performance of L-PBF 316LSS due to the enhanced ductility.

Exploiting lack of fusion defects for microstructural engineering in additive manufacturing

Rapid cooling rates and stochastic interactions between the heat source and feedstock in additive manufacturing (AM) result in strong anisotropy and process-induced defects deteriorating the tensile ductility and fatigue resistance of printed parts. We show that by deliberately introducing a high density of lack of fusion (LoF) defects, a processing regime that has been avoided so far, followed by hot isostatic pressing (HIP), we can print Ti-6Al-4V with reduced texture and combinations of strength (TS=1.0 ± 3E-2 GPa) and ductility (ε failure =20 ± 1%) surpassing that of wrought, cast, forged, annealed, solution-treated and aged counterparts. Such improvement is achieved through the formation of low aspect ratio α-grains around LoF defects upon healing, surrounded by α-laths. This occurrence is attributed to surface energy reduction and recrystallization events taking place during the healing of LoF defects via HIP post-processing. Our approach to design duplex microstructures is applicable to a wide range of AM processes and alloys, and can be used in the design of damage tolerant microstructures.

Hypo-toxicity and prominent passivation characteristics of 316 L stainless steel fabricated by direct metal laser sintering in a simulated inflammation environment

3D-printing is an emerging technology that challenged wrought counterparts by one-step manufacturing for complicated biological devices. However, the material properties and surface features due to manufacturing parameters play an important role on the corrosion behaviour and influence the toxicity of the material as an implant. In this paper, the improvement of pitting potential was observed by electrochemical experiments as the result of grain refinement of DMLS 316 L at 200 W laser power. The ICP results verified the supressed release of toxic cations after the formation of the passive film with enhanced characteristics. However, the pores from DMLS 316 L have the potential to develop into pits when polarised above pitting potential, promoting the risk of using 3D-printed 316 L as implant materials.

Effect of High Laser Energy Density on Selective Laser Melted 316L Stainless Steel: Analysis on Metallurgical and Mechanical Properties and Comparison with Wrought 316L Stainless Steel.

The austenitic 316L stainless steel (SS) is used extensively for marine applications as well as in construction, processing, and petrochemical industries due to its outstanding corrosion resistance properties. This study investigates the density, microhardness, and microstructural development of 316L SS samples fabricated by selective laser melting (SLM) under high laser energy densities. The selective laser melted (SLMed) specimens were fabricated under high laser energy densities (500, 400, and 333.33 J/mm3) and their metallurgical and mechanical properties were compared with the wrought specimen. SLMed 316L SS showed excellent printability, thereby enabling the fabrication of parts near full density. The porosity content present in the SLMed specimens was determined by both the image analysis method and Archimedes method. SLMed 316L specimens fabricated by the SLM process allowed observation of a microhardness of 253 HV1.0 and achieved relative density up to 98.022%. Microstructural analysis using optical microscopy and phase composition analysis by X-ray diffraction (XRD) has been performed. Residual stresses were observed using the XRD method, and compressive stress (-68.9 MPa) was noticed in the as-printed specimen along the surface of the build direction. The microstructure of the as-built SLMed specimens consisted of a single-phase face-centered cubic solid solution with fine cellular and columnar grains along the build direction. The SLMed specimens seemed to yield better results than the wrought counterpart. IRB approval and Clinical Trial Registration Number are not applicable for this current work.

Notched fatigue of additive manufactured metals under axial and multiaxial loadings, part II: Data correlations and life estimations

Fatigue performance of additive manufactured (AM) metals is highly sensitive to the existence of defects. This becomes more challenging when mechanically induced stress concentrations due to notches or complexities in the geometries are also present. Multiaxial stress states are additional important consideration in designing parts for critical load carrying applications. In this work, notched uniaxial and multiaxial fatigue and cracking behavior of two commonly used AM metals (Ti-6Al-4V and 17–4 PH stainless steel) with different post fabrication heat treatments and surface roughness conditions were studied. In part I of these two-part paper series, AM notch test results were compared to AM unnotched as well as to the notched and unnotched wrought counterparts. In this second part, both crack initiation (i.e., critical plane-based method) and crack growth (i.e., fracture mechanics-based method) approaches are used for uniaxial and multiaxial data correlations and life estimations. Both approaches are shown to result in satisfactory estimations of the experimental results.

