Joint Evolution Research Articles

Combined strengthening mechanism of solid-state bonding and mechanical interlocking in friction self-piercing riveted AA7075-T6 aluminum alloy joints

A recently developed friction self-piercing riveting (F-SPR) technique based on the combination of friction stir processing and riveting has been reported to possess both solid-state bonding and mechanical fastening characteristics. However, there is still a lack of quantitative understanding of the hybrid enhancement mechanism, hindering its engineering application. To fill in this gap, the current research investigated the microstructure evolution, microhardness distribution, and miniature-tensile performance of the aluminum alloy AA7075-T6 F-SPR joints by experiments. An accurate numerical simulation model was established to quantitatively evaluate the individual contributions of microstructure, local bonding strength, and macro interlocking to the performance of the joint, which could well explain the experimental results. It was found that due to the friction stirring of the rivet, solid-state bonding driven by dynamic recrystallization is realized between the trapped aluminum in the rivet cavity and the bottom aluminum sheet. The solid-state bonding zone has 75% yield strength, 81% ultimate tensile strength, and 106% elongation compared to the base material. This solid-state bonding enables the internal interlocking between the trapped aluminum and the rivet to withstand the additional load, which forms a novel dual-interlock fastening mechanism and increases the peak cross-tension force by 14.3% compared to the single-interlock joint.

Microstructure evolution and mechanical properties of Cu-Sn intermetallic joints subjected to high-temperature aging

In this work, full Cu3Sn joints were aged at 500°C, 550°C, and 600°C for various durations. Microstructure evolution and grain morphology of the joints were systematically investigated by OM, SEM, EBSD, and TEM technology. In addition, the mechanical properties of hardness and shear strength for the CuSn intermetallic compounds were studied using a Micro Vickers hardness tester and uniaxial tensile equipment. Upon heating to 500°C, the phase transformation pathway was Cu3Sn–Cu41Sn11–α(Cu); aging at 550°C demonstrated a pathway of Cu3Sn–Cu41Sn11–Cu41Sn11&α(Cu)–α(Cu); and finally, heating to 600°C resulted in a phase transformation pathway of Cu3Sn–Cu20Sn6–Cu20Sn6&α(Cu)–α(Cu). It was found that longer aging times and higher temperatures had effects on grain size and morphology. For the 500°C group, the average grain size of the fully formed Cu41Sn11 phase increased from 1.93 μm at an aging time of 20 min to 4.20 μm at 30 min. The grain sizes of the fully formed Cu41Sn11 phase in joints subjected to 550°C for 11 min was lower than 2.36 μm, which were in turn smaller than joints treated at 500°C for 30 min (4.2 μm). Treatment with higher temperatures resulted in more cores and a higher growth rate of Cu41Sn11 grains formed on the interface of Cu/Cu3Sn, which eventually led to the small grain size of the Cu41Sn11 phase. The grain morphologies of the formed Cu41Sn11 phase and Cu20Sn6 phase were mainly column grain. After an aging time of 11 min for the 550°C group, the grain morphology of α(Cu) included in a two-phase microstructure (Cu41Sn11&α(Cu)) mainly exhibited dendritic properties at the grain boundary of Cu41Sn11, its crack tendency broke the hardenability of single grains for the Cu41Sn11 phase, which resulted in a decline in strength from 107 MPa to 42 MPa for Cu41Sn11&α(Cu) compared to the Cu41Sn11 phase. Nevertheless, for the joints treated at 600°C after an aging time of 7 min, the dispersion of α(Cu) particles mainly followed granular shapes within the one large Cu20Sn6 grain in the two-phase microstructure (Cu20Sn6&α(Cu)). This blocked the extension of dislocation, which finally led to an enhancement in strength for the Cu20Sn6 & α(Cu) phase (51 MPa) compared to the Cu20Sn6 phase (73 MPa).

