Optimal Joining Research Articles

Advancements in joining Al-Zn-TiC-Mg composites using friction stir welding process: Influence of traverse speed

In this article, dissimilar magnesium and aluminum alloys were welded with a Zn interlayer and TiC nanoparticles by friction stir welding. Optimal joining conditions were achieved by a combination of three traverse speeds (30, 45, and 60 mm/min) and constant rotational speeds (1050 rpm). The best microstructure evolution and mechanical properties were achieved for specimens joined at rotational and traverse speeds of 1050 rpm and 45 mm/min, respectively. The grain size decreases as the traverse speed increases from 30 to 45 mm/min due to a reduction in heat input, an improvement in reinforcing distribution, and high intermixing of materials, then increases from 45 to 60 mm/min due to inadequate heat input for recrystallization process. It was shown that the TiC particles play a prominent role in the microstructure modification and enhance mechanical properties of weld samples while the Zn foil interlayer plays a vital in avoiding the formation of Al-Mg IMC phases. The obtained result under optimal welding parameters indicates that MgZn2, Mg-Al-Zn compounds, Mg and Al solid solution, were the main detected common phases in the stir zone instead of the brittle and hard Al-Mg IMCs formation. The average hardness values of 232 Hv were achieved, while the strength of the weld specimen experiences the 189 MPa value. In addition, a combination of brittle and ductile modes was observed based on the fracture surface of the weld sample after the tensile test.

Distributionally Robust Observable Strategic Queues

This paper presents an extension of Naor’s analysis on the join-or-balk problem in observable M/M/1 queues. Although all other Markovian assumptions still hold, we explore this problem assuming uncertain arrival rates under the distributionally robust settings. We first study the problem with the classical moment ambiguity set, where the support, mean, and mean-absolute deviation of the underlying distribution are known. Next, we extend the model to the data-driven setting, where decision makers only have access to a finite set of samples. We develop three optimal joining threshold strategies from the perspectives of an individual customer, a social optimizer, and a revenue maximizer such that their respective worst-case expected benefit rates are maximized. Finally, we compare our findings with Naor’s original results and the traditional sample average approximation scheme. Funding: This research was supported by the National Science Foundation [Grants 2342505 and 2343869].

Fabrication of reinforced Al–Mg composite by TiC particles via FSW: Microstructure and tribology study

In this article, wear and microstructure behaviors of dissimilar Al–Mg joints made by friction stir welding (FSW) with the addition of TiC nanoparticles under different welding parameters were investigated. Optimal joining conditions were achieved by a combination of various rotational and traverse speeds and the best microstructure evolution, mechanical properties, and tribological behaviors were achieved for specimens joined at rotational and traverse speeds of 850 rpm and 50 mm/min, respectively. It was shown that the TiC particles play a prominent role in the microstructure modification and enhanced mechanical properties of weld samples due to the recrystallization process and pinning effect. The maximum hardness and strength values of 241 HV and 168 MPa, respectively were obtained for the weld sample made by optimal condition. The friction coefficient and wear volume loss for weld samples fabricated at 850 rpm and 50 mm/min is comparatively less compared to other conditions due to the highest grain refinement, homogenous distribution of particles, and highest hardness characteristics. The major wear happened because of adhesion, abrasion, plastic deformation, delamination, and oxidation.

Estimation of stationary optimal transport plans

Abstract We study optimal transport for stationary stochastic processes taking values in finite spaces. In order to reflect the stationarity of the underlying processes, we restrict attention to stationary couplings, also known as joinings. The resulting optimal joining problem captures differences in the long-run average behavior of the processes of interest. We introduce estimators of both optimal joinings and the optimal joining cost, and establish consistency of the estimators under mild conditions. Furthermore, under stronger mixing assumptions we establish finite-sample error rates for the estimated optimal joining cost that extend the best known results in the iid case. We also extend the consistency and rate analysis to an entropy-penalized version of the optimal joining problem. Finally, we validate our convergence results empirically as well as demonstrate the computational advantage of the entropic problem in a simulation experiment.

