Effect of Industrial Heat Treatment and Barrel Finishing on the Mechanical Performance of Ti6Al4V Processed by Selective Laser Melting

Additive manufacturing is now capable of delivering high-quality, complex-shaped metallic components. The titanium alloy Ti6Al4V is an example of a printable metal being broadly used for advanced structural applications. A sound characterization of static mechanical properties of additively manufactured material is crucial for its proper application, and here specifically for Ti6Al4V. This includes a complete understanding of the influence of postprocess treatment on the material behavior, which has not been reached yet. In the present paper, the postprocess effects of surface finish and heat treatment on the mechanical performance of Ti6Al4V after selective laser melting were investigated. Some samples were subjected to barrel finishing at two different intensities, while different sets of specimens underwent several thermal cycles. As a reference, a control group of specimens was included, which did not undergo any postprocessing. The treatments were selected to be effective and easy to perform, being suitable for real industrial applications. Tensile tests were performed on all the samples, to obtain yield stress, ultimate tensile strength and elongation at fracture. The area reduction of the barrel-finished samples, after being tested, was measured by using a 3D scanner, as a further indication of ductility. Experimental results are reported and discussed, highlighting the effect of postprocessing treatments on the mechanical response. We then propose the optimal postprocessing procedure to enhance ductility without compromising strength, for structures manufactured from Ti6Al4V with selective laser melting

calibration and prediction assessment of different ductile damage models on ti6al4v and 17 4ph additive manufactured alloys

Abstract Nowadays, metal additive manufacturing is becoming always more popular, being able to deliver complex shaped high quality products. Though many studies have been conducted on the high cycle fatigue behavior of these materials, yet ductile failure has still not been completely investigated, to identify the failure limits under static complex stress states. In the present study, the calibration of three ductile damage models on two popular additive manufactured alloys was carried out. The selected alloys were Ti6Al4V, processed via Electron Beam Melting, and 17-4PH fabricated with Selective Laser Melting technology; both broadly used in actual industrial applications. For each material a set of samples, was fabricated to perform a thorough static mechanical characterization, involving tensile tests on round smooth bars, notched bars, tests under plane strain conditions and torsion tests. The stress state in the critical points was retrieved relying on FEM simulations, and the data collected via the hybrid experimental-numerical procedure subsequently used to tune the damage models. Specifically, the selected models are the Rice and Tracey, the Modified Mohr-Coulomb by Wierzbicki and the one proposed by Coppola and Cortese. While the former does not take into account the effect of Lode parameter, the latter two consider its influence on fracture onset. A minimization algorithm was used for their calibration, and different optimization strategies were adopted to check the robustness of identified parameters. The resulting strains to fracture as a function of damage parameters were plotted for each formulation. The failure prediction accuracy of all models was assessed and compared to the others

A nonlinear model for ductile damage accumulation under multiaxial non-proportional loading conditions

In this paper, a nonlinear model for ductile damage accumulation is presented and applied to predict failure under non-proportional loading conditions. To this purpose, several tension-torsion tests were executed on samples of an isotropic Grade X65 steel, using a custom biaxial equipment. The experiments were carried out on hollow cylindrical specimens with two different gauge axial lengths. Non-proportional loading paths were achieved changing the tension to torsion ratio during runs. The dependence of damage accumulation on the equivalent plastic strain was modelled by means of a nonlinear function of the triaxiality and the deviatoric component of the stress state. The devised model, suitable to predict failure under complex loading paths, was implemented in a commercial FE code. All experimental tests were simulated in order to monitor the stress and strain evolution within the specimen, and to determine the onset of fracture. In the numerical simulations, the constitutive behavior was described using both a J2 and a J2J3 plasticity models. Thanks to the applied testing conditions, different stress states were induced in the material. A straightforward calibration of the accumulation model was achieved from few experiments, then the transferability of the formulation was validated on different tests. The proposed method showed appreciable improvements in final fracture prediction over a linear approach, as well as over other nonlinear approaches from the literature

A direct methodology for the calibration of ductile damage models from a simple multiaxial test

In the present work a straightforward calibration procedure of ductile damage models is proposed. The direct methodology involves the use of a simple multiaxial specimen, to be tested with a universal testing machine, capable to reproduce different stress states in the material. The specimen geometry was the one proposed by Driemeier et al. [1]. In addition, a numerical-analytical procedure was devised for the identification of material strains to fracture and corresponding stress states, directly from experimental tests. This allowed to overcome the use of Finite Element Analysis and inverse methods usually adopted to retrieve the local parameters representative of the material ductility