Effect of preheat & post-weld heat treatment on the microstructure and mechanical properties of 6061-T6 aluminum alloy welded sheets

MIG welding is one of the most predominantly welding techniques in manufactories due to its high automation. However, aluminum alloy welded joints fabricated using MIG welding often exhibit a reduction in mechanical properties. In this paper, an attempt has been made to combine preheat and post-weld aging treatment in double-pulse MIG (DP-MIG) welding process to produce 6061-T6 aluminum alloy welded sheets with high mechanical properties. Response surface methodology (RSM) was used to study the main effect and interactive effect of heat treatment parameters including preheat temperature, aging temperature and time on mechanical properties. On this basis, the optimum heat treat parameters that maximize both strength and ductility of welded joints have been determined using multi-objective optimization algorithm. Additionally, the influence of heat treatment parameters on the microstructural evolution and mechanical properties of welded joints was further investigated, by systematic experimental and theoretical analysis. The results showed the preheat temperature affects the grain size and precipitation of the welded joint, while the aging heat treatment mainly affects the precipitation and transformation of the strengthen phase. Since the aging temperature is of greater significance for the improvement of mechanical properties of welded joints, compared with other heat treatment parameters and their interactions, indicating the strengthening of precipitates determines the mechanical properties of welded joints.

An Observational Investigation of Spatiotemporally Contiguous Heatwaves in China From a 3D Perspective

AbstractUnderstanding the evolution of heatwaves is important for their prediction, mitigation, and adaptation. While most studies focused on either their temporal variability at individual station (or grid point) or the spatial variation over fixed durations, their daily joint evolution over space and time remains largely unexplored. Here, we investigate multi‐dimensional characteristics of spatiotemporally contiguous heatwaves across China during 1961–2018 using observational data from a 3D perspective (i.e., latitude × longitude × time). Results obtained by the 3D identification show that large contiguous heatwaves in China exhibit distinct characteristics in different geographical locations and time periods. Heatwaves in northern areas prefer to move from west to east at a faster speed, persist longer, and have greater extent and intensity than southern heatwaves, which are generally originated from the east and move westward. It is also found that contiguous heatwaves are growing in frequency, magnitude, areal extent, duration, and traveling distance across China.

Research on Reliability of Ni/Sn/Cu(Ni) Copper Pillar Bump Under Thermoelectric Loading

Abstract With the development of packaging devices toward high performance and high density, electronic devices are subjected to thermo-electric stresses under service conditions, which has become a particularly important reliability problem in micro-electronics packaging. The reliability of the chip under thermo-electric stresses is studied in this paper. First, thermo-electric coupling experiments were carried out on two solder joint structures of Ni/Sn3.5Ag/Cu and Ni/Sn3.5Ag/Ni. The interface evolution of solder joints under different current densities was analyzed. The reliability of the two structures under thermo-electric stresses was compared and analyzed. After that, three-dimensional finite element analysis was employed to simulate the current density, Joule heat, and temperature distribution of the flip chip. Finally, through the combination of experiment and simulation, the distribution of Joule heat and temperature of the chip was analyzed. The results show that the Ni/Sn3.5Ag/Ni structure has better reliability than the Ni/Sn3.5Ag/Cu structure under thermal–electric coupling. In addition, when the Ni layer was used as the cathode side, the constant temperature applied on the chip was 150 °C, and the current density was higher than 5 × 104 A/cm2, the dissolution failure of the Ni layer occurred in two structures. Because the higher current density generated a large amount of Joule heat where the current was crowded, resulting in excessively high temperature and rapid dissolution of the Ni barrier layer.