Tailoring the microstructure and mechanical property of laser powder bed fusion fabricated Ti–6Al–2Sn–4Zr–2Mo via heat treatment

The heat treatment effect on the microstructure evolution and mechanical properties of the laser powder bed fusion fabricated Ti–6Al–2Sn–4Zr–2Mo (Ti6242) alloy was investigated in this study. The highly dense Ti6242 samples comprising acicular α΄ martensite and deformation twinning could be fabricated. The as-built alloy exhibited a high yield strength (σy, 1296 ± 44 MPa) yet a limited tensile elongation (ɛf, 5.7 ± 1.1%). Four different heat treatment profiles were conducted to tailor the microstructure and tensile properties. The direct annealing at 630 °C and 850 °C caused martensite decomposition. The annealing at 630 °C/8 h/air cooling led to the formation of β nano-particles and layers, leading to a slight increase of σy and a reduction of ɛf. Annealing at 850 °C/3 h/air cooling led to coarsening of α lamellae and growth of β phase, resulting in a decreased σy and enhanced ɛf compared with those at 630 °C. The two-stage heat treatment and thermal cycling triggered the formation of partially globularized primary α and Widmanstatten structure (secondary αs + β). These two heat treatments gave rise to a balanced combination of strength and ductility that could be comparable to those of wrought counterparts. This work demonstrates that a combination of laser powder bed fusion and post heat treatment could allow for the fabrication of complex-shaped Ti6242 components with little compromise of mechanical property for industrial applications.

Microstructural characterization of 15-5PH stainless steel processed by laser powder-bed fusion

The 15-5PH (UNS S 15500) stainless steel combines high mechanical strength, ductility, and good corrosion resistance for aircraft and aerospace applications. This set of properties and its high added-value applications make it an excellent choice for additive manufacturing processes such as laser powder-bed fusion (LPBF). However, there is a need to understand the complex microstructure developed in LPBF-processed parts, which may show particularities such as columnar solidification, preferential orientation, different kinds of porosities, chemical segregation along the melt pools, metastable phases, and oxide nanoinclusions. We report the microstructural characterization of LPBF-processed 15-5PH stainless steel in the as-built condition and after aging. Low porosity (<1%), high hardness (420 HV0.1), and a large amount of retained austenite (15%) were found in the as-built samples. The microstructure shows the usual “fish scale”-like morphology. Grain size and hardness vary depending on the location within the melt pools. Reconstruction of the parent grains from EBSD maps indicates a microstructural refinement due to in situ reaustenitization of parts of the previous consolidated layers, where these heat-affected zones are harder and present larger amounts of retained austenite. After aging, as-built samples were harder and more resistant to overaging than annealed or wrought counterparts. Transmission electron microscopy reveals a large amount of nanometric crystalline silicon-oxide inclusions, indexed as cristobalite. The combination of fine-grained martensite, coherent Cu-rich clusters, nanometric oxide particles, and retained austenite makes LPBF-processed 15-5PH stainless steel a very promising material for high-end structural applications.

Ultra Low-Cycle Fatigue Behavior Comparison between Additively Manufactured and Rolled 17-4 PH (AISI 630) Stainless Steels

This study investigates the mechanical behavior of additively manufactured (AM) 17-4 PH (AISI 630) stainless steels and compares their behavior to traditionally produced wrought counterparts. The goal of this study is to understand the key parameters influencing AM 17-4 PH steel fatigue life under ULCF conditions and to develop simple predictive models for fatigue-life estimation in AM 17-4 steel components. In this study, both AM and traditionally produced (wrought) material samples are fatigue tested under fully reversed (R = −1) strain controlled (2–4% strain) loading and characterized using micro-hardness, x-ray diffraction, and fractography methods. Results indicate decreased fatigue life for AM specimens as compared to wrought 17-4 PH specimens due to fabrication porosity and un-melted particle defect regions which provide a mechanism for internal fracture initiation. Heat treatment processes performed in this work, to both the AM and wrought specimens, had no observable effect on ULCF behavior. Result comparisons with an existing fatigue prediction model (the Coffin–Manson universal slopes equation) demonstrated consistent over-prediction of fatigue life at applied strain amplitudes greater than 3%, likely due to inherent AM fabrication defects. An alternative empirical ULCF capacity equation is proposed herein to aid future fatigue estimations in AM 17-4 PH stainless steel components.

Characterization of the Microstructure and Surface Roughness Effects on Fatigue Life Using the Tanaka–Mura–Wu Model

Additive manufacturing (AM) has drawn tremendous interest in engineering applications because it offers almost unlimited possibilities of innovative structural design to save weight and optimize performance. However, fatigue properties are one of the limiting factors for structural applications of AM materials. The recently developed Tanaka–Mura–Wu (TMW) model is modified to include the microstructure and surface roughness factors, in addition to the material’s elastic modulus, surface energy and Burgers vector, to predict the fatigue curves as functions of stress or plastic strain for several typical AM materials as well as their conventional (wrought) counterpart. Furthermore, with statistical characterization of the microstructural effect, the model can be established to evaluate fatigue design allowables.

