With the development of fields such as aviation， aerospace， and navigation， the service conditions for high-end equipment have become increasingly stringent， placing higher demands on the manufacturing industry. Additive manufacturing technology， also known as 3D printing technology， has significant advantages over traditional manufacturing techniques in producing complex shapes and structures， and it is expected to achieve specific location printing and structural printing with unique properties in three-dimensional space. Wire-based laser directed energy deposition （W-LDED） technology， as an important branch of additive manufacturing， has notable advantages such as high efficiency， high precision， and high material utilization， making it promising for applications in the manufacturing of high-end equipment. Despite the many advantages of W-LDED technology， there are still numerous challenges regarding the selection of process parameters， multiple thermal cycles， and the precise control and repeatability of the manufacturing process. The deposition quality and manufacturing stability are influenced by various factors， and addressing these current challenges is a key focus of research both domestically and internationally. Based on this， this paper provides a detailed introduction to the current research status of W-LDED technology from three aspects： process parameter optimization， deposition quality analysis， and microstructural composition control. It analyzes the impact of different parameters on forming quality and manufacturing stability， proposes optimization strategies， summarizes the current application scenarios of W-LDED technology， and presents ideas for the future development trends of this technology，including material innovation and the development of multifuctional composites，research on forming mechanisms，establishing predictive models for process-defect-microstructure property relationships， new hybrid additive/subtractive manufacturing methods，and the development of large-scale，high-precision，and multifuctional equipment.

Addressing the need for high-precision shape control and low-damage property control in formed parts， as well as the goal of achieving deep decoupling of thermal， mass， and force aspects of the heat source while increasing the deposition rate， multi-electrode arc welding/additive manufacturing （AM） technology has gradually become a focal point of interest in both academia and industry. This paper systematically reviews the development history of multi-electrode arc processes， comprehensively summarizes the cutting-edge research achievements in the field of multi-electrode arc welding and AM， and categorizes and summarizes different types of coupled arcs in multi-electrode arcs. The multi-electrode arc system achieves finer control over the thermal-mass-force transfer process of the hybrid arc by introducing multiple electrodes， which helps optimize the forming quality of deposited layers， reduce defects， and improve manufacturing precision. This study highlights the differences in heat source and electrode arrangement configurations among various types of multi-electrode arc processes， and their distinct thermal-mass-force decoupling transfer characteristics， summarizes the influence mechanisms of welding process parameters on the stability of hybrid arcs. Finally， this paper proposes multi-electrode arc characteristics suitable for wire arc additive manufacturing， explores the high-performance manufacturing of composite components， and establishes a process database for novel multi-electrode arc technologies， which provide valuable insights for the application and promotion of coupled arc and multi-electrode arc AM technologies.

Wire arc additive manufacturing （WAAM） employs an arc as the energy source to melt metal wire layer by layer， making it suitable for the rapid fabrication of medium to large-sized complex metal components. Owing to its high forming efficiency， low manufacturing costs， and exceptional material utilization rates， WAAM has extensive application prospects in the aerospace and defense sectors. The regulation of residual stress and distortion in metal components is a key scientific and technical problem that must be solved to promote the efficient and high-quality development and application of WAAM. The mechanisms and influencing factors of residual stress and distortion in WAAM are explored. The experimental measurement and numerical simulation methods are analyzed and compared， and strategies for reducing residual stress and distortion at different stages of the WAAM process （before， during， and after deposition） are systematically summarized. Finally， it is pointed out that numerical simulations， machine learning， in-situ diagnosis， and control are the key research directions for controlling residual stress and distortion in WAAM in the future.

Multi-wire arc additive manufacturing technology has the advantages of low cost and high efficiency， especially high flexibility in composition design and regulation， and has become the mainstream technology for the preparation of large-scale complex metal structural parts. Multiple wires （same or different） are fed at the same time to realize in-situ alloying in the molten pool. This method provides a feasible path for the preparation of advanced metal materials with complex compositions. This paper discusses the research progress of multi-wire arc additive manufacturing in the preparation of traditional materials such as high-performance titanium alloys， aluminum alloys， and stainless steels， as well as advanced metal materials such as functionally graded materials， high-entropy alloys， and intermetallic compounds. The problems of uneven microstructure， anisotropy of mechanical properties and insufficient forming accuracy of multi-wire arc additive manufacturing components are discussed. The development directions of multi-wire arc additive manufacturing process window， multi-process coupling， and forming process monitoring and control system are proposed， which provide a theoretical basis for the improvement and development of the multi-wire arc additive manufacturing process.