Design and Optimization of Dynamic Test Samples for Ductile Damage Assessment

The present research aims at assessing and comparing the damage evolution in a structural steel, mainly used in pipeline applications, both under quasi-static and dynamic conditions. Accordingly to the core of the literature related to plastic damage modelling, two key parameters must be controlled in the tests: the stress triaxiality and the Lode angle, both depending on the stress state. Either strongly affect the material strain to failure. Hence, different specimen geometries are needed to test the material in the desired ranges of these parameters. In this work, three kinds of geometries typically used in static tests, i.e. round and notched cylindrical, and thin rectangular, have been considered and adapted to an available Hopkinson bar facility. The shape of the specimens (diameter, fillet/notch radius, thickness, gauge length) and the incident pulse intensity have been studied within a multi-objective optimization scheme, in order to achieve similar strain rates for the three kinds of tests, with nearly constant time histories of strain rate, triaxiality and Lode angle during deformation. More specifically, the adopted solutions permitted to achieve an average strain rate of 3500 s-1, with varying triaxialities from 0.5 to 1.2, Lode angles from 0.5 to 1 and strains to failure from 0.8 to 1.5

Additive manufacturing structural redesign of hip prostheses for stress-shielding reduction and improved functionality and safety

Nowadays, the total hip arthroplasty (THA) is a widespread surgical procedure, representing the best option to restore hip joint mobility in patients suffering from trauma or joint diseases. One of the well-known possible drawbacks of THA is the stress-shielding phenomenon. Some years after the surgery, the femur starts to degrade because of its persistent unloaded condition induced by the high prosthesis stiffness, which carries the great part of the load normally taken by the bone. This condition is particularly invalidating in younger patients, with longer life expectation after the operation, requiring one or multiple additional operations to restore the proper prosthesis-bone firm connection. The present study tries to address this issue proposing an innovative prosthesis design, taking advantage of the shape freedom ensured by Additive Manufacturing techniques. Additionally, the structural integrity of the novel prosthesis is assessed using a ductile damage numerical approach. Different prosthesis geometries were investigated: one conventional and commercially available already, and two more innovative geometries. For each one, a bulk solution was compared to a lighter version characterized by an inner reticular structure with a body-centred cubic unit cell and by an equivalent density of about 5%, only feasible through the additive manufacturing fabrication. Extensive Finite Element numerical simulations were carried out to compare the percentage of the induced stress shielding for the different prosthesis geometries. Pros and cons of each geometry were pointed out and eventually the most promising solution in limiting the stress shielding phenomenon was chosen. At the same time, the structural integrity of the selected design was ensured, embedding a ductile damage model in the Finite Element analysis, calibrated on a SLM Ti6Al4V, the biocompatible alloy for the prosthesis fabrication. Structural safety was evaluated under four different loading conditions: walking, stumbling, the exceptional overload due to hammering insertion during surgery and the force which induced the collapse of the implant. Additionally, the safety margin was quantified through the definition of an overall safety factor under the maximum expected load

Full Field Strain Measurement of Dissimilar Laser Welded Joints

Laser welded tailored-blanks (TWBs) made of dissimilar advanced high strength steel (AHSS) sheets are used in the automotive industry to produce resistant and lightweight components. However, the stamping of AHSS TWBs poses some issues concerning the different formability of the parent materials and of the weld seams. Moreover, the weldment exhibits an inhomogeneous mechanical behavior due to the presence of a fusion zone and of a heat affected zone. For this reason, it is crucial to be able to assess the local mechanical properties of the welded joints in order to optimize the laser welding parameters for the fabrication of AHSS TWBs, and hence to guarantee proper formability of TWBs in the final stamping process. In this paper, digital image correlation is employed to investigate the mechanical properties of laser welded joints. Overall and local behavior are studied by means of two image acquisition systems working at different magnification levels and framing the front and the back side of a loaded specimen in a tensile test. The use of two image grabbing systems has been introduced to measure the strain field as close as possible to the weld seam where high strain gradients are present. To this purpose, it has been necessary to tune the parameters of the image correlation processing algorithm. This is because the method, working on image subsets on which the displacement field is modeled with low order shape functions, leans to smooth displacement gradients and strain singularities. By varying post-processing parameters an elaboration strategy has been suggested to capture the strain gradients correctly. Theapproach, here applied to laser welded joints, may be also used to measure the surface strain field wherever defects or inhomogeneity create strain discontinuities. © 2015 The Authors