Effects of Powder Atomization Route and Post‐Processing Thermal Treatments on the Mechanical Properties and Fatigue Resistance of Additively Manufactured 18Ni300 Maraging Steel

In this work the effect of microstructural homogenization and solution annealing prior to ageing on mechanical properties and fatigue resistance of 18Ni300 maraging steel parts processed by laser powder bed fusion (L‐PBF), using two different feedstock powders with different N and O contents is investigated. The static mechanical properties are not significantly affected by the selection of the different feedstock powders. This is ascribed to the extremely fine size of the nonmetallic inclusions, characteristic of L‐PBF process, even by using feedstock powders containing higher O and N contents. The tensile properties in both “directly aged,” and “homogenized, solution annealed and aged” samples are comparable. The tensile strength and ductility are slightly improved compared with the vacuum melted wrought counterparts. The dynamic mechanical properties are affected by the O and N contents of the powders. Heat treatment schedule imposed a significant effect on the fatigue properties. The obtained fatigue strength in directly aged samples is already comparable to the wrought counterparts. Fatigue resistance is improved by using feedstock powders with lower oxygen and nitrogen levels. Proper homogenization heat treatments prior to ageing led to a further improvement of fatigue strength because of the elimination of intercellular microsegregation.

Mechanical properties and microstructural characteristics of additively manufactured C103 niobium alloy

C103 is a refractory-based niobium alloy for high temperature applications in aerospace. This paper aims to evaluate the compatibility of C103 alloy as a choice of material for laser-based additive manufacturing (AM) technologies. Crack free AM specimens with relative densities of greater than 99% were successfully fabricated via laser powder bed fusion (L-PBF) method with horizontal and vertical build orientations. Static strength levels of the stress-relieved specimens exceeded that of their wrought counterparts. Higher yield and ultimate tensile strength of AM specimens were attributed to high dislocation density and the formation of dislocation cell structures, which was revealed by transmission electron microscopy. Loss of ductility in AM specimens was partially due to high density of dislocations (i.e., shorter mean free path for dislocations), while the presence of defects may also have compromised the total elongation to failure. Electron backscattered diffraction analysis revealed formation of sharp 001||BD rod texture in AM samples, which resulted in anisotropic mechanical behavior. Uniaxial tensile and compressive deformation led to the development of (001) <110> parallel to tensile direction and <111> parallel to build direction, respectively.

Laser powder bed fusion of Ni-based Hastelloy X superalloy: Microstructure, anisotropic mechanical properties and strengthening mechanisms

Ni-based Hastelloy X (HX) superalloy is comprehensively employed in the aerospace industry because of its excellent oxidation resistance and high-temperature strength. Nevertheless, due to the insufficient overlapping feature of the process, extremely high porosity and surface roughness are formed between layers, which are primarily affected by processing parameters. In this study, statistical models regarding the effect of process parameters on density and surface roughness for laser powder bed fusion (LPBF)-fabricated HX were established via a response surface method, and interactions among density, surface roughness and processing parameters were systematically investigated. A multiple quantitative analysis of anisotropic strengthening mechanisms was established in terms of the grain boundary, solid solution and dislocation strengthening mechanisms. Experimental results demonstrated a maximum density (8.294 ± 0.003 g/cm3) and minimum surface roughness (6.81 ± 0.12 μm) within a set of optimised parameters (laser power = 165 W, scanning speed = 1000 mm/s and scan spacing = 0.103 mm) using a multi-response optimisation approach. For the LPBF-fabricated HX, the XOY plane (perpendicular to build direction) exhibited higher strength (yield strength = 696 MPa; ultimate tensile strength = 892 MPa) than compared with the YOZ plane (parallel to the build direction) along the build direction, which was primarily attributed to high dislocation density (1.28 × 1014/m2) and refined grain size (13.9 μm). Multiple quantitative analyses indicated that the difference in dislocation strengthening dominated the anisotropy in the mechanical properties between the two planes of the LPBF-fabricated HX, whereas solid solution and grain boundary strengthening contributions dominated in the wrought counterparts. This work offers new insights for meeting the requirements for a multi-degree freedom of design within Ni-based parts with customised performance in additive manufacturing.

Mechanical properties, corrosion behavior and cytotoxicity of Ti-6Al-4V alloy fabricated by laser metal deposition

Ti-6Al-4V (TC4) is a widely used biomaterial due to the combination of excellent mechanical properties, corrosion resistance and biological properties. Laser metal deposition (LMD), an efficient and rapid additive manufacturing technology, is very suitable for biomaterial Ti alloy based implants for a customized design. The microstructure evolution and mechanical properties of TC4 samples fabricated by LMD with different scanning approaches were systematically studied in comparison with its wrought counterpart. Large-area mapping (LAM) electron backscatter diffraction (EBSD) results show that the one-way scanning produces finer grains and stronger texture index in the 〈 0001 〉 direction than those of the cross scanning equivalents. Moreover, the former has higher ultimate tensile strength and plasticity. Electrochemical experiments and in vitro biosafety evaluations were carried out to verify the feasibility of TC4 for long-term services in the body with respect to MC3T3-E1 cells. This work could promote the employment of LMD to build high-quality metal implants for medical applications. • Large temperature gradients lead to the formation of martensite in the LMD process. • One-way scanning has higher strength and ductility than cross scanning. • Corrosion resistance of one-way scanning is better than that of cross scanning. • The good biocompatibility of LMD TC4 samples is confirmed in vitro.