The 18Ni350 maraging steel（M350） straight-wall component is fabricated using wire arc additive manufacturing（WAAM） process. By employing direct aging heat treatment， the microstructure and mechanical properties of M350 are controlled. The effect of different aging conditions （aging temperature and aging time） on the microstructure and performance of M350 is studied. The results show that the solidification microstructure of M350 fabricated by wire arc additive manufacturing consists of columnar dendrites and cellular dendrites， with segregation of Ni， Mo， and Ti elements observed in the interdendritic regions.During the direct aging process， a reverse transformation occurs in the interdendrite region where Ni， Mo， and Ti elements segregate，leading to the conversion of martensite phase to austenite phase. With the increase of aging temperature and aging time， the size and quantity of reversed austenite increase. The microhardness， yield strength （YS）， and ultimate tensile strength （UTS） first increase and then decrease. The peak microhardness （534HV）， YS （1600 MPa）， and UTS （1658 MPa） are achieved at 530 ℃ aging for 3 h. At the same time， the elongation after fracture remains a value above 13%. In addition， the M350 fabricated by wire arc additive manufacturing demonstrates mechanical anisotropy， with the anisotropy difference peaking under the optimal aging conditions， exhibiting a YS difference of 360 MPa and a UTS difference of 287 MPa.

In order to obtain NiTi alloy with excellent properties， dual-wire arc additive manufacturing technology is used to control the wire feed speed of Ni and Ti wires， and precisely adjust the atomic ratio and phase composition of Ni alloy. The results show that when the Ni/Ti atomic ratio is 8∶10 in the center of the longitudinal cladding passage， the deposited NiTi alloy is mainly composed of Ti₂Ni phase accompanied by a small number of Ti-rich particles， and the microhardness and compressive strength reach 560HV and 1600 MPa， respectively. When the Ni/Ti atomic ratio is 11∶10， the Ti₂Ni phase is included in the NiTi phase， and the irrecoverable strain of 1.6% appears in the cyclic compression process. When the atomic ratio of Ni/Ti is 15∶10， the cluster Ni₃Ti phase is formed in the NiTi phase， the longitudinal fracture strain is close to 40%， and the irrecoverable strain is only 1.2% after cyclic compression， showing good superelasticity. In addition， compared with the central region of the longitudinal cladding passage， the microstructures of the transverse lapping region of the samples with different Ni/Ti atomic ratios show obvious grain coarsening and component segregation， and the compressive strength and plastic deformation ability are significantly reduced.

A wire-based friction stir additive remanufacturing （W-FSAR） method is proposed to address large cracks and material loss in aluminum alloy components during production and service. The W-FSAR tools consist of a wire feeding device， a stationary sleeve， and a screw-structured stirring head. This method effectively fills and repairs 10 mm-width and 2 mm-depth groove defects in aluminum alloy components. The results indicate the repaired sample has high repair efficiency， smooth morphology， homogeneous microstructure， and excellent mechanical properties. The dynamic recovery and recrystallization processes refine the grain size to 1.59 μm. The ultimate tensile strength and elongation of the repaired samples are （410±8） MPa and （11.9±0.9）%， respectively， which increase by 26% and 159% compared to the worn-out specimens. There are numerous dimples on the fracture surface， exhibiting typical ductile fracture characteristics.

During the wire-arc additive manufacturing process， Al5356 straight-wall components are fabricated by imparting lateral swings of varying frequencies and amplitudes to the welding gun. The impact of these swinging arcs on the forming quality， pore distribution， microstructure， and mechanical properties of the components is evaluated through surface waviness calculations， microstructural analysis， and mechanical tensile tests. The results show that incorporating the arc swing technique in the manufacturing process significantly enhances the forming accuracy， compactness， microstructural uniformity， and mechanical properties of the straight-wall samples. Within the experimental parameters， applying an arc swing reduces the surface waviness of Al5356 straight-wall samples by 60% compared to those produced without an arc swing. Additionally， the porosity and maximum pore diameter are decreased from over 0.65% and 33 µm to below 0.20% and 10 µm， respectively. The average tensile strength in both the X-direction （deposition direction） and Z-direction （build direction） increases by approximately 13% and 15%， respectively， while the average elongation improves by about 27% and 25%， respectively. Notably， the frequency of the arc swing has a more pronounced effect than the amplitude in enhancing surface quality， pore dispersion， and pore diameter reduction in the deposited components. High-frequency arc oscillation exerts a potent stirring effect on the melt pool， leading to a more uniform temperature distribution across the transverse direction of the deposited weld path. The observed enhancement in mechanical properties is primarily attributed to the reduction of pore defects and the homogenization of the microstructure. Therefore， the proper application of the arc swing technique in wire-arc additive manufacturing holds significant promise for improving the forming quality and mechanical properties of components.

