Enhanced low cycle fatigue properties of selective laser melting Ti-6Al-4V with fine-tuned composition and optimized microstructure

Improving the low-cycle fatigue(LCF)properties of additively manufactured Ti-5.6Al-3.8V alloy is critical in ensuring its service safety and represents a significant research challenge.This work discusses a so-lution that optimizes the alloy's microstructure and ductility by precisely controlling the over-saturated strengthening elements and heat treatment.This was accomplished using selective laser melting(SLM),heat treatment at 800 ℃ for 2 h,and furnace cooling on a Ti-5.6Al-3.8V alloy with tightly controlled Al,V,and O concentrations in a lower range.The results showed that the SLM-fabricated Ti-5.6Al-3.8V alloy,post-heat treatment,exhibited α laths with a width of～1.4 μm and β columnar grains with a diameter of～126 μm,without experiencing coarsening or variant selection phenomena.The alloy bal-anced strength and ductility post-heat treatment with a UTS of 1015 MPa and an EL of 16.5％relative to the as-deposited state(UTS of 1199 MPa and EL of 11.9％).Notably,the LCF properties of the heat-treated SLM Ti-5.6Al-3.8V alloy are superior to those of other Ti-6Al-4V alloys produced by additive manu-facturing and comparable to traditional forgings.At high strain amplitudes(1-1.5％),the fatigue life of this alloy was twice that of the Ti-6Al-4V forgings.Furthermore,we comprehensively analyzed the mi-crostructure,strength,and ductility of the SLM Ti-5.6Al-3.8V alloy to elucidate the factors influencing its LCF properties.These findings provide a solid foundation for improving the LCF properties of additively manufactured Ti-6Al-4V alloy,thereby contributing to its safe and reliable use in critical applications.     

Microstructural banding of directed energy deposition-additively manufactured 316L stainless steel

The microstructures of 316L stainless steel created by rapid solidification are investigated by comparing the similar microstructures of individual hatches of directed energy deposition additive manufacturing(DED-AM) and those of single, laser surface-melted tracks formed on a solid plate. High recoil pressure,which is exponentially dependent on the laser beam power density, induces convection of the melt pool,which causes formation of microstructural bands in the as-solidified microstructure. The microstructural bands are associated with changes in the chromium concentration and are a significant component of the inhomogeneous microstructure of DED-AM.     

The effect of laser energy input on the microstructure,physical and mechanical properties of Ti-6Al-4V alloys by selective laser melting

Selective laser melting is an advanced manufacturing process which can control the microstructure evolution and mechanical properties of as-manufactured products via various processing parameters. In this study, the porosity/relative density, surface quality, microstructure and mechanical properties were investigated on the selective laser melted Ti-6Al-4V alloy specimens fabricated with a wide range of laser energy inputs. It was found that the microstructure of selected laser melted Ti-6Al-4V alloys is typical of acicular martensites α′. Quantitative analysis reveals that the relative density, martensitic lath size and microhardness increase with the laser energy input. The surface quality is also substantially affected by the energy input.     

Towards the hydrogen pore in additively manufactured AlMgScZr alloy:Influencing factors,formation kinetics mechanism

Hydrogen pores in laser additive manufacturing(LAM)aluminum alloys have long been a major challenge for its application.In this study,the formation mechanisms of hydrogen pores and its kinetic behaviors were systematically investigated in laser directed energy deposited(L-DED)AlMgScZr alloy.The results indicated that hydrogen pores accumulated layer by layer at the melt pool boundaries,due to the high nucleation rate and insufficient time to escape of hydrogen pores at the melt pool boundaries.Dring the AlMgScZr powder at 150 ℃/4 h enabled the significant decrease of moisture content from 0.0520 wt％to 0.0096 wt％,while the porosity of the L-DED sample only decreased from 0.51％to 0.36％.The increase of laser power decreased the porosity of L-DED sample.Except the L-DED parameters,the alloy composition exerted a more significant influence on hydrogen porosity.When the Mg content increased from 9 wt％to 14 wt％,the porosity exhibited an increase from 0.36％to 0.52％.Conversely,the incorporation and subsequent augmentation of Si content resulted in a significant reduction in porosity,reaching as low as 0.15％with an increase to 2 wt％.The solubility and diffusion coefficient of[H]in Al-Mg-Sc-Zr melt with different Mg and Si contents were calculated by density functional theory(DFT).It was observed that increasing the Si content led to a decrease in the nucleation zone for hydrogen pores and reduced poros-ity.A comprehensive porosity inhibition strategy that combines powder drying,process optimization,and composition regulation have been developed.     

