Joint Efficiency Research Articles

Joint design and joint performance of Plain Concrete Pavements (JPCP) - Investigations and experiences in Germany -

Unreinforced concrete pavements have to be equipped with longitudinal and transversal kerbs to guarantee controlled cracking (JPCP). The designs of such joints depending on the technical requirements are given in respective specifications. Main features for the design are aiming first for the controlled cracking at the foreseen position during young age of the concrete as well as avoidance of wild cracking during service life, second for sufficient load transfer at the joint to reduce edge load effects and third for a good long time performance, which requires joint sealing. This paper focus on research works performed in the field of joint efficiency (dowelling) of transversal joints, which is especially related to the design parameters and the positioning of dowels. New developments and alternative solutions concerning the today’s usage of coated steel bars must be done within the area of conflicts of following headings: Load transfer efficiency, longitudinal resistance, long-time behaviour (corrosion). FEM models and laboratory tests offer evaluation criteria for modified parameters. Especially the coating-material foreseen to cover the steel bar must be evaluated within the area of conflicts shown above. Additionally the actual dowel position and alignment is of importance. Non-destructive inspection tools are under development. The outputs of these investigations are strengthening the background for the updating work concerning the respective specifications.

ANALYSIS OF FULL-DEPTH PCC PATCHES FOR JOINTED CONCRETE PAVEMENTS

This study investigates the effectiveness of full-depth Portland Cement Concrete (PCC) patches for rehabilitating jointed concrete pavements, focusing on load transfer efficiency, stress distribution, and deflection characteristics under heavy axle loads. The deterioration of jointed PCC pavements, primarily at transverse contraction joints due to heavy traffic and environmental conditions, necessitates maintenance strategies like Concrete Pavement Restoration (CPR) which includes full-depth patching among other techniques. The research emphasizes the importance of dowel bar size, spacing, and concrete thickness in the design of full-depth patches to ensure effective load transfer and minimize pavement deflections and stresses. Key findings reveal that deflections along transverse joints increase with fewer dowel bars, while joint efficiency improves with thicker slabs and more dowel bars. Tensile stresses were found to be less influenced by the number and size of dowel bars, not exceeding rupture limits, indicating stresses alone are insufficient for patch design. The study highlighted that bearing stresses are critical, with six and seven dowel bars configurations exceeding allowable stresses, suggesting that a minimum of eight dowel bars of at least 1 1/4-inch diameter is necessary for effective design. The optimum design proposed involves a 10-inch thick patch with eight dowel bars positioned strategically across the transverse joint for improved load transfer and reduced bearing stress, thus enhancing pavement longevity and performance. (Abstract generated by AI tool ChatGPT 4)

Experimental study on seismic behavior of a novel RC slab- SC wall joint based on UHPC and rebar – steel plate lap splices

Steel plate concrete shear walls (SC walls) have been widely used in practical projects because of great mechanical properties. For the application in nuclear power plant containment, the joint of specially shaped reinforced concrete slab and SC walls is difficult to design and construct. By using ultra-high performance concrete (UHPC), a new type of joint with the non-contact lapping method is proposed in this paper, which has advantages of simple construction, superior performance, suitable cost, and wide application range. Five RC-UHPC slab-shear wall joints were designed and tested under reciprocating load to study the effects of structures of lapping steel plate, shear-resisting methods, and interfacial reinforcing methods on the mechanical properties of the joints. The test results showed that the joints adopting the new structures were damaged for the flexural failure of the concrete area, with no damage to the joint zone, while the other specimens failed in the shear failure of the joint zone. It can be demonstrated that the ribs formed by bars and studs on the lapping steel plates are effective measures for non-contact force transferring, and interfacial reinforcing bars can improve the strength of the interface between concrete and UHPC. The force transferring efficiency of the new joints was close to or more than 90%, which indicates that the proposed joint structures can effectively realize the non-contact lapping force transferring. The design formulas and suggestions are proposed based on the analysis of the mechanism of new structures.

Formation mechanism of ZrC-SiC joint prepared by pulsed-current assisted diffusion bonding

For pulsed-current assisted diffusion bonding of ZrC-SiC composite, the required joining temperature and joint formation mechanism showed great difference with grain-size. Compared to base-materials with a grain-size of 3–5 μm (m-ZS), the joining temperature of n-ZS (grain-size ∼1 μm) could be reduced by 100 °C while maintaining the same joint efficiency. Moreover, the straight bond-line of m-ZS joint was eliminated by strain-induced grain-boundary migration, but causing by the straight interface due to the pining effect of interfacial SiC, shear strength of the joint bonded at 1700 °C is only 81.7 % of the substrate. In contrast, pulsed-current enhanced the vaporization-deposition mechanism in n-ZS joint, resulting in massive formation of nano-scale ZrC particles at interface. At 1700 °C, the cross-interface growth of nano-particles contributed to a seamless joining, and the joint microstructure and shear strength are consistent with those of the base-materials.