NiTi shape memory alloys （SMAs） have found widespread applications due to their unique superelasticity and shape memory effects. However， traditional manufacturing methods face challenges in fabricating complex geometries and precisely controlling the microstructure NiTi alloys. Wire arc additive manufacturing （WAAM）， with its layer-by-layer deposition characteristics， offers a novel solution for NiTi alloy fabrication. This paper reviews the research progress in WAAM NiTi shape memory alloys， with emphasis on the influence of process parameters on microstructure， phase transformation behavior， and mechanical properties. The advantages and disadvantages of different arc processes （such as gas metal arc welding， gas tungsten arc welding， and cold metal transfer） in NiTi alloy fabrication are analyzed， along with recent achievements in forming quality， phase transformation temperature control， and mechanical properties through WAAM technology. Particular attention is given to the significant microstructural heterogeneity and oxidation issues arising from high heat input， low cooling rates， and repeated thermal cycling during the layer-by-layer deposition process， which adversely affect mechanical properties and superelastic performance. To address these challenges， strategies including process optimization， active cooling， third element addition， and heat treatment are proposed to improve material homogeneity. Furthermore， this paper discusses the heterogeneous structure design of NiTi alloys with other metals， highlighting the potential of WAAM in fabricating multi-material composite structures for high-performance devices. While WAAM demonstrates advantages in fabricating complex geometries and multi-material structures， challenges remain regarding oxidation， element vaporization， and poor interlayer bonding. Future research should focus on heat treatment optimization and microstructural control， development of novel multi-metal composites， and exploration of innovative approaches to enhance interfacial bonding and oxidation resistance， thereby further improving NiTi alloy performance and expanding their application domains.

Transient liquid phase（TLP）bonding is an optimised bonding process that joins nickel-based superalloys. The post-bond heat treatment（PBHT）refers to a heat treatment process for the joint after the connection. This article summarizes the adverse effects of TLP thermal cycling on the tissue properties of nickel-based superalloys and the harmful phase problems of joints after TLP joining. The article explains the mechanism， types and current status of the PBHT process， analyses and discusses the systems and effects of the PBHT process where it is applicable. Finally， the development prospects and future research directions are presented. For future PBHT process research， it is recommended to continue to develop in the direction of multi-level and multi-pass precision heat treatment， and explore the TLPB-PBHT integrated process.

As lightweight and high-performance structural materials， nano-Al₂O₃ reinforced aluminum matrix composites can achieve lightweight energy saving and emission reduction， and have broad application prospects in aerospace， automotive industry， shipbuilding， national defense， and 5G electronic communication. In this paper， high energy ball milling powder metallurgy method， ultrasonic assisted casting method， friction stir method， additive manufacturing method， in-situ reaction method and other nano-Al₂O₃ reinforced aluminum matrix composite preparation technologies are introduced. The effects of nano-Al₂O₃ reinforcement， the interface microstructure between the reinforcement and aluminum matrix， the size and content of the reinforcement， the grain size of the aluminum matrix，the dispersion of the reinforcement， and the microstructure design on the mechanical properties of nano-Al₂O₃ reinforced aluminum matrix composites are analyzed and summarized. The main strengthening mechanisms of nano-Al₂O₃ reinforced aluminum matrix composites and the coupling forms of each strengthening stress are also summarized. Finally， the future development direction of nano-Al₂O₃ reinforced aluminum matrix composites in the aspects of large-size preparation technology with high reinforcement volume fraction， heterogeneous configuration optimization， and the integration of high-strength and heat-resistant structure and function are prospected.