Herd Behaviors in Epidemics: A Dynamics-Coupled Evolutionary Games Approach

The recent COVID-19 pandemic has led to an increasing interest in the modeling and analysis of infectious diseases. The pandemic has made a significant impact on the way we behave and interact in our daily life. The past year has witnessed a strong interplay between human behaviors and epidemic spreading. In this paper, we propose an evolutionary game-theoretic framework to study the coupled evolution of herd behaviors and epidemics. Our framework extends the classical degree-based mean-field epidemic model over complex networks by coupling it with the evolutionary game dynamics. The statistically equivalent individuals in a population choose their social activity intensities based on the fitness or the payoffs that depend on the state of the epidemics. Meanwhile, the spreading of the infectious disease over the complex network is reciprocally influenced by the players’ social activities. We analyze the coupled dynamics by studying the stationary properties of the epidemic for a given herd behavior and the structural properties of the game for a given epidemic process. The decisions of the herd turn out to be strategic substitutes. We formulate an equivalent finite-player game and an equivalent network to represent the interactions among the finite populations. We develop a structure-preserving approximation technique to study time-dependent properties of the joint evolution of the behavioral and epidemic dynamics. The resemblance between the simulated coupled dynamics and the real COVID-19 statistics in the numerical experiments indicates the predictive power of our framework.

High Temperature Mechanical Properties and Microstructure Evolution of Ti-6Al-4V Alloy Linear Friction Welding Joints

The combination of linear friction welding (LFW) and hot forming processes is limited due to a lack of research on the high-temperature flow behavior and microstructure evolution of welded joints. In this paper, an electric-assisted high-temperature uniaxial tensile test platform based on digital image correlation (DIC) is built, and uniaxial tensile tests of Ti-6Al-4V alloy with LFW joints are performed at different temperatures (923–1023 K) and different strain rates (0.001 s−1–0.01 s−1). Then the microstructure of the LFW joints under different hot deformation conditions have been analyzed by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that the high-temperature flow behavior of LFW joints shows an obvious correlation between temperature and strain rate: the yield stress decreases from 203 MPa at 923 K to 105 MPa at 1023 K, and increases from 85 MPa to 130 MPa when the strain rate increases from 0.001 s−1 to 0.01 s−1 at 973 K. The hot deformation mechanisms with deformation conditions have been characterized, which changes from the mechanism of dislocation creep to the mechanism of self-diffusion as the deformation temperature increases from 923 to 1023 K. Especially, the fraction of high angle boundaries (HABs) rapidly rise from 49.2% to 64.1% with the increasing temperatures, the discontinuous dynamic recrystallization (DDRX) become the primary mechanism of nucleation during high-temperature deformation of LFW joints.

Microstructure evolution and hardness distribution of linear friction welded AA5052-H34 joint and AA5083-O joint

The microstructure evolution of the AA5052-H34 joint and AA5083-O joint fabricated by linear friction welding (LFW) was evaluated in a wide range of interfacial temperatures, i.e., applied pressures. The grain refinement of Al alloys with high stacking fault energy was found to be mainly dominated by discontinuous dynamic recrystallization (DDRX) at low temperatures. While, it was clear that grain refinement at high temperatures was dominated by continuous dynamic recrystallization (CDRX). In the joint of work hardened AA5052-H34 fabricated at low applied pressure of 50 MPa (above the 0.5 Tm), the annealing effect was more significant than the grain-boundary strengthening effect at the WC, leading to the softening. However, the increased applied pressure to 100 MPa resulted in the flat hardness distribution due to the increased strengthening by grain refinement. On the other hand, in the joint of annealed AA5083-O, the flat hardness was obtained at the lower pressure of 50 MPa, because both the low angle grain boundary length per unit area and dislocation density at the WC was almost equivalent to that of base material.

Iterative Machine Learning-Aided Framework Bridges Between Fatigue and Creep Damages in Solder Interconnections

Costly and time-consuming approaches for solder joint lifetime estimation in electronic systems along with the limited availability and incoherency of data challenge the reliability considerations to be among the primary design criteria of electronic devices. In this article, an iterative machine learning framework is designed to predict the useful lifetime of the solder joint using a set of self-healing data that reinforce the machine learning predictive model with thermal loading specifications, material properties, and geometry of the solder joint. The self-healing dataset is iteratively injected through a correlation-driven neural network (CDNN) to fulfill the data diversity. Outcomes show a very significant enhancement in lifetime prediction accuracy of the solder joint within a very short time. The effects of solder alloy and solder layer geometry are separately evaluated on the creep-fatigue damage evolution of the solder joint. The results reveal that Sn–Ag–Cu-based solder alloy generally has a better performance. Moreover, the creep and fatigue damage evolutions are found dominant, respectively, in Sn–Pb- and Sn–Ag–Cu-based solder alloys. The proposed framework offers a tool allowing for the reliability-driven design of electronic devices in the early stage of fabrication.