Effects of Temperature and Strain Rate on the Ductility of an API X65 Grade Steel

In the last few decades, great effort has been spent on advanced material testing and the development of damage models intended to estimate the ductility and fracture of ductile metals. While most studies focused on static testing are applied at room temperatures only, in this paper, multiaxial tests have been executed to investigate the effects of dynamic action and temperature on the mechanical and fracture behavior of an API X65 steel. To this end, a Split Hopkinson Bar (SHB) facility for dynamic tests, and a uniaxial testing machine equipped with a high-temperature furnace, were used. Numerical simulations of the experiments were setup for calibration and validation purposes. Based on the experimental results, the Johnson–Cook and Zerilli–Armstrong plasticity models were first tuned, resulting in a good experimental–numerical match. Secondly, the triaxiality and Lode angle dependent damage models proposed by Bai–Wierzbicki and Coppola–Cortese were also calibrated. The comparison of the fracture surfaces predicted by the damage models under different loading conditions showed, as expected, an overall significant increase in ductility with temperature; an appreciable increase in ductility was also observed with the increase in strain rate, in the range of low and moderate triaxialities

Chainsaw vibrations, a useful parameter for the automatic tree volume estimations and production assessment of felling operations

An innovative approach for the automatic operational monitoring of motor-manual felling activities with chainsaw is here described and discussed. This new system of assessment can be considered as a solution for Precision Forestry (PF) applications and can be employed as a ICT tool for the management of the forest companies. Aim of the proposed system is to manage operational information such as: a) positioning of each felling operation inside the forest, b) measurement of the time spent to carry out the felling, c) estimation of the size of every felled tree, and in the end, d) the analysis of the productivity of the felling operations. This experience is based on the operative principle which considers that the lumberjack, during the actual cutting, drives the chainsaw with the engine at the highest number of rpm. During this action, the generated vibrations reach the maximum amplitude. Some preliminary experiments were conducted during thinning operations in a spruce stand (Picea abies), where a dedicated device composed by a triaxle accelerometer (sampling frequency of 10Hz) and a GNSS module were installed on the filter cover of a professional chainsaw (Husqvarna 560 XP). During the test, for each cut, the duration of the vibration with the maximum amplitude and the section cut area were measured and collected, with the assumption that a strong positive correlation exists between these two parameters. The test objectives were to validate such a correlation and to provide a methodology to estimate the volume of each cut plant. To this aim, once entered the diameter of each plant (derived from the estimated section), the volume for each felled tree was estimated through a specific single entry table commonly used by forestry manager. During two consecutive days of thinning operations, 30 trees were felled and monitored and all the operative parameters through the proposed automatic system were recorded. Simultaneously, a manual time study and diameter measurement using respectively watch and calliper, were performed. 20 of these records were used to create a mathematical model for the volume estimation, while the other 10 were used for its validation. Comparing the results obtained by the automatic time study with those from the manual survey, and with the identification of the felling location, very good correlation values were obtained (R2 >0.75). Beside this, a difference lower than 5% resulted by the comparison between the estimation volume carried out through the new approach and the classic one

Bending Fatigue Behavior of 17-4 PH Gears Produced by Additive Manufacturing

The introduction of Additive Manufacturing (AM) is changing the way in which components and machines can be designed and manufactured. Within this context, designers are taking advantage of the possibilities of producing parts via the addition of material, defining strategies, and exploring alternative design or optimization solutions (i.e., nonviable using subtractive technologies) of critical parts (e.g., gears and shafts). However, a safe and effective design requires specific resistance data that, due to the intrinsic modernity of additive technologies, are not always present in the literature. This paper presents the results of an experimental campaign performed on gear-samples made by 17-4 PH and produced via Laser Powder Bed Fusion (PBF-LB/M). The tests were executed using the Single Tooth Bending Fatigue (STBF) approach on a mechanical pulsator. The fatigue limit was determined using two different statistical approaches according to Dixon and Little. The obtained data were compared to those reported in the ISO standard for steels of similar performance. Additional analyses, i.e., Scanning Electron Microscopy SEM, were carried out to provide a further insight of the behavior 17-4PH AM material and in order to investigate the presence of possible defects in the tested gears, responsible for the final failure