Enhancing the tensile properties of laser repairing Ti-6Al-4V alloys:Optimization of strain distribution based on composition fine-turning

In the context of laser repairing damaged forging titanium(Ti)alloys,a common challenge is the sig-nificant reduction in elongation of the repaired samples compared to that of the substrate.In this work,directed energy deposition(DED)technology was employed to repair the TC4(Ti-6Al-4V)forgings by ma-nipulating the Al and V contents of the repaired zone(RZ).Subsequent evaluation encompassed the mi-crostructure,microhardness.and tensile properties across the laser repair deposition samples(LRDs).The results revealed that despite the LRD TC4-0Ti's strength reaching 97.80％of the substrate,its elongation is only 43.93％of the substrate.Upon appropriately reducing the Al and V contents of RZ,the LRD TC4-5Ti demonstrates a strength of 935.04 MPa and an elongation of 14.59％,achieving 98.70％and 82.38％of the substrate,respectively.As the Al and V contents of RZ are further decreased,the strength of the LRDs gradually diminishes,falling below the forging standards.Utilizing digital image correlation(DIC)technology,the deformation behavior of different zones during the tensile process of these LDRs was in-vestigated.The results indicated a concentration of strain distribution within either RZ or the substrate zone(SZ)of the LRDs during the tensile process,which signifies the mismatch of deformation capacity between these two zones.Consequently,the tensile properties of the LRDs were adversely affected.By judiciously adjusting the Al and V contents of RZ,the abovementioned mismatch phenomenon can be ameliorated,which facilitates a synergistic strain behavior between SZ and RZ during the tensile process,aiding in the homogenization of strain distribution and consequently enhancing the tensile properties of the LRDs.     

Effect of mean stress on fretting fatigue of Ti-6Al-4V on Ti-6Al-4V

Fretting fatigue tests of Ti‐6Al‐4V on Ti‐6Al‐4V have been conducted to determine the influence of stress amplitude and mean stress on life. The stress ratio was varied from R=−1 to 0.8. Both flat and cylindrical contacts were studied using a bridge‐type fretting fatigue test apparatus operating either in the partial slip or mixed fretting regimes. The fretting fatigue lives were correlated to a Walker equivalent stress relation. The influence of mean stress on fretting fatigue crack initiation, characterized by the value of the Walker exponent, is smaller compared with plain fatigue. The fretting fatigue knockdown factor based on the Walker equivalent stress is 4. Formation of fretting cracks is primarily associated with the tangential force amplitude at the contact interface. A simple fretting fatigue crack initiation metric that is based on the strength of the singular stress field at the edge of contact is evaluated. The metric has the advantage in that it is neither dependent on the coefficient of friction nor the location of the stick/slip boundary, both of which are often difficult to define with certainty a priori.     

Critical review of the state of the art in multi-material fabrication via directed energy deposition

Directed energy deposition (DED) is an additive manufacturing technique that employs laser melting to permit for the free form manufacturing of components from many input materials from powder or wire form of feedstock. The nature of DED processes and advancements thereof has led to research that has been aimed at leveraging this additive manufacturing technology for the fabrication of high-value metallic alloys in addition to the exploration of multi-material composites and components – many of which are only possible to produce via DED. Herein, a critical review of the present body of research regarding DED of multi-materials is presented. Assessed are the developing research trends, design methodologies, common issues and challenges, as well as identification of the areas where further research should be directed.     

A finite element analysis of fretting fatigue crack growth behavior in Ti-6Al-4V

There is a need for methodology(ies) to analyze the crack growth behavior under fretting fatigue condition since its experimental determination is a challenging task. A finite element sub-modeling method was used to estimate the crack propagation life in titanium alloy, Ti-6Al-4V specimens. Two contact geometries, cylinder-on-flat and flat-on-flat, were analyzed. The computed crack propagation lives were combined with the results of an experimental study where total fatigue lives were measured. The combined numerical-experimental approach provided the crack initiation lives. The crack propagation life increased with increasing applied cyclic bulk stress in similar manner for both contact geometries. Almost 90% of the fretting fatigue life was spent during the crack nucleation and initiation phases in the high cycle fatigue regime. A parametric study was also conducted to investigate the effects of contact load, coefficient of friction and tangential force on the crack growth behavior. The crack propagation life decreased with increase of these three parameters. This decrease was similar for the contact load and the tangential force in both contact geometries, however, the decrease in the case of coefficient of friction was relatively more in the cylindrical pad than in the flat pad.     