Quantum process in probability representation of quantum mechanics

In this work, the operator-sum representation of a quantum process is extended to the probability representation of quantum mechanics. It is shown that each process admitting the operator-sum representation is assigned a kernel, convolving of which with the initial tomogram set characterizing the system state gives the tomographic state of the transformed system. This kernel, in turn, is broken into the kernels of partial operations, each of them incorporating the symbol of the evolution operator related to the joint evolution of the system and an ancillary environment. Such a kernel decomposition for the projection to a certain basis state and a Gaussian-type projection is demonstrated as well as qubit flipping and amplitude damping processes.

Research on Microstructure Characteristics of Welded Joint by Magneto-Optical Imaging Method

This paper proposes an approach for analyzing the microstructure evolution of laser welding seam by magneto-optical imaging (MOI). The Faraday magneto-optical effect and magnetic domain theory are used to account for the MOI mechanism. The influence of laser welding on the welded joint was inspected by the analysis of color, grayscale and brightness of magneto-optical images. The relation between the brightness of magneto-optical image and grain size of microstructure is discussed as well, and the characteristics of magneto-optical images of weld microstructure were compared with those of scanning electron microscope (SEM) images. Experimental results show that three regions, including the weld zone (WZ), the heat-affected zone (HAZ), and the base metal (BM) are distributed in the magneto-optical image of the welded joint. The MOI method can investigate the microstructure evolution of welded joints and provide a theory and experimental basis for detecting weld defects.

The occurrence of deformation induced ferrite transition (DIFT) during back heating assisted friction stir welding pipeline steel: The influence on the toughness of welded joint

A pipeline steel was friction stir welded without and with preheat at 150 °C and 300 °C via back-heating plate. The microstructural evolution of the welded joints was studied and related to impact toughness. At conventional welding without preheating, bainite and martensite formed in the welded joint and semicircular ferrite phase band formed on the top surface of the welded joint because of the occurrence of deformation induced ferrite transition (DIFT). The semicircular band ferrite phase in the center and particularly at the advancing side embedded into the welded joint because of the spiral downward movement of the material. While, the shape of ferrite phase at the retreating side was flat on the top surface of the welded joint. The ferrite phase band split the integrity of the welded joint with embedding into the top surface. The crack easily propagated along the ferrite phase, leading to its dramatic deflection at the advancing side of welded joint during impact test, which eventually increased impact toughness greatly. But the crack propagated straightly at the center and retreating side and particularly at the retreating side, and the impact toughness was quite low with the existence of brittle martensite phase. With preheating to 150 °C and 300 °C, only bainite phase formed in welded joints, and the DIFT gradually weakened with the decrease of frictional force between the stirring tool and the top surface of the welded joint. As a result, the formation of semicircular ferrite phase band weakened, and only a little of it formed on the top surface of 300 °C preheating welded joint. Finally, the dramatic deflection of the crack at the advancing side occurred at 150 °C preheating welded joint, but disappeared at 300 °C preheating welded joint. Without the formation of brittle martensite phase, all positions in the stir zone of 150 °C and 300 °C preheating welded joints showed excellent impact toughness. However, as high preheating temperature led the coarsening of bainite lath, the impact toughness of 300 °C preheating welded joint was slightly lowered than that at 150 °C. Hence, medium preheating temperature is recommended in back heating assisted friction stir welding pipeline steel.