Unveiling the influence mechanism of splat cooling on the microstructure evolution and mechanical properties of friction stir welded ZK61M magnesium alloy

The application of cooling assistance during the friction stir welding (FSW) process can effectively improve the yield strength of the joint, thereby causing more strain to occur in the base material on both sides of the weld seam, inhibiting strain localization in the joint, and achieving the effect of simultaneously improving the strength and toughness of the joint. However, there are currently limited studies on cooling-assisted FSW of magnesium alloys, especially for ZK series magnesium alloys. Therefore, conventional FSW and novel splat cooling assisted friction stir welding (SCaFSW) were carried out on ZK61M magnesium alloy in this study. It was found that the application of splat cooling slightly increases the traverse force, reduces the area of the stir zone (SZ) and refines the grain size. After applying splat cooling at a welding speed of 60 mm/min, the ultimate tensile strength (UTS) of the joint increased from 210.4 MPa to 257.1 MPa, achieving a joint efficiency of 90.7 %. The yield strength (YS) of the joint also improved, rising from 138.6 MPa to 185.0 MPa, representing a 33.4 % improvement, which can be attributed to the reduction in grain size and basal slip Schmid factor (SF). When splat cooling is applied, due to the weakened deformation effect of the newly formed shear band on the deposited shear band, a large number of recrystallized grains are preserved. At the same time, the reduced annealing effect leads to the retention of more dislocation structures. Ultimately, both the degree of recrystallization and dislocation density can be simultaneously increased in the SCaFSW joint. It revealed that the application of splat cooling is an effective method for improving the joint properties of FSWed ZK series Mg alloy.

Friction stir welding of Haynes 282 Ni superalloy by using a novel hemispherical tool

In the present investigation, friction stir welding (FSW) of a gamma prime (γ′) strengthened Haynes 282 nickel-base superalloy by using a novel hemispherical tool is documented. A joint efficiency of ~ 96% was achieved in the as-welded condition, which further increased to ~ 100% after two-step post-weld aging heat treatment. An extremely fine (~ 2 μm) grained microstructure was observed in the FSWed region compared with the coarse (~ 48 μm) grain base metal region. The carbides (MC and M23C6) and γ′ precipitates are identified as the important microstructural constituent phases observed in the welded region. The aging heat treatment resulted in the precipitation of the γ′ strengthening phase and altered the morphology of the M23C6 carbide phase in the welded region from a continuous film to discrete particles at the grain boundaries. The preliminary results concerning tool life suggest that no significant tool wear occurs at least up to a welding distance of 200 mm. Therefore, the novel hemispherical tool design allows the successful FSW of high-temperature application materials without any significant tool wear or high heat generation.

Investigate on dissimilar welding of high-entropy alloy and 310S with various fillers

This study investigates the microstructure and mechanical properties of non-equimolar CoCrFeMoNi high-entropy alloy (HEA) and SUS310S stainless steel dissimilar welds using different filler materials. The dissimilar welding was performed on gas tungsten arc welding with Inconel 82, Inconel 625, and 316L stainless steel fillers. The non-equimolar CoCrFeMoNi HEA is shown effective weldability with these fillers, maintaining distinguished phase stability and structural integrity. Microstructural analysis revealed varied grain morphologies influenced by the heat in the fusion zone and heat-affected zone. Mechanical properties were evaluated through tensile and hardness tests, showing that welds with Inconel 625 filler exhibited superior strength and ductility. The weld hardness value was maintained at 170 HV, with a yield strength of 301 MPa and an ultimate tensile strength of 587 MPa, resulting in a joint efficiency of 106 %.