Modeling the fatigue crack growth behavior of Ti-6Al-4V by considering grain size and stress ratio

Ti-6Al-4V is a commonly used titanium base alloy in aerospace applications. The increasing demand for damage-tolerant designs of such components necessitates a detailed knowledge of its crack growth behavior. The aim of this research was the characterization and phenomenological modeling of long crack growth behavior with respect to microstructure and stress ratio. Therefore, the long crack propagation was characterized for eight different heat treatment conditions and four stress ratios. For comparison, physically short crack growth tests were also performed. The long crack growth threshold was found to be dominated by roughness-induced crack closure, and the fracture surface roughness is controlled by the primary α-grain size. The reason for this correlation is a near-threshold crack propagation mode, which is dominated by the transcrystalline fracture of α-grains. This correlation was used to model the crack growth threshold with respect to microstructure. A linear relation was determined between the stress ratio and the threshold value, which was also found in this approach. Further presented models cover the crack growth behavior in the near-threshold (Stage I) and mid-growth rate regions (Stage II).     

Behaviour of fatigue cracks emanating from circular notches in Ti-6Al-4V under bending

This paper is aimed at evaluating the behaviour of small cracks emanating from notches in the Ti‐6Al‐4V alloy. Pulsating four point bending tests were performed at a nominal stress ratio of 0.1 and a frequency of 15 Hz on prismatic specimens with a central hole. The conditions of initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 56.6 and 100% of the 0.2% yield stress of the material. Microstructural effects were discussed. To this purpose a specific device based on the ‘in situ’ detection of cracks by photomicroscopy was developed. Corresponding results were analysed quantitatively considering the effect of the yielded region at the notch tip by elastic–plastic finite element modelling. Furthermore, information regarding the sites of fatigue crack initiation and propagation path were discussed on the basis of careful fractographic analysis of the specimens. The importance of the two phase α, β microstructure on the material damage was highlighted and correlated to the observed oscillations in the crack growth rate. Mechanically and microstructurally long cracks were correlated by linear‐elastic fracture mechanics.     

Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures

Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V,which were subjected to a bi-lamellar heat treatment,which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions.The bi-lamellar microstructure was obtained as (i)a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization.For the bi-lamellar treatment with β-homogenization,cooling rates in the range 1-500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 ℃.The microstructures were characterized using various microscopical techniques.Mechanical testing with Vickers hardness indentation and tensile testing was performed.The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure,because of the presence of fine α-platelets inside the β-lamellae.Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions.The age hardened bi-lamellar microstructure shows a similar hardness as the very fine,as-built martensitic microstructure.The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition,which has high strength,but poor ductility.After the bi-lamellar heat treatment,the elongation was improved by more than 250 ％.Due to the very high strength of the as-built condition,loss of tensile strength is unavoidable,resulting in a reduction of tensile strength of～18 ％.     

Correlation of fatigue properties and microstructure in investment cast Ti-6Al-4V welds

The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime.     

Advances in additively manufactured titanium alloys by powder bed fusion and directed energy deposition:Microstructure,defects,and mechanical behavior

Ti and its alloys have been broadly adopted across various industries owing to their outstanding proper-ties,such as high strength-to-weight ratio,excellent fatigue performance,exceptional corrosion resistance and so on.Additive manufacturing(AM)is a complement to,rather than a replacement for,traditional manufacturing processes.It enhances flexibility in fabricating complex components and resolves machin-ing challenges,resulting in reduced lead times for custom designs.However,owing to distinctions among various AM technologies,Ti alloys fabricated by different AM methods usually present differences in mi-crostructure and defects,which can significantly influence the mechanical performance of built parts.Therefore,having an in-depth knowledge of the scientific aspects of fabrication and material properties is crucial to achieving high-performance Ti alloys through different AM methods.This article reviews the mechanical properties of Ti alloys fabricated by two mainstream powder-type AM techniques:powder bed fusion(PBF)and directed energy deposition(DED).The review examines several key aspects,en-compassing phase formation,grain size and morphology,and defects,and provides an in-depth analysis of their influence on the mechanical behaviors of Ti alloys.This review can aid researchers and engi-neers in selecting appropriate PBF or DED methods and optimizing their process parameters to fabricate high-performance Ti alloys for a wide range of industrial applications.     