In this study， 0.95MgTiO₃-0.05CaTiO₃ powders are synthesized via a high-temperature solid-phase reaction， utilizing TiO₂，CaCO₃，and Mg（OH）₂ as the primary reactants. Guided by the phase diagram， sintering aids comprising Li₂B₄O₇-Al₂O₃， which possess a low melting point， are formulated to facilitate the low-temperature sintering process of 0.95MgTiO₃-0.05CaTiO₃ microwave dielectric ceramics. The study comprehensively investigates the influence of the synthesis temperature （900-1100 ℃） on the phase composition of calcium magnesium titanate powder， as well as the effects of sintering temperature （1175-1250 ℃） and aid content （1%-5%， mass fraction， the same below） on the density， microstructure， dielectric constant， quality factor， and frequency-temperature coefficient of calcium magnesium titanate ceramics. The results indicates that 0.95MgTiO₃-0.05CaTiO₃ powders could be successfully synthesized at a reaction temperature of 1100 ℃. The incorporation of sintering aids effectively reduced the sintering temperature of the ceramics. However， excessive addition （5%） resulted in decreased density and dielectric properties. Optimal performance is achieved with an aid content of 3% and a sintering temperature of 1225 ℃， producing 0.95MgTiO₃-0.05CaTiO₃ ceramic with a relative density of 98.70%， a dielectric constant of 20.38， a quality factor of 37240 GHz， and a frequency-temperature coefficient of -9.6×10^-6 ℃^-1.

The oxidation behavior of SiC_f/SiC composites， fabricated through the melt infiltration process， is meticulously investigated in a water vapor corrosion environment. The findings reveal that after exposure to water vapor corrosion at 800 ℃ and 1200 ℃ for 400 h， the flexural strength retention of uncoated samples is 78.8% and 74.9% respectively， whereas coated samples maintain flexural strengths of 95.9% and 93.0% respectively. The application of environmental barrier coatings effectively shield the material from direct contact with the corrosive water vapor medium， thereby mitigating the substantial decline in mechanical properties of the SiC_f/SiC composites. Notably， the oxidation of the BN interfacial layer emerge as the primary factor contributing to the deterioration of the mechanical properties. Specifically， uncoated samples exhibit partial disappearance of the interfacial layer and the formation of holes between the fibers and the matrix after 400 h of corrosion at 1200 ℃， thereby compromising the protective role of the interface. Simultaneously， parts of the interface layer continue to bond the fiber and the matrix. The interplay between the oxidation of the BN interfacial layer and the SiC matrix is identified as the main cause for the decline in the mechanical properties of the SiCf/SiC composites.

A photocatalyst g-C₃N _x @CN with defective carbon nitride （g-C₃N _x ） encapsulated by CN shells is prepared by in-situ growth and high-temperature dezincification. The structural， morphological， and compositional characterization of g-C₃N _x @CN has been analyzed and characterized by various analytical means. First， g-C₃N _x with nitrogen defects is prepared by high-temperature polycondensation of melamine and high-temperature denitration of Mg powder， followed by in-situ growth of ZIF-8 by loading ZnO nanoparticles. Finally， g-C₃N _x @ZIF-8 is dezincified at high temperature， and the CN shell-encapsulated visible light catalyst material g-C₃N _x @CN with double defects （N， Zn） with ZIF-8 is prepared. The study demonstrates that the g-C₃N _x @CN catalyst exhibits strong visible photocatalytic activity and effectively degrades methylene blue and 2，4-dichlorophenol within 240 min， with the best performance of the g-C₃N _x @CN-5∶4 composite. The single linear oxygen （1O₂） plays a dominant role in the reaction system. In the cycle test， the g-C₃N _x @CN shows excellent recycling and light stability. This study extends the study of defective carbon nitride materials in visible light absorption and provides a viable method for the derivation of metal catalysts to inorganic non-metal catalysts.