Effect of microstructure induced anisotropy on fatigue behaviour of functionally graded Inconel 718 fabricated by additive manufacturing

In this paper, the effect of microstructural anisotropy on the fatigue crack growth behaviour of the functionally graded Inconel 718 fabricated through laser powder bed fusion (L-PBF) is investigated. Different manufacturing parameters, including low and high laser powers, were used to produce a variety of non-graded (NG) and functionally graded (G) specimens in two build directions, vertical and horizontal. In addition, a group of heat treated wrought samples was tested as a reference. It was observed that the different manufacturing parameters result in various grain size, crystallographic textures, precipitates and Laves phases, porosity, and un-melted particles. Three-point bending fatigue tests were conducted to measure the threshold stress intensity factor (ΔK th ) and fatigue crack growth rate (FCGR),da/dN. Only the lower laser power L-BPF Inconel material was found to have comparable to the wrought heat treated material fatigue crack growth behaviour. Furthermore, a new approach of automatically controlling ΔK as a function of the crack length was employed for graded specimens to investigate the crack growth rate as a function of local microstructure. The FCGR value of the vertical L-PBF samples, in which the crack direction was perpendicular to the build direction, remained constant. In contrast, the da/dN value of the horizontal samples with the crack direction parallel to the build direction increased constantly with the increase of the crack length. This behaviour is in good agreement with the hardness profile of the graded materials. Melt pool boundaries, graded interface boundaries, and grain orientations close to 〈001〉 were found to deflect the crack path. Additionally, it was found that L-PBF material is more affected (at a low stress ratio of R = 0.1) by the roughness-induced crack closure than the wrought counterparts. This study has successfully demonstrated the feasibility of using an additive manufacturing process to fabricate functionally graded materials featuring tailorable fatigue response of the local microstructures. • The hardness profiles of the functionally graded materials found to be highly affected by the build direction. • The Non-graded lower laser power material showed a comparable fatigue crack growth behaviour to the wrought material at a low stress ratio of R = 0.1. • The fatigue crack growth rate of the functionally graded materials along the crack length found to be almost constant for the V samples while it had an increasing trend for the H samples.

Microstructural evolution and corrosion behavior of Ti–6Al–4V alloy fabricated by laser metal deposition for dental applications

In this study, microstructural evolution and corrosion behavior of Ti–6Al–4V (TC4) alloy fabricated by laser metal deposition (LMD) were investigated in simulated saliva at 37 °C as a function of scanning strategies. One-way scanning possessed a higher corrosion resistance than cross-scanning, but both of those were inferior to the wrought one. Compared with conventional wrought counterpart, the higher content of β phase improved the corrosion resistance. Results of Mott–Schottky and potentiostatic tests indicated that the passive film upon the surfaces of all the samples exhibited an N-type semiconductor structure and instantaneous nucleation. Of these, the LMD-ed parts displayed a lower donor concentration, higher vacancy diffusion coefficient, and thicker passive film than the wrought samples. XPS analysis revealed that the double-layer structure of passive film leads to a dense protective layer on the surface while the oxide contents of the three were different.

The possible role of nano sized precipitates on the mechanical properties of additively manufactured IN 718 superalloy

In the present study, three dimensional parts of Inconel 718® (IN 718) superalloy were fabricated using laser powder bed fusion (L-PBF) additive manufacturing (AM). The soundness of the as-built parts was checked by measuring the relative density using Archimedes method. Tensile tests were carried out at the strain rate of 4.6 × 10-3 s-1 at ambient temperature on cylindrical samples extracted from the large size 3D blocks of AM IN 718. Tensile tests were also carried out employing the same test parameters on the wrought counterpart of IN 718 (W IN 718) so as to compare their mechanical properties. The AM IN 718 exhibited about 15% increase in 0.2% yield strength (0.2%YS) as compared to W IN 718. Microstructures of AM IN 718 and W IN 718 were characterized using electron back scattered diffraction. The dislocation substructure of pre-deformed samples was examined using transmission electron microscope (TEM). Bright field TEM images revealed nano-sized precipitates in AM IN 718 and carbides in W IN 718. The marginal increase in 0.2%YS in AM IN 718 is attributed to the impediment to dislocation movements by nano-sized precipitates and high initial dislocation density.