Microstructural influences on the growth of small cracks in Ti-6Al-4 V

ABSTRACT The effects of microstructure on the growth of small cracks in Ti-6Al-4 V under fatigue loading are presented. The small crack growth is compared with large crack growth. Two large crack tests were performed at a stress ratio of 0.4 and a frequency of 15 Hz. For small crack growth tests, double edge notch specimens were loaded under constant amplitude at four maximum stresses with a stress ratio of 0.4 and a frequency of 15 Hz. A plastic replication technique was used to monitor the small fatigue crack growth rate. The microstructure consists of bands of α and β phases. The present study indicates that the crack growth direction and shape are dependent upon the grain size and grain orientations, and that the crack growth rate seems to be affected by the spacing of α-rich and β-rich bands. Small cracks are propagated at stress intensity factors well below the large crack threshold stress intensity factor.     

Growth mechanism of the tubular TiB crystals in situ formed in the coatings laser-borided on Ti-6Al-4V alloy

Coatings containing borides were fabricated on titanium alloy by laser boriding treatment. SEM images indicate that some rod-like TiB crystals with a hollow core (filled with α-Ti before etched) are formed in the coatings. The formation of rod-like hollow TiB is attributed to the following aspects: first, because the chemical composition in some areas in the laser melt pool is hypoeutectic and in some other areas is hypereutectic due to the boron atoms dissolved in the melt distributing unevenly during rapid solidification, β-Ti firstly crystallizes as primary phases in the hypoeutectic areas of the melt pool and acts as a heterogeneous nucleus for TiB to crystallize on it, and second, because the bond between B-B is stronger than that between Ti-B and between Ti-Ti. TiB grows along the B-B zig-zag chain as well as normal to the direction of the B-B zig-zag chain to form a tubular structure with a β-Ti core in it.     

Plain fatigue and fretting fatigue behaviour of plasma nitrided Ti-6Al-4V

Fatigue tests with and without fretting against unnitrided fretting pads were conducted on unnitrided and plasma nitrided Ti-6Al-4V samples. Plasma nitrided samples exhibited higher surface hardness, higher surface compressive residual stress, lower surface roughness and reduced friction force compared with the unnitrided specimens. Plasma nitriding enhanced the lives of Ti-6Al-4V specimens under both plain fatigue and fretting fatigue loadings. This was explained in terms of the differences in surface hardness, surface residual stress, surface roughness and friction force between the unnitrided and nitrided samples.     

Relative performance of hydrogenated, argon-incorporated and nitrogen-incorporated diamond-like carbon coated Ti-6Al-4V samples under fretting wear loading

Hydrogenated diamond-like carbon (DLC) (H-DLC), argon-incorporated DLC (Ar-DLC) and nitrogen-incorporated DLC (N-DLC) coatings were deposited on flat rectangular Ti-6Al-4V samples. The DLC coatings were characterised by Raman spectroscopy and nanoindentation. Fretting wear tests were conducted on uncoated and DLC coated samples with an alumina ball as the counterbody. As the Ar-DLC and N-DLC coatings had relatively more sp² network compared to the H-DLC coating, they exhibited lower values of hardness and elastic modulus. At both loads of 4.9 N and 14.7 N, all DLC coated specimens showed lower values of tangential force coefficient (TFC), wear volume and specific wear rate compared to the uncoated samples. While the Ar-DLC coated sample exhibited the lowest TFC, wear volume and specific wear rate at 4.9 N load, the N-DLC coated specimen exhibited the lowest TFC, wear volume and specific wear rate at 14.7 N load.     