Bi-objective optimization and reliability analysis in unreliable M/G/1 retrial queues with rational customers, reserved idle times and setup times

ABSTRACT Energy consumption arising from no-loads and failures is a common and challenging problem in the design of service systems. To analyze the impact of no-load consumption and failure consumption on the energy consumption simultaneously, this paper considers an M/G/1 retrial queue with reserved idle times, setup times and server breakdowns. First, the steady-state condition and performance measures of the system are obtained by the embedded Markov chain and the supplementary variable method. Then, the reliability indicators of the system are analyzed, and an algorithm for solving the system reliability R ( t ) is constructed based on the Monte Carlo method. In order to better optimize the system, the customer’s equilibrium joining strategy is derived, and the socially optimal joining strategy is obtained by the particle swarm optimization (PSO) algorithm. From the decision-maker viewpoint, a bi-objective optimization model is constructed to minimize the expected energy consumption and the expected waiting time of retrial customers, and solved by the non-dominated ranking genetic algorithm II (NSGA-II). Finally, we illustrate an application of the proposed analytical approach in the cloud computing data center, which makes it possible to achieve an appropriate balance between energy consumption and quality of service (QoS).

Optimal joining strategies in a repairable retrial queue with reserved time and N-policy

Optimal joining strategies in a repairable retrial queue with reserved time and N-policy

Joining of additively manufactured fiber-reinforced thermoplastic and metals by ultrasonic energy: Mechanical and corrosion behavior

Ultrasonic joining (U-Joining) is applied to create reinforced hybrid joints between unreinforced or fiber-reinforced polymers and surface-structured metals. While the feasibility of joining additively manufactured (AM) materials was already demonstrated, a detailed description of the process parameters’ effects is still missing. Therefore, this study aims to define the influence of the U-Joining parameters on the mechanical and corrosion properties of AM 20% short-carbon-fiber-reinforced poly-ether-ether-ketone (PEEK-20CF) and 316L stainless steel (316L SS) hybrid joints. Optimal joining parameters were determined via Box-Behnken design of experiments to maximize the joints’ ultimate lap shear forces (ULSF) and displacement at break (DaB), where the results increased from 1.4 ± 0.2 kN to 3.6 ± 0.3 kN (2.6 times) and from 1.2 ± 0.3 mm to 2.4 ± 0.1 mm (2 times), respectively. Microstructural analyses revealed a strong micromechanical interlocking between the parts due to the filling of crevices at the as-built AM metallic part’s rough surface by consolidated PEEK-20CF, which resulted in a mixture of adhesive, cohesive and net tension failure during the lap-shear tests. Finally, the corrosion resistance was also evaluated by potentiodynamic polarization curves, showing that the energy input during the joining cycle slightly reduced the pitting corrosion resistance of the 316L part.

Analysis of strategic customer behavior in observable M/G/1 queues with impatience

ABSTRACT In this paper, we propose a queueing-game-theoretic model and analyze the strategic behavior of customers and social optimization in an observable M / G / 1 queue, in which arriving customers decide whether to join the system or balk based on a new binary and random reward-cost structure. Each incoming customer to the queue has a relative tolerance time. If the customer’s service does not begin (or end) within his or her relative tolerance time, the customer will incur a cost for his or her waiting. We first derive closed-form solutions for customers’ equilibrium and socially optimal joining strategies using the technique of the Laplace-Stieltjes transform. Furthermore, some representative numerical experiments are performed to visualize the theoretical results. The numerical scenarios illustrate the influence of the relative tolerance time on equilibrium strategy and socially optimal strategy. Finally, we compare the effect of relative tolerance time on social welfare in observable and unobservable queues. The numerical results show that observable queues lead to higher social welfare. This study provides guidance for system providers in designing more economical and sustainable public service systems.

Low-resistance joints for YBCO-coated conductors with Ag nanoparticle paste

Due to the limited available piece length of YBCO-coated conductors (i.e. tapes or wires) and the different requirements for magnetic field, joints are inevitable for manufacturing high-temperature superconducting magnets. In this study, a sintering nano-silver (Ag) process was developed and used to connect YBCO tapes stabilized by anAg layer with low-temperature and short-time sintering of Ag nanoparticle (NP) paste. The thermodynamic characteristics of Ag NP paste were explored using a TG/DSC setup. The effects of sintering temperature, mechanical pressure and lapped length on microstructures and electrical properties of joints were comprehensively investigated. It is found that the pre-volatilization of low-boiling-point solvent in the paste is beneficial to improve the densification of sintered structure, thus contributing to increasing the critical current I c of the joint. With increasing sintering temperature, the I c of the joint will be close to that of the virgin tape, and the joint resistance experiences small fluctuations, but joint connectivity is enhanced. As the temperature reaches 205 °C, I c decreases to 84% of the virgin tape, and joint resistance increases obviously. In addition, the axial tension strength at room temperature is improved with the increase in mechanical pressure, while the resistance does not demonstrate distinct variation. Considering the electromechanical properties, the optimal joining process is determined as sintering at 180 °C and 30 MPa for 10 min. The joint with this technology possesses a closely connected interface and a well-sintered nano-Ag microstructure with pores. By further extending the lapped length, a YBCO joint resistivity as low as ∼10.56 nΩ cm2 is obtained, which is around a quarter of that of the soldering joint, and the process is much easier than that of the Ag diffusion joint.

A single server retrial queue with event-dependent arrival rates

In this work, we consider a novel single-server retrial queue with event-dependent arrival rates. Contrary to other related works, the primary customers’ arrival rates depend on the last realized event, which refers either to a departure, or to an arrival of either type, or to when a customer arrives during a busy period, compared with others. Our motivation stems from the modeling of service systems, in which the customers express their willingness to join the system based on the last realized event. We investigate the stability conditions, and derive the stationary distribution both at service completion epochs, and at an arbitrary epoch. We also study the asymptotic behaviour under high rate of retrials. Performance measures are explicitly derived, and extensive numerical examples are performed to investigate the impact of event-dependency. Moreover, constrained optimisation problems are formulated and solved with ultimate goal to derive optimal joining probabilities.

Equilibrium and social optimality in queues with service rate and customers’ joining decisions

ABSTRACT This paper treats a single server Markovian queueing system with changeable service rates and customer choice. When a customer arrives at an empty system, he is served by the server at a low service rate. This low service rate continues until the queue length reaches a threshold. Then the server switches to a high service rate to serve customers who join the system. The high service rate continues until the system becomes empty and a low service rate starts again. Under a service reward and waiting cost structure, customers decide to join or to balk based on their net utility of joining the system. The customers’ equilibrium and socially optimal joining strategies are studied under two information scenarios which are observable and unobservable queues, respectively. Such a model fits a stochastic service system where the server can make his service rate decision and customers can make their joining or baking decisions and thus has wide applications in service industries. Our focus is on examining the equilibrium performance of the queue under the interaction between the server’s service rate and customers’ joining decisions. We develop the steady-state distribution and performance measures of the system by formulating two-dimensional continuous-time Markov chains.

Advances in simulation and experimental study on intermetallic formation and thermomechanical evolution of Al–Cu composite with Zn interlayer: Effect of spot pass and shoulder diameter during the pinless friction stir spot welding process

In this study, the pinless friction stir spot welding of aluminum–copper composite with Zn interlayer was analyzed under experimental measurement and finite element method. The role of shoulder diameter, 16, 18, and 20 mm, and the number of spot pass, one to three pass, on microstructure, mechanical properties, and thermal history of weld samples were investigated. Based on the obtained results, there is a good agreement between numerical data and experimental analyses. It was shown that the heat source, due to plastic deformation and friction, increased as the shoulder diameter was increased, whereas the stress distribution in the weld samples was reduced. In addition, the thickness of the Zn interlayer at the joint interface changes when shoulder size increases from 16 to 20 mm, due to high temperature and intermixing between materials. From the microstructure analysis, the grain size in the joint zone gradually decreases as the spot pass increases from 1 to 3. It was concluded that the shoulder diameter of 16 mm with three spot passes showed the best result of 7.45 kN. Depending on X-ray diffraction analyses of the fracture surfaces, three primarily intermetallic phases including the Al2Cu, CuZn5, and CuZn2 were determined in the weld interfaces. Based on finite element method analysis, the axial compressive stresses showed the lowest profile as the shoulder diameter of the tool is 16 mm. Finally, the ductile fracture was detected as the main fracture mechanism for the joint sample with optimal joining conditions.

Energy harvesting cognitive radio networks with strategic users: A two-class queueing model with retrials

Motivated by the rapid growth in wireless communication systems, we develop a single-server queueing model with service retrials for the cognitive radio network (CRN). A key appeal of CRN is its flexible allocation scheme of limited spectrum for two user groups: (1) the licensed user, hereby referred to as primary user (PU), and (2) unlicensed users, referred to as secondary users (SUs). The PU take priority over SUs: when a PU arrives and observes a busy server currently occupied by an SU, it will take over the server by preempting the SU out of service. SUs are strategic and delay sensitive: they decide immediately upon arrivals on whether to join the CRN or to balk, based on the anticipated expected utility (service reward minus delay cost). In case the server is unavailable, SUs temporarily join an orbit queue where they wait for future accesses to service. Another realistic feature of our model is that, after successfully serving a user (PU or SU), the server becomes unavailable and undergoes an energy harvesting phase of a random time. We characterize the equilibrium system performance in the following three steps: First, we obtain the equilibrium joining behavior for utility-maximizing SUs; second, we derive the optimal joining strategy from the perspective of a social planner; and finally, we enforce that users adopt the socially optimal strategy by imposing an admission fee. We also investigate the impact of the energy harvesting time on users?? equilibrium strategies and report numerical experiments to provide qualitative and quantitative insights of our results.

Strategic joining rules in unobservable queues with dynamic service rate

Abstract This paper considers an unobservable queue with adjustable service rate and strategic customers. A queueing-game-theoretic model is built to capture the interaction between the server’s service rate and customers’ joining decisions. Iterative and recursive methods are used to derive the steady-state distribution and the expected sojourn time in the queue. We obtain customers’ equilibrium and socially optimal joining strategies under two information scenarios which are unobservable and partially unobservable queues, separately. It is found that there are four equilibrium joining strategies at most in the fully unobservable queue and two equilibrium joining strategies in the almost unobservable queue. Interestingly, the social optimal arrival rate is between the minimum and maximum stable equilibria. Thus, in most cases, managers need to charge a price to induce the social optimal customers’ behaviour. However, if the minimum equilibrium is achieved, managers are required to provide a subsidy to maximize the social welfare. unobservable queue; $(m, N)$ policy; equilibrium strategy; social welfare.

Material-specific predicting of the optimal joining parameters for the screw blind rivet joining process

The screw-blind riveting combines direct screwing with the blind riveting. The novel joining element can be integrated into every injection-molded component like a standard screw boss without significantly increasing the processing time. The screw-blind-rivet (SBR) consists of a screw boss connected to a substrate by a thin sleeve. The joining process of screw-blind-riveting can be divided into two phases. The first is the screw-in phase. The screw is secured into the SBR. The second phase is the forming phase. Further turning of the screw will cause the sleeve to buckle out and form an undercut. To qualify the connection element for industrial use, in addition to the strengths of the design, suitable connection parameters are required for the user. The independent parameters of the joining process are the contact pressure of the screw and the screwing-in speed. The rate is reduced to avoid damage and ensure a proper alignment. The speed after the headrest is an additional influencing parameter. The most influencing parameter for the undercut is the rotation angle after the initial screw placement as it affects the shape of the undercut. The larger the undercut is, the more stable the rivet. However, the larger the undercut is, the more deformations and material damage occur. Thus, the optimal angle for the forming depends on the material of the rivet. Consequently, the deformation angle must be selected depending on the material properties. For this, various materials were tested. The results show a well predictability of the optimum joining parameters by using the elongation at yield stress of the used material.

Laser-assisted joining of carbon fiber reinforced polyetherketoneketone thermoplastic composite laminates

Carbon-fiber-reinforced thermoplastic (CFRTP) materials have drawn considerable interest owing to their high strength-to-weight ratio. Laser joining technologies have been reported to join CFRTP to metals or plastic materials. However, the direct laser joining of CFRTP/CFRTP is challenging. Herein, we demonstrated the feasibility of laser direct joining of 2.2-mm-thick CFRTP/CFRTP by laser heat conduction. A thulium fiber laser (1940 nm) was used under the optimal joining conditions of 20 W power (beam diameter = 5.5 mm, approximately) and 3 min laser irradiation time. We observed that an increase in the laser irradiation spots per unit area results in a higher joining strength. The joining strengths of carbon fiber (CF)-polyetherketoneketone (PEKK)/CF-PEKK composites were evaluated by single lap shear tests, and the highest average interlaminar shear strength result was 30.9 MPa. Our study indicates that the laser-assisted joining of CF-PEKK/CF-PEKK laminates is viable and worthy of further exploration for industrial applications.