This study investigates the dynamic compressive mechanical properties of three-dimensional woven carbon/carbon composites under both room temperature（25 ℃） quasi-static and from 25 ℃ to 900 ℃ dynamic compression conditions using both a materials testing machine and a split Hopkinson press bar device equipped for simultaneous high-temperature loading. The experimental results reveal that the strength of the composites is influenced by three key factors： fiber orientation， strain rate， and temperature. Specifically， under consistent strain rates and temperatures， the composites exhibit higher strength in the Z-direction compared to the XY-direction. As the strain rate escalates， the strengths of the composites in both the XY-direction and Z-direction increase， indicating a positive correlation with the strain rate. Upon heating from room temperature to 900 ℃， the strengths of both XY-direction and Z-direction composites initially rise， peaking at 600 ℃， and then gradually decline. Under both static and dynamic loading conditions， XY-direction composites undergo shear failure， albeit with a smaller shear fracture angle in the dynamic case compared to the quasi-static scenario. An increase in the strain rate results in a transition in the fracture mode of Z-direction composites， shifting from shear failure to a combination of matrix crushing and partial fiber fracture.

The enrichment of nitrate pollutants in water will bring great harm to ecology and seriously threaten human life and health. It is significant to study the electrocatalytic materials of high efficiency electrocatalytic nitrate reduction to ammonia for ecological protection and formation of the nitrogen cycle. In this study， a self-supporting dendritic Cu/CF （D-Cu/CF） electrode is deposited on the copper foam （CF） skeleton by the hydrogen bubble dynamic template （DHBT）. The orientation tip of the three-dimensional dendrite structure greatly increases the number of active sites and intrinsic activity when it is used in synthesizing ammonia by nitrate electroreduction. The effects of deposition time and different potentials on electrochemical performance are investigated. Under the optimum conditions， D-Cu/CF electrode shows an ammonia production rate of 0.379 mmol·h^-1·cm^-2 and a Faraday efficiency of 92.8%. The ammonia production rate is stable and the Faraday efficiency is above 90% after 6 cycles of nitrate electroreduction. Furthermore， the D-Cu/CF electrocatalyst exhibits good electroreduction performance in the actual water sample test， showing great potential practical application value.

Al₂O₃-based eutectic ceramics have excellent high-temperature mechanical properties，showing great application prospects in the field of extreme environments. Therefore， it is of great significance to explore the preparation method with low energy consumption and simple process for its industrial application. The Al₂O₃-GdAlO₃-ZrO₂ eutectic ceramics are successfully prepared by flash sintering technique. The effects of electric field strength， current limit and flash sintering time on flash sintering behavior， phase and microstructure morphology are studied. The results show that the flash sintering temperature decreases with the increase of electric field strength. Under the electric field strength of 600， 700， 800， 900 V/cm， the flash temperature of Al₂O₃-Gd₂O₃-ZrO₂ mixed powder is 994， 958， 943， 911 ℃， respectively. Al₂O₃-GdAlO₃-ZrO₂ eutectic ceramics are obtained at an electric field strength of 900 V/cm and a flash temperature of 911 ℃. With the increase of current limit and flash sintering time， the irregular eutectic structure transforms into a regular eutectic structure， and the eutectic structure becomes finer. The electric field strength has no significant effect on the morphology of the eutectic structure. The Joule heating effect plays an important role in the formation of eutectic structure. However， it is difficult to fully explain the formation of eutectic structure in the flash sintering process only by the Joule heating effect.

Epoxy composites that possess high thermal conductivity and synergistic flame resistance are anticipated to achieve efficient heat dissipation and minimal fire risk in electronic equipment， thereby exhibiting promising application prospects in electronic products. In this study， SnO₂ nanoparticles are modified using γ-aminopropyltriethoxysilane， and the resultant modified SnO₂ （m-SnO₂） is further integrated with boron nitride nanowires （BNNS） through electrostatic self-assembly to produce BNNS@m-SnO₂ hybrid fillers. Subsequently， thermally conductive and flame-resistant composites with a specific orientation structure are fabricated using the blade-casting method， with epoxy resin serving as the polymer matrix. The results indicate that the Zeta potential of the modified SnO₂ nanoparticles shift from -19.1 mV to 28.7 mV， enabling their combination with BNNS （Zeta potential of -27.8 mV） through electrostatic interaction. The incorporation of BNNS@m-SnO₂ hybrid fillers significantly enhances the thermal conductivity and flame resistance of the epoxy composites. Notably， the thermal conductivity of the EP/BNNS@m-SnO₂-10%（mass fraction） composites reach 3.79 W·m^-1·K^-1， while also demonstrating a higher peak combustion temperature （410.9 ℃） and a lower peak heat release rate （302.2 W·g^-1）.

A medium carbon alloy steel 40Cr3Mn3Ni3Si2Mo is designed and prepared， and tempering is carried out after hot rolling， hot rolling plus refrigerating treatment， as well as quenching plus refrigerating treatment respectively. Microstructural characterization and properties testing are conducted on the heat-treated samples to investigate microstructural evolution. The relationship between processing， microstructures and mechanical properties is established， and the mechanism of strengthening and toughening is illuminated. Consequently， the principles for composition design and process optimization of quenching and partitioning （Q&P） typed ultra-high strength steels with high strength and plasticity are figured out. The results show that multiphase microstructures of tempered martensite and carbon-enriched austenite are achieved through tempering on the hot-rolled testing steel， while its strength fails to reach the level of ultra-high strength steels due to high volume fraction of retained austenite. The match of strength and ductility of the hot-rolled testing steel increases significantly through low temperature tempering after refrigerating treatment because of the improved phase proportion and distribution. When the testing steel is refrigerating treated and tempered instantly after oil quenching to weaken the effect of austenite stabilization， more excellent comprehensive properties are achieved with 1506 MPa in yield strength， 1895 MPa in ultimate tensile strength and 16.7% in elongation. Moreover， a feasible approach to increase the strength and plasticity of 1800-1900 MPa graded ultra-high strength steels is proposed： through controlling the martensitic phase transformation kinetics by alloying design and process optimization， austenite of about 20% in volume fraction is retained within martensite after incomplete quenching， its stability is reinforced by tempering assistant partitioning. Thus， the elongation increases to 15%-18%， with the yield strength lowered slightly to 1400-1600 MPa.

The oxidation kinetics and microstructure of S30815 heat resistant stainless steel， both in its base material and welded joint， are analyzed at different service temperatures by the constant temperature oxidation method. High temperature oxidation kinetic curves are obtained by the mass gain method. The morphology， composition， and microstructure of the oxide films are studied by using scanning electron microscopy （SEM）， energy dispersive spectroscopy （EDS）， and X-ray diffraction （XRD）， respectively. The results show that under constant temperature oxidation conditions at 780 ℃， there is less mass gain and a relatively lower oxidation rate. The oxidation products are mainly Cr₂O₃， Fe₂O₃， and Fe₃Mn₃O₈， exhibit sheet， strip， and polyhedron shapes. At 880 ℃， the mass gain and the oxidation rate significantly increase. The oxidation product at this temperature comprises a mixed oxide of Cr₂O₃， Fe₃O₄， MnFe₂O₄， and Ni （Cr₂O₄）， which is mainly in thin strips and sheets. The oxidation kinetics curves of S30815 follow a parabolic rule. With the increase of oxidation time， the oxidation rate gradually decreases and eventually tends to be stable， showing a good oxidation resistance at high temperature. A larger amount of dense Cr₂O₃ oxide film is generated on the surface of the welded joint， exhibiting a lower average oxidation rate and better oxidation resistance compared to the base material.

The surface microstructure of Inconel 625 alloy is refined through the application of laser shock peening （LSP） technology， aiming at optimizing its surface morphology.The microstructural characteristics are examined using laser scanning confocal microscopy， electron backscatter diffraction （EBSD）， and transmission electron microscopy （TEM）. Vickers microhardness testing is employed to analyze hardness variations. The results show that suitable LSP conditions can eliminate impact pits and enhance surface quality. Furthermore， the microstructure gradually transitions， with the surface layer evolving into ultrafine lamellae and grains under LSP treatment. The surface layer is predominantly composed of substructures and deformed grains， whereas the transition layer features a mix of deformed grains and recrystallized structures. As the depth increases， the population of deformed grains decreases， and recrystallized grains increases. A quantitative assessment of the geometrically necessary dislocation density （ρ _GND） for the alloy after five LSP treatments show that the surface ρ _GND reaches 2.91×10¹⁴ m^-2， compared to 0.61×10¹⁴ m^-2 for the untreated layer， indicating a significant increase post LSP. This alteration in dislocation density results in a shift in microhardness， which escalates with the number of LSP cycles and diminishes with depth. This trend can be attributed to LSP-induced changes in the proportion of large-angle grain boundaries， with the grain boundary strengthening effect in Inconel 625 alloy adhering to the Hall-Petch relationship.