Shearing Damage Evolution of Natural Rock Joints with Different Wall Strengths

Shearing Damage Evolution of Natural Rock Joints with Different Wall Strengths

Labour market protection across space and time: A revised typology and a taxonomy of countries’ trajectories of change

We measure and interpret the evolution of labour market protection across 21 high-income countries over three decades, employing as conceptual foundations the ‘regime varieties’ and ‘trajectories of change’ developed by Esping-Andersen, Estevez-Abe, Hall and Soskice, and Thelen. We measure labour market protection considering four institutional dimensions – employment protection, unemployment protection, income maintenance and activation – and the evolution of the workforce composition. This measurement accounts for the joint evolution of labour market institutions, their complementarities and their relation to outcomes, and mitigate the unrealistic Average Production Worker assumption. We handle the multi-dimensional nature of labour market protection with Principal Component Analysis and capture the characteristics of countries’ trajectories of change with a composite score. We contribute to the literature in three ways. (1) We portray a revised typology that accounts for processes of change between 1990 and 2015, and that clusters regime varieties on the basis of coordination and solidarity levels, that is, Central/Northern European, Southern European, liberal. (2) We illustrate that, despite a persistent gap, a large majority of Coordinated Market Economies experiencing a decline in the level of labour market protection became more similar to Liberal Market Economies. (3) We develop a fivefold taxonomy of countries’ trajectories of change (liberalization, dualization, flexibility, de-dualization and higher protection), showing that these trajectories are not always path-dependent and consistent with regime varieties previously developed in the literature.

Microstructural Evolution of Cu/Sn-58Bi/Sn-3.0Ag-0.5Cu Composite Solder Joint during Isothermal Aging

A composite solder joints made of Cu/Sn-58Bi/Sn-3.0Ag-0.5Cu (in weight percent) was successfully fabricated. The composite solder was made from combining a Sn-3.0Ag-0.5Cu solder ball and Sn-58Bi solder paste. The composite solder was reflowed on Cu-OSP (copper-organic solderability preservatives) substrate to form a composite solder joint. The microstructure evolution of the composite solder joints under different reflow temperature and thermal aging conditions were investigated using an optical microscope. This study shows that a composite solder joint of SAC305/Sn-58Bi can be assembled at lower temperature (160 °C) and gradually mixed through isothermal aging.

Study of Influences of Groove Type on Welding Residual Stress Evolution in Bimetallic Clad Plate Joints during Multi-layer and Multi-pass Welding

Study of Influences of Groove Type on Welding Residual Stress Evolution in Bimetallic Clad Plate Joints during Multi-layer and Multi-pass Welding

Failure mechanisms and damage evolution of hard rock joints under high stress: Insights from PFC2D modeling

Rock joints can be subjected to high normal stress in deep rock projects such as deep mining, tunnelling, and oil and gas exploitation. The spatial and temporal damage evolution and distribution of rock joints cannot be directly observed due to the opaque nature of rocks. Discrete element modeling provides a good alternative solution to visualize the development of micro-cracks under complex geological conditions. In this study, 2D Particle Flow Code (PFC) modelling is conducted to shed light on the failure mechanism and damage evolution of hard rock joints, especially under high stress. The influence of normal stress, initial joint roughness and grain size on the shear behavior and micro-damage are examined, and the spatial and temporal distribution of micro-cracks are also compared and quantified. Results indicate that more cracks develop for a joint sheared under a higher normal stress, and a rougher joint also tends to generate more cracks. On the other hand, the spatial distribution of shear-induced micro-cracks becomes more unevenly distributed with increasing normal stress and roughness. Three stages, namely slow increase, fast increase and slow increase can be divided on the temporal cumulative crack development curve. The micro-cracks grow faster in the fast growth stage for a rougher joint sheared under a higher normal stress, which is consistent with the acoustic emission monitoring in the laboratory direct shear tests. Both peak shear strength and damage zone size are affected by the particle size, which is closely associated with the more interlocked effect of large grains on the two sides of the shear interface. The maximum length of the crack is dependent on the magnitude of contact force, rather than the shear displacement, and the maximum contact force tends to be larger for the rougher joint.

A continuous heterogeneous-agent model for the co-evolution of asset price and wealth distribution in financial market

The heterogeneous agent model (HAM) is a powerful tool to study the stock price dynamics and stylized anomalies in financial markets, such as the volatility clustering and long-range dependence of stock returns. The existing HAMs often require the perfect arbitrage condition and relatively strong behavioral assumptions on agents’ expectation formation, which are not sufficient to capture the complexity in real finance market. To fill the theoretical gap, we propose a novel continuous version of HAM that relies on weaker assumptions, in which heterogeneous agents are identified continuously by their wealth and the ex-post price expectations no matter how they are formed. Given the non-parametric distribution on the continuous agent space and the bounded arbitrage condition, we establish the joint evolution equations for stock price and wealth distribution. Within the family of mixed-exponential distributions, we derive an explicit expression for the equations and prove that the evolution equation of wealth distribution has the fascinating splitting-mixing property. An endogenous generating mechanism for agent types arises naturally from the splitting-mixing property, which links the continuous HAM to the classical HAMs with finite agent types. By numerical simulation, we show that the wealth dynamics tends to positively associate the weight of each generated agent type with their averaged investment performance, while the price dynamics experiences the period-doubling bifurcation and chaotic bifurcation. By introducing random shocks into the continuous HAM, we fit the resulting return series via a GARCH model. Based on the significance of the fitted GARCH parameters, a wide range of model parameters are identified by which the continuous HAM can simultaneously generate volatility clustering, long-range dependence and other stylized anomalies in financial markets.

Microstructural evolution and fatigue performance of dissimilar solid-state joints of SUS316L and SUS410

Dissimilar stainless steels 316L (SUS316L) and SUS410 in a cylindrical butt configuration are solid-state joined along the cylinder axis by electrically assisted pressure joining (EAPJ). In EAPJ, different electric current densities are applied during compressive plastic deformation to investigate the effect of electric current density on joint performance. The microstructure analysis confirms that solid-state joints without macro- and micro-defects are fabricated through elemental diffusion and recrystallization during EAPJ. The effect of electric current density on the formation of martensite and subsequently on the variation of hardness is revealed by the grain average image quality maps and their relevant area distribution. The results of quasi-static tensile tests show that the fracture behavior of the joint is strongly affected by the applied electric current density. The fatigue behavior of joints with a higher tensile strength (SUS410 base metal fracture) is also evaluated by the S–N curve under a certain confidence level using two-parameter Weibull distribution. Finally, the fracture mechanism is investigated by examining the fracture surfaces from fatigue failure. The fracture morphology under high stress level exhibits a mixed fracture mode of base metal and interface fracture, while the fracture under low stress level exhibits interface fracture.

Microstructure evolution and mechanical properties of continuous drive friction welded dissimilar copper-stainless steel pipe joints

In the present investigation, the microstructure evolution and mechanical properties of dissimilar materials Copper-Stainless Steel pipe joints welded by continuous drive friction welding under two different processing conditions are analyzed. The processing conditions of friction welding for copper-stainless steel joints are varied by two friction times of 10 s and 15 s while keeping other processing parameters constant. The welded specimens are analyzed for materials characterizations and mechanical testing using optical microscopy and scanning electron microscopy, electron dispersive x-ray spectroscopy, electron backscatter diffraction analysis, X-ray diffractions, tensile testing, and microhardness measurements. The results revealed that the major microstructural evolution is observed at the Cu side with dynamic recrystallized zones. Enhanced metallurgical bonding between Cu-SS materials is obtained with microstructural evolutions (such as full dynamic recrystallized zone at Cu side and quenching zone at SS side) near to Cu-SS interface, in case of weld made by friction time of 15 s. Superior interatomic diffusion leading to enhanced metallurgical bonding is evidenced for weld made by friction time of 15 s. The reaction layer thickness influences the bonding and mechanical properties of Cu-SS friction welds. The reaction layer thickness of 17.28 μm is observed for the weld made by friction time of 10 s, whereas the reaction layer thickness of 1.21 μm is observed for the weld made by friction time of 15 s. The ultimate tensile strength of 181.05 MPa is obtained for Cu-SS friction weld.