Enhancing performance through friction welding of dissimilar aluminium 2014 and 7075 hybrid metal-matrix composite: insights into material characterization, crystallographic texture, and mechanical properties

Abstract Welding high-strength, age-hardenable aluminium alloys from the 2XXX and 7XXX series poses challenges when using fusion-based welding processes. Common issues such as hydrogen porosity, hot cracking, stress corrosion cracking, and solidification cracking can lead to a reduction in mechanical properties due to the loss of strengthening precipitates and alloying elements. Consequently, friction stir welding (FSW) has emerged as a promising solid-state welding technique for joining dissimilar aluminium alloys and composites, particularly in aerospace applications where a combination of strength, corrosion resistance, and other desirable properties is required. This study investigates the FSW of dissimilar aluminium alloys, specifically AA2014 and a hybrid metal-matrix composite of AA7075 reinforced with silicon carbide and graphite particles. The microstructural and mechanical properties of the welded joints were characterized, revealing an average grain size of 2.39 μm ± 1.2 in the stir zone (SZ). A high angle grain boundary volume fraction of 47.25% was observed in the SZ, compared to 2.01% in the AA7075 composite and 15.03% in the AA2014 base metal. The presence of shear textures and the distribution of cube, S, and H textures indicate a homogeneous mixing of the base materials and a high shear strain rate during welding. The hardness of the SZ increased by 13%, and the ultimate tensile strength, yield strength, and elongation of the joint were measured at 251 MPa, 183 MPa, and 6.44%, respectively. The flexural strength of the joints improved significantly, with the CW13 sample showing a 67% increase compared to CW1. The dissimilar joint CW13, welded at 12.5 mm min−1, achieved a maximum joint efficiency of 95%.

Effect of Temperature Control and Rotational and Traverse Speeds on the Mechanical Properties of Friction Stir-Welded Polypropylene Plates.

In the present study, the effects of temperature and rotational and traverse speeds on the mechanical properties of polypropylene joints that are welded by friction stir welding using a non-rotational shoulder and a heat-assisted welding process is investigated. Tensile properties, microhardness measurements, microscopy observations, and thermal analysis are carried out in the present research to evaluate the effect of the welding parameters on the mechanical properties of welded joints. The experiments are conducted and analyzed by means of a central composite design using an analysis of variance (ANOVA). Variations in pre-heating temperature from 60 °C to 80 °C, rotational speed from 800 to 1500 rpm, and traverse speed from 20 mm/min to 100 mm/min are made for observations. A remarkable joint efficiency of 94% is achieved with joints that are free of discontinuities and defects. The fractured surfaces are observed to identify ductile and brittle zones. The crystallinity is measured, and a correlation between crystallinity and joint strength is discussed. The sample with highest efficiency shows 65% crystallinity and a ratio of 37.9% of ductile zone-total fractured area.

Analysis of mechanical properties and microstructure of single and double-pass friction stir welded T-joints for aluminium stiffened panels

This study investigates the application of Friction Stir Welding (FSW) for fabricating stiffened structures in AA2219-T31 using a T-lap configuration. These structures are vital in various applications where weight is a crucial factor, including aircraft fuselages, railway cars and automotive parts. This study assesses the formation dynamics of lack of bonding in single-pass welds, including the examination of microstructure, hardness, mechanical properties, and fracture locations. A second welding pass was employed over the initial weld to eliminate the lack of bonding. The implementation of a second welding pass significantly improves joint efficiency, achieving up to 90 % of the ultimate tensile strength in the skin direction and 95 % in the stiffener direction, establishing a new benchmark for AA2219-T31 T-joints. Additionally, no significant differences in grain size were found between single- and double-pass welds, highlighting the lack of bonding as key factor affecting the strength of the joints. The proposed method offers valuable insights for future industrial applications to avoid defects such as lack of bonding.

Linear friction welding of T-Joints in low carbon steel: Effect of welding parameters on joint quality

Linear friction welding (LFW) is a well-established solid-state joining technique. However, its application in T-joint configurations remains unexplored. This study investigated the effects of welding parameters, such as oscillation direction, upset, and applied pressure after oscillation, on linear friction welded (LFWed) T-joints using low-carbon steel SM490A. The flash ejection behavior, flash profiles, microhardness, microstructure at the welding interface, and tensile properties of the joints were evaluated under various welding conditions. The results indicated that flash symmetry was lower along the oscillation direction and higher perpendicular to it. Short-side oscillation produced more homogeneous flash ejection compared to long-side oscillation. No distinct softening zones were observed in the hardness profiles of the LFWed T-joints. The microstructure at the welding interface consisted of martensite, bainite, and ferrite, indicating that the weld region reached temperatures above the A1 temperature. The martensite fraction and hardness increased with higher upset and applied pressure after oscillation. Tensile tests revealed 100 % joint efficiency across all welding conditions, with ductile fracture occurring in the base metal due to the absence of welding defects and increased hardness at the interface.

Evaluations of nanomechanical and indentation creep of diffusion-bonded aluminum and austenitic stainless steel joint interface

Diffusion bonding of aluminum alloy (AA-7075) with austenitic stainless steel (AISI 304L) was performed in the environment of Argon inert gas. The resultant bonding interface was investigated for its mechanical, nanomechanical, microstructural, and compositional characteristics using lap shear tests, nanoindentation and indentation creep, as well as optical (OM) and scanning electron microscopy (SEM), with energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results revealed that intermetallic compounds (IMCs) of FeAl3, FeAl2, FeNi3, and Fe3Ni2 were formed at the bonding interface. Similarly, a nano hardness of 3.5 GPa occurred at the bonding interface which was 15% and 454% higher as compared to base metal (BM) of AISI 304 and AA-7075 respectively. Additionally, indentation creep showed that the bonding interface has higher creep resistance compared to other bonding zones (BZ). Lastly, an average shear strength of 67 MPa was achieved with a joint efficiency of 58%.

Effect of high pressure-low plasticity burnishing on the mechanical and microstructural behaviour of copper foil interlayered FSW aluminium alloys

Abstract This study delves into the impact of high pressure-low plasticity burnishing (HP-LPB) on the mechanical and microstructural behaviour of friction stir welding (FSW) joints, specifically involving AA5052 and AA6082 aluminium alloys. Notably, copper foil (CF) is introduced as a novel element in the HP-LPB process to enhance the weld strength. The experiments were conducted with the tool rotational speeds (TRS) of 1000, 1100, and 1200 RPM, each paired with the welding speeds (WS) of 20, 25, and 30 mm min−1, and tool tilt angle (TTA) of 0°, 1°, and 2°. Mechanical behaviour is assessed through tensile testing along with microhardness measurements, revealing the advantages of HP-LPB with CF in enhancing joint strength and toughness. The microstructural analysis reveals the dissolution of precipitates, highlighting the influential role of copper foil in the improvement of joint efficiency. From the weld without a CF interlayer, a maximum tensile strength of 188 MPa was achieved at a TRS of 1200 RPM, WS of 25 mm min−1 and TTA of 2°. The post-processed FSW sample interlayered with copper foil, exhibited an improvement in joint efficiency by 87% at the optimum process parameter. This research demonstrates that the use of copper foil interlayers combined with HP-LPB treatment can substantially enhance the mechanical properties and structural integrity of FSW aluminum alloys, offering a valuable solution for advanced industrial applications.

Effect of precipitate evolution on galvanic corrosion behavior of friction stir welded AA6061-T6 joint after post-weld aging

The microstructural transformation of FSW AA6061-T6 joint after post-weld T6 and its effects on the mechanical properties and corrosion behavior was investigated through electrochemical tests, immersion experiments and transmission electron microscopy (TEM) observation. The research results show that the joint efficiency and strength can be enhanced by post-welding aging. The hardness of SZ is recovered to a level close to that of matrix due to the sufficient precipitation of the effective strengthening phase β''. The main β' precipitate in over-aged HAZ coarsens during post-weld aging and cannot effectively restore strength, resulting in the lowest hardness in HAZ and limited tensile strength of the joint. The presence of macro-galvanic effects in the whole joint was confirmed. In SZ, the highest level of aging characteristics is caused by the highest aging drive due to fully solidified atoms, and is confirmed by the continuous grain boundary precipitates (GBPs) and dense matrix precipitates, which lead to the most positive potentials. Whereas, in HAZ, discrete GBPs and coarsened matrix precipitates indicate over-aged features, which lead to the most negative potentials. The potential difference between SZ and HAZ results in a macro-galvanic effect in the joint, where SZ is protected as the cathode and HAZ is corroded faster as the anode.

Influence of multiple friction stir welding passes on the microstructure and mechanical properties of AA2024 age-hardened aluminum alloy

Friction stir welding (FSW) is a promising joining process for age-hardened aluminum alloys. However, when the process is used for welding age-hardened aluminum alloys, the hardness and joint efficiency are poor mainly because the strengthening precipitates become incoherent and/or get dissolved during the welding process. In this novel work, efforts have been made to enable the reprecipitation of the strengthening precipitates using multi-pass FSW. After producing FSW welds on AA2024 alloy, the welds have been processed by the same tool and at the same parameters several times to facilitate the reprecipitation of strengthening precipitates. As the precipitation kinetics of age-hardened aluminum alloys are temperature-dependent, a thermal imaging camera was utilized to record the temperature history during welding and subsequent cooling of the welded plates. The metallurgical examination of welded samples was carried out using optical microscopy, Scanning Electron Microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and microhardness test. It was observed that grain refinement occurs continuously due to dynamic recrystallization and severe plastic deformation up to the third FSW pass, after which there is no significant change in grain size. Generally, the microhardness value was higher in the stir zone than the heat-affