The tensile flow properties of austenitic (S316-L) and martensitic (S410-L) stainless steel wall structures deposited by powder-fed laser additive manufacturing (LAM) process are evaluated. The ...properties obtained by the LAM process are compared to commercial rolled sheets of these metals. Strain-rate sensitivity, work hardening, and fracture behavior are assessed by conducting uniaxial tensile testing at different strain rates (0.001, 0.01, 0.1, and 1.0 sec−1). Moreover, a correlation between the final microstructure and mechanical properties is established for the LAM products through detailed analyses of grain structures and hardness indentation measurements. The results indicate a strong dependency for the strain rate in martensitic alloys compared to austenitic alloys produced by the LAM process. Interestingly, the tensile strength of commercial rolled martensitic stainless steel sheet doubles (∼100% increase) by increasing the strain rate, while preserving its elongation to failure. Comparing the two manufacturing methods, a lower strain-rate sensitivity factor is recorded for the additive manufacturing material (m of ∼0.0336) compared to the commercial sheet (m of ∼0.0775). This lower sensitivity is attributed to coarser grain structure and greater microstructural heterogeneity of the LAM product, which stems from directional solidification and cooling phenomenon during the layer-upon-layer deposition process. In contrast, the work hardening exponent (n value) varies little (0.1834–0.2854) for the different materials and manufacturing methods. Fractographic studies reveal that the fracture mode varies from ductile rupture towards ductile-brittle with the formation of greater martensitic phases, which is in combination with the failure component changing from shear to tensile at high strain rates.
A phase-field model of alloy solidification is coupled to a new heat transfer finite element model of the laser powder deposition process. The robustness and accuracy of the coupled model is ...validated by studying spacing evolution under the directional solidification conditions in laser powder deposition of Ti–Nb alloys. Experimental Ti–Nb samples reveal the microstructure on a longitudinal section with significant change in the size of the dendrites across the sample. Quantitative phase-field simulations of directional solidification under local steady-state conditions extracted from the results of the finite element thermal model confirmed this behavior. Specifically, the phase-field simulations agree with the results of the analytical model of Hunt in predicting a minimum spacing value, which is due to the mutual effects of the increasing temperature gradient and decreasing solidification velocity towards the bottom of the microstructure. This work demonstrates the potential of coupling the phase-field method to complex heat transfer conditions necessary to simulate topologically complex microstructure morphologies present in laser powder deposition and other industrially relevant casting conditions.
In cable-driven parallel manipulators (CPMs), cables can perform only under tension, and therefore, redundant actuation, which can be provided by redundant limbs, is needed to maintain the cable ...tensions. By optimizing the distribution of the forces in the cables and the redundant limbs, the average size of actuators can be reduced resulting in lower cost. Optimizing the force distribution in CPMs requires consideration for the inequality constraints imposed on the cable forces as a result of the unilateral driving property of the cables. In this study, a projection method is presented to calculate optimum solutions for the actuators force distribution in CPMs. Two solutions are presented: 1) a minimum-norm solution that minimizes the 2-norm of all forces in the cables and redundant limbs and 2) a solution that minimizes the 2-norm of the forces in the cables only. The optimization problem is formulated as a projection on an intersection of convex sets and the Dykstra's projection method is used to obtain the solutions. This method is successfully applied to a 3-DOF CPM.
Cable-actuated parallel manipulators (CPMs) rely on cables instead of rigid links to manipulate the moving platform in the taskspace. Upper and lower bounds imposed on the cable tensions limit the ...force capability in CPMs and render certain forces infeasible at the end effector. This paper presents a geometrical analysis of the problems to 1) determine whether a CPM is capable of balancing a given wrench within the cable tension limits (feasibility check); 2) minimize the 2-norm of the cable tensions that balance feasible wrenches; and 3) check for the existence of an all-positive nullspace vector, which is a necessary condition to have a wrench-closure configuration in CPMs. The unified approach used in this analysis is systematic and geometrically intuitive that is based on the formulation of the static force equilibrium problem as an intersection between two convex sets and the application of Dykstra's alternating projection algorithm to find the projection of a point onto that intersection. In the case of infeasible wrenches, the algorithm can determine whether the infeasibility is because of the cable tension limits or the non-wrench-closure configuration. For the former case, a method was developed by which this algorithm can be used to extend the cable tension limits to balance infeasible wrenches. In addition, the performance of the algorithm is explained in the case of incompletely restrained cable-driven manipulators and the case of manipulators at singular poses. This paper also discusses the algorithm convergence and termination rule. This geometrical and systematic approach is intended for use as a convenient tool for cable tension analysis during design.
Laser cladding is an advanced material processing technology that has potential to deposit various materials locally on highly non-planar and complex surfaces. It can be used to refurbish or improve ...corrosion, wear and other surface related properties of components. The laser cladding of WC–Co using continuous wave (CW) laser has been tried and problems, like—cracks, porosity, poor bonding, partial melting of WC particles in the Co matrix, etc., have been observed. To resolve these issues, the successful laser cladding with alternate binder materials, like—Ni, Fe, Co–Cr, Ni–B–Si, etc., have been reported. In the present study, a pulsed Nd:YAG laser was used to deposit multi-layer overlapped cladding on low carbon steel substrate using dynamic powder blowing technique. Thus, produced laser cladding samples were subjected to various mechanical tests and metallurgical analyses. The results showed that fully dense and crack free clad surfaces of WC–Co with an excellent metallurgical bonding and low dilution were deposited. No melting of WC particles in the Co matrix was observed during the microscopy. The average microhardness at the clad surface was about 1350
HV, while that at substrate was 200
HV. The observed adhesion strength of the WC–Co cladding to the substrate was about 60
MPa.
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In this research, powder-fed laser additive manufacturing based on directed energy deposition (DED) technology is utilized for 3D printing/fabrication of dissimilar alloy walls. ...Three-dimensional fabrication of metallic layers (A410-L stainless steel, A316-L stainless steel, and zirconium) with different crystallographic structures such as body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) are studied. Solidification cracking of the HCP metal structure inhibits the manufacturing of a sound dissimilar wall during direct deposition of zirconium on stainless steel. Different techniques are employed to eliminate cracking during the deposition of such dissimilar walls. First, a functionally graded transition layer is employed to gradually transition from a BCC or FCC layer to a zirconium layer. However, several longitudinal and horizontal cracks are observed in the functionally graded structure, deteriorating the overall integrity of the wall. Subsequently, inter-layers are applied to reduce the metallurgical differences between the BCC, FCC, and HCP metal structures. These inter-layers are designed to suppress the formation of brittle intermetallic compounds and decrease the level of thermal stress leading to cracking. Several metallic powders including nickel, titanium, vanadium, and copper are tested and analyzed as inter-layer materials during layer by layer fabrication of the dissimilar wall. Cross-sectional examinations are performed for each case, in order to study the feasibility of fabricating a sound dissimilar wall without deleterious phases. Embrittlement and presence of nickel-rich intermetallics within the Zr matrix restricts the successful fabrication of a dissimilar alloy wall through the formation of vertical cold cracks. Samples with titanium and vanadium inter-layers demonstrate horizontal hot solidification cracking at the stainless steel interface, which is attributed to the large solidification temperature range. Nonetheless, best results are attained with the copper inter-layer due to its high compatibility with both of the stainless steel and Zr metals.
This paper develops multiple fixed model blending based adaptive parameter identification schemes for multi-input multi-output (MIMO) systems with polytopic parameter uncertainty. The developed ...identification schemes are proven to be asymptotically stable for uncertain linear time-invariant (LTI) MIMO systems, and is shown to provide fast adaptation for even uncertain linear time-varying (LTV) systems. Furthermore, utilizing the proposed parameter identification schemes, a linear-quadratic (LQ) optimal multiple-model adaptive control (MMAC) scheme is developed for linear MIMO systems with polytopic uncertainties. The proposed MMAC scheme is proven to be asymptotically stable for LTI MIMO systems and applied to tracking control of uncertain lateral vehicle dynamics. A set of simulation test results are presented to verify the stability, effectiveness, and comparison of the proposed MMAC scheme.
The present work is the first to show the effect of processing cooling rate on phase structure and the resulting nano-scale deformation mechanisms in functionally graded stainless steels produced by ...directed energy deposition. To this end, fabrication by laser additive manufacturing involved implementing a closed-loop feedback control approach to monitor and control the peak temperature of the molten pool and the corresponding cooling rate. Single-track wall structures were produced from 316-L austenitic and 410-L martensitic stainless steels deposited using open-loop conditions with varying processing parameters (laser power and beam scanning speed) to produce graded microstructures. These were then compared to uniformly deposited layers where a constant cooling rate was maintained to generate homogenous microstructures using these two alloys, containing well-aligned columnar austenitic grains and random martensitic packets. Microstructural aspects (grains and crystallographic texture) across different layers (bottom, middle, and top) of the consolidated stainless steel walls (austenitic and martensitic) were compared under various processing conditions (without and with closed-loop or feedback control) and characterized using electron backscattering diffraction analysis. The nano-scale plastic deformation of these austenitic and martensitic stainless steel structures were assessed using nanoindentation testing over a range loading rates from 1 to 50 mN/s. The indentation response using varying strain rates was investigated based on the plastic flow indenting transition behavior at a high loading rate in order to correlate their dependence on microstructural features (predominantly for the martensitic phase). According to the rate sensitivity analyses, the gradient in grain structure (size and morphology) in the case of additively manufactured austenitic microstructure was not sensitive to loading rate, while the martensitic grains had a high local loading rate sensitivity.
Accurate information on vehicle longitudinal and lateral velocities is vital for the efficient operation of many vehicle control systems. In this paper, an estimation structure to simultaneously ...estimate vehicle velocities in longitudinal and lateral directions is developed and experimentally validated. This structure includes two parallel estimators: The first estimator is a kinematic-based observer for longitudinal velocity estimation, and the second is a combination of a kinematic-based observer and an inverse tire model to estimate vehicle lateral velocity. The proposed structure can effectively handle the additive biases, which are common in a vehicle's stock accelerometers' signals, and provide an accurate estimate of vehicle velocities when one (or more than one) wheel experiences excessive slippage. Additionally, the proposed structure is not sensitive to changes in parameters of the tire model and vehicle mass. The performance of this estimation structure is validated by experimental studies.
Vibration control plays a crucial role in many structures, especially in the lightweight ones. One of the most commonly practiced method to suppress the undesirable vibration of structures is to ...attach patches of the constrained layer damping (CLD) onto the surface of the structure. In order to consider the weight efficiency of a structure, the best shapes and locations of the CLD patches should be determined to achieve the optimum vibration suppression with minimum usage of the CLD patches. This paper proposes a novel topology optimization technique that can determine the best shape and location of the applied CLD patches, simultaneously. Passive vibration control is formulated in the context of the level set method, which is a numerical technique to track shapes and locations concurrently. The optimal damping set could be found in a structure, in its fundamental vibration mode, such that the maximum modal loss factor of the system is achieved. Two different plate structures will be considered and the damping patches will be optimally located on them. At the same time, the best shapes of the damping patches will be determined too. In one example, the numerical results will be compared with those obtained from the experimental tests to validate the accuracy of the proposed method. This comparison reveals the effectiveness of the level set approach in finding the optimum shape and location of the CLD patches.
