Materials Science Forum Vol. 1016

Abstract: The rapid development of cold spray technology has made it a viable option to repair and remanufacture damaged components as well as to create novel materials for biomedical applications. One of the most influential parameters of this distinctive process is the deposition velocity, which ultimately controls the degree of material deformation and material adhesion. Although the majority of materials can be successfully deposited at relative low deposition velocity (<700m/s), this is not representative of Ti alloys which have high yield strength. The amount of deformation and resultant properties of the coating are related to the velocity, temperature, and tensile strength of the particles. The ability to predict the deformation and resultant properties helps in developing process parameters and tailoring coatings to get the desired properties. In the current study, the particle deformation behavior and bonding with the substrate was investigated over a range of impact conditions. The effects of deposition velocity, gas temperature, gas pressure and nozzle stand-off distance were studied using cold sprayed splats of spherical Ti-6Al-4V powder deposited on to 316 SS substrate utilizing helium as a carrier gas. Finite element modeling of the impacted particles was conducted using Johnson-Cook high-strain-rate properties in a Lagrangian analysis to predict the overall deformation and estimated stress state of the impacted particles. Particle temperature due to impact was also predicted. Overall predictions were in good agreement with experimental results. Optical microscopy, scanning electron microscopy (SEM) and focused ion beam (FIB) were used to identify three distinct regions within the impact morphologies; these include the initial impact region, the jetting region, and the upper splat region.

Abstract: In this study, the relationship between hydrogen embrittlement resistance (HER) and the microstructure of low temperature tempered martensite was investigated using steel sheets which were controlled by carbon content and tempering conditions. Focusing on transition carbides and interstitial carbon content which are peculiar microstructures to low temperature tempered martensite, microstructure was evaluated by synchrotron radiation X-ray diffraction (SR-XRD). The HER was evaluated by U-bending and fracture surface was observed after the slow strain rate test (SSRT). As the result, the HER was improved and fracture morphology was changed from intergranular to quasi-cleavage when the high carbon content and high temperature tempering were adopted. In the steels improved the HER, the increase of the volume fraction of transition carbides and the decrease of interstitial carbon content was confirmed. Hydrogen trapping by the transition carbides could explain the change of the HER and fracture morphology. These results suggested that the hydrogen trapping by the transition carbides was effective to improve the HER of the low temperature tempering martensitic steels.

Abstract: There is a growing demand for more efficient and economic manufacturing process to improve product quality, reduce production cost, reduce lead time and increase productivity. The application of computer aided design and manufacturing (CAD/CAM) techniques to forging is becoming increasingly popular as the resulting improvements in yield and productivity. Modeling and simulation have become a major concern in recent and advanced research. In this paper die design for forging of an automobile component “Stub Axle” is presented. In die forging process, complex shape component cannot be made in one stage and therefore, the use of preform die becomes essential. The initial preform design was carried out by conventional method. The simulation has been carried out using software DEFORM-3D. The main goal of this study is to design an optimal preform shape resulting an optimal initial billet selection. Keywords:CAD/CAM, Preform, DEFORM-3D, Simulation, Forging

Abstract: Modern advanced high strength steels (AHSS) for the automotive sector often contain retained austenite which promotes remarkable combinations of strength and ductility. These high strength steels may however be subject to a risk of hydrogen embrittlement. For the current contribution, hydrogen trapping and embrittlement behaviour were investigated in AHSS compositions having different levels of retained austenite. Hydrogen permeation tests revealed that hydrogen diffusion was slower for increased levels of retained austenite, being controlled most likely by reversible trapping at austenite-matrix interfaces. External hydrogen embrittlement tests via step loading also revealed that resistance to hydrogen was lower for increased levels of retained austenite. It was suggested that during step loading the hydrogen accumulated at austenite-matrix interfaces, leading to cracking when the applied stress was high enough.

Abstract: Coatings deposited by detonation spraying equipment CCDS2000 using alumina powder are studied. CCDS2000 is characterized by a computer control system, robot compatible spraying unit (gun barrel), a portable chiller, two powder feeders, and other peculiar properties. This installation allows to deposit coatings on complex shape surfaces and thin-walled parts under optimal conditions. Studies of the coating properties included measurements of coating microstructure, porosity, microhardness, adhesion, cohesion, abrasive and erosive wear, and dielectric properties (specific resistivity and dielectric strength). The detonation sprayed coatings have an adhesion of 60-70 MPa, cohesion of 100 MPa, microhardness of 1500 HV₁₀₀, porosity of about 1% (measured on microsections of coatings using OLYMPUS Stream Image Analysis software). Impregnation of coatings with silicone oil showed that the real open porosity of coatings is up to 6%. Dielectric strength of the ceramic layer with the thickness of 200 μm exceeds 30 kV/mm. Specific resistivity depends on atmospheric humidity and when the relative humidity is less than 60%, the specific resistivity is greater than 10¹³ Ω·cm.

Abstract: In the present study, two different yield criteria were investigated to model and compare the yield thresholds functions for the plastic behavior of rolled sheets. These two different yield criteria as described via Hill48 yield quadratic and F. Barlat Yld2000-2d non-quadratic criterion. For this purpose, an experimental device of simple tensile test and the studied material are described. The experimental results in terms of Yield stress and Anisotropic coefficient are estimated from the Associated Flow Rule (AFR) and Non-Associated Flow Rule (NAFR). However, it is found that the criterion of Yld2000-2d is the most appropriate model in comparison with the experimental results.

Abstract: Although quenched and partitioned (Q&P) steels are traditionally alloyed with Si, its precise role on microstructural mechanisms occurring during the partitioning process is not thoroughly investigated. In this study, a systematic investigation has been carried out to reveal the influence of Si on austenite decomposition, phase transformation and carbide precipitation during Q&P treatment. Using a Gleeble thermomechanical simulator, three medium carbon steels with varying Si contents (0.25, 0.70 and 1.5 wt.%) were hot-rolled, reaustenitized, quenched into the M_s -M_f range, retaining about 20% austenite at the quench-stop temperature (T_Q), and held for 1000 seconds above T_Q in the temperature range of 200-300°C in order to better understand the mechanisms operating during partitioning. Dilatometric measurements combined with microstructural characterization using SEM-EBSD, TEM, and XRD clearly revealed the occurrence of various mechanisms. The effect of partitioning temperature/time on the hardness of the Q&P samples was correlated with the microstructural features. Steel containing low Si content (0.25%) was incapable of promoting carbon enrichment of austenite during partitioning, leading to its continuous decomposition into isothermal martensite and/or bainite without any detectable austenite retained even holding at 300°C. In comparison, 1.5% Si content promoted retention of about 19% austenite under similar Q&P conditions. Small fractions of bainite and high-carbon martensite formed during final cooling in both steels after partitioning at 200°C. Moreover, carbide precipitation was strongly retarded by high Si content.

Abstract: The microstructures and shape memory behaviors of Ti-18Nb-6Zr (at.%) alloy subjected to different heat treatments were investigated through optical microscopy (OM), X-ray diffraction (XRD), Transmission electron microscopy (TEM) and tensile tests. Recrystallization occurs in cold-rolled Ti-18Nb-6Zr alloy after solution treatment at 600~800 °C. The cooling rate after solution treatment at 800°C shows a dramatic effect on the microstructure of the alloy. The microstructures for the water quenching (WQ) and oil quenching (OQ) specimens are composed of single α'' martensite phase, while for the air cooling (AC) specimen, the microstructure consists of predominant β phase and a small amount of fine athermal ω phase. During tensile deformation, two-stage yielding is observed in the alloy subjected to 800°C/0.5h/WQ heat treatment. The stress for martensite variants reorientation and the yield stress for plastic deformation are 310MPa and 455MPa, respectievely, and the maximum shape memory strain of 3.1% is obtained with pre-strain of 6%.

Abstract: Abrasive Water Jet (AWJ) machining has proven to be an effective and versatile technique for milling various kinds of materials, even with low machinability such as aerospace grade titanium alloy Ti6Al4V. Many studies have been performed in order to master this technology and produce geometrically accurate shapes. However, in the context of bonding repairs which require surfaces free from foreign bodies, AWJ machining presents a significant drawback in form of abrasive grit embedment. The goal of this present work is then to investigate the effect of a post-AWJ machining cleaning operation using Plain Water Jet process (PWJ – i.e. without abrasive particles) on the surface quality and material properties. For this, several characterization techniques were employed. It was concluded that the contamination has been reduced by 65% without noticeable changes in depth of cut and crater volume. The AWJ milling operation produced surface and subsurface hardening as well as biaxial compressive residual stress, mostly piloted by the jet pressure. PWJ cleaning reduced the depth of hardening without clear modification in surface hardness.

Abstract: Several ferritic stainless steel grades are widely studied and used in solid oxide fuel cells (SOFCs) technology as interconnect materials. Their high-temperature oxidation behavior is interesting to evaluate their applicability at SOFCs operating conditions and to design degradation tests and models predicting the lifetime of a SOFC stack. In this work the AISI441 grade was oxidized in static air at 850°C to study its oxidation kinetic by weight gain measurements. It was found a parabolic growth with a rate constant of 9.42 x 10^-14 g²cm^-4s^-1. Data calculated using the diffusion coefficients of the species involved in the oxidation process resulted in higher weight gain. Discrepancies between the measurements and the model were corrected taking into account the chromium volatilization.