Strategic access in a Green IoT(Internet of Things) system with an unreliable server

The present research aims to study the strategic behaviour of users in an Internet of Things(IoT) system that is impeded by an unreliable server. In the system, an IoT device is equipped with an energy harvesting unit. The IoT system under study is employed as a Markovian retrial queueing system that is used in a single server, subject to active failures. The failed server is repaired immediately, while the user who was served before the system’s breakdown event waits in the server until the server is repaired. The generating function approach is used to identify important system performance metrics. Moreover, queueing theory concepts are incorporated into game theory for the analysis of the users’ strategies. The users in the IoT system, act to maximize their expected benefit without being perturbed by other users in the system and determine their equilibrium joining strategies. Furthermore, taking into account social welfare factors, the IoT device acts as a social planner, and as a consequence, socially optimal joining strategies are also analysed. Finally, the analytical findings are validated with numerical examples.

Transition between flow-drill screwing systems considering joining process and joint characteristics

Flow-drill screwing is one of the key joining technologies for car body structures in multi-material lightweight design. In the course of technological developments and subsequent volume production of a product, different assets are used to obtain the same joints, assuming that similar processes yield the same joint characteristics. Since a simple transfer of the process parameters for joining the same materials is usually not possible, a remarkable experimental effort is required to meet manufacturing requirements. In this study the transition to an enhanced flow-drill screwing system and its effects on the joint is investigated. For this purpose, two flow-drill screwing systems typically used in the automotive industry are considered. An application-oriented approach for determining the joining parameters is shown. First, the optimal joining parameters for the target system were determined based on the process curves and parameters of the initial system by fulfilling the requirements for the joint. The joints were evaluated by using cross sections and single-lap shear tests. On this basis, the results of both flow-drill screwing systems were compared. Due to the further development of the flow-drill screwing system the process times can be significantly shortened while achieving the same mechanical properties and better process control at the same time.

Efficient Joining Sequence Variation Analysis of Stochastic Batch Assemblies

Abstract Geometric variation causes functional and aesthetic problems in the assemblies. The challenge is predicting the moments of the distribution of geometric deviations of assemblies to evaluate compliance with the set requirements. The joining operation, i.e., resistant spot welding (RSW), is one of the most crucial steps in the assembly process of nonrigid components, imposing forces on the parts and causes bending and deformation during the assembly, consequently contributing considerably to the final geometric outcome of the assembly. To model the behavior of the assembly realistically and achieve accurate simulation results, considering the sequence of joining is essential. In a digital twin of the assembly process, joining sequences need to be provided for the optimal geometric outcome of the batch of assemblies. The sequence optimization of the joining processes is a time-consuming combinatorial problem to solve. Variation analysis of nonrigid assemblies with stochastic part inputs, including optimal joining sequences, requires an extensive amount of the computational effort. More efficient approaches for evaluating assembly geometric variation are desired. In this article, a computationally efficient approach is proposed for geometric variation analysis and optimization of nonrigid assemblies with stochastic part inputs with respect to the RSW sequences. A clustering approach is proposed categorizing the incoming parts based on the part variation. Sequence optimization is performed, and geometric variation is analyzed for each cluster. The results show that the proposed method drastically reduces the computation time needed for sequence optimization compared to individualized optimization for each assembly.

On the role of input welding parameters on the microstructure and mechanical properties of Al6061-T6 alloy during the friction stir welding: Experimental and numerical investigation

The Taguchi method was employed to find the optimum values of friction stir welding parameters including welding speed, rotating speed, and tilt angle for joining AA6061-T6 aluminum alloys. The combined influences of these parameters were entirely analyzed. Statistical outcomes were investigated by the study of variances and signal-to-noise ratios. A Coupled Eulerian and Lagrangian technique is implemented to simulate and verify the optimal parameters during the friction stir welding. To verify results, a comparison between the welding process under optimized parameters with experimental and non-optimized parameters was simulated for the friction stir welding process. The material flow, strain rate, thermal behaviors, and mechanical properties of samples fabricated with optimal welding parameters are higher than those produced from the non-optimal parameters. It was also concluded that the grain size of the stir zone under optimal welding parameters (6–8 µm) is finer than that of non-optimal welding parameters (11–13 µm). Low uniform distribution of material element and coarse microstructure were some of the results of welding with non-optimized parameters. Based on residual stress analysis, the application of optimal joining conditions can decrease the peak tensile residual stress by about 38.3%. The much desirable results obtained in terms of microstructure and mechanical properties could be of great significance to the welding industry.