The twisting mechanism of subsurface fatigue cracking in Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy

The initiation phenomenon of a subsurface fatigue crack was examined by monitoring of the acoustic emission (AE) effects revealed by the slightly surface-hardened specimens of titanium-based VT3-1 (Ti-6Al-2Sn-4Zr-2Mo-0.1Si) alloy under fatigue tests. The circumferentially notched round-bar specimens were cyclically stretched in the stress range of 720-680 MPa, with the stress ratio varying as 0.31-0.36, at 35-Hz loading frequency. The AE monitoring has shown the subsurface cracking events as concomitant with the unloading (compressive) portions of the cyclic load trajectory. In the several fractographically examined areas of subsurface fatigue fracture, mode III (rotational or twist-like) way of crack opening was found to dominate on the short-crack path; during the damage-accumulation period, creative of the first smooth crack-origin facets, such a crack-opening pattern appeared mainly related to the unloading portions of the loading trajectory. Internal residual stresses appear to control such a deformation mode in the locally compressed material. Stress-release effects should follow from the occurrence of the first crack facet favoured by gas diffusion. Therefore, the subsurface crack origination is actually a synergetic problem. The dominant mode-III deformation is also creative of a plastic zone around the now stress-released area of the first fracture facet. This plastic zone is to be involved in the mode-I (tension) opening of the subsurface crack propagating from the first subsurface crack facet. The subsurface crack opening occurs as facilitated by the dissolved-gas diffusion toward a material discontinuity. Consequently, a short crack can expand around the first facet through a combined modes III and I opening to form a growing subsurface-fracture area. Such a model of the twist-controlled subsurface cracking is proposed and discussed here in terms of the well-known numerical data on the subsurface stress-state evolution as well as on the progress of plastic mesoscopic-scale level deformation in tensile-loaded metals.     

Fretting fatigue behaviour of shot-peened Ti-6Al-4V at room and elevated temperatures

Fretting fatigue behaviour of shot‐peened titanium alloy, Ti‐6Al‐4V was investigated at room and elevated temperatures. Constant amplitude fretting fatigue tests were conducted over a wide range of maximum stresses, σ_max= 333 to 666 MPa with a stress ratio of R= 0.1 . Two infrared heaters, placed at the front and back of specimen, were used to heat and maintain temperature of the gage section of specimen at 260 °C. Residual stress measurements by X‐ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Elevated temperature induced more residual stress relaxation, which, in turn, decreased fretting fatigue life significantly at 260 °C. Finite element analysis (FEA) showed that the longitudinal tensile stress, σ_xx varied with the depth inside the specimen from contact surface during fretting fatigue and the largest σ_xx could exist away from the contact surface in a certain situation. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from FEA results to characterize fretting fatigue crack initiation behaviour. It showed that stress relaxation during test affected fretting fatigue life and location of crack initiation significantly. MSSR parameter also predicted crack initiation location, which matched with experimental observations and the number of cycles for crack initiation, which showed the appropriate trend with the experimental observations at both temperatures.     

Additive manufacturing of copper-stainless steel hybrid components using laser-aided directed energy deposition

Combining dissimilar materials in a single component is an effective solution to integrate diverse material properties into a single part.Copper-stainless steel hybrid components are attracting more and more attention since the high thermal conductivity of copper can greatly enhance the thermal performance of stainless steel,which benefits its applications in many industries.However,direct joining of copper and stainless steel such as SS316 L is challenging since they preserve significant dissimilarities in physical,chemical,and thermo-mechanical properties.This paper aims to fabricate well-bonded copper-SS316 L hybrid parts using a laser-aided directed energy deposition (DED) process.A nickel-based alloy Deloro 22 (D22) is introduced between copper and SS316 L to address the detrimental issues in copper-SS316 L direct joints.Using this technique,defect-free interfaces are achieved at both the D22-SS316 L and copper-D22 transition zones.Tensile testing of Cu-D22-SS316 L and D22-SS316 L hybrid parts shows the fracture occurs at pure copper and SS316 L region,respectively,indicating an excellent bonding at the interfaces.Ascending in the building direction,a transition of grain structure is observed.A significant diffusion zone is obtained at both the D22-SS316 L and the Cu-D22 interfaces.The large diffusion distance results in a smooth variation in microhardness over the dissimilar materials.The microhardness increases from SS316 L to D22 with the highest value of 240 HV and then decreases from D22 to Cu with the lowest value of 63 ± 4 HV.Testing of thermophysical properties of the Cu-D22-SS316 L system indicates there is a ～300 ％ increase in thermal diffusivity and a ～200 ％ increase in thermal conductivity when compared to pure SS316 L.The significant increase in thermal diffusivity and conductivity validates the enhanced thermal performance of SS316 L when it is joined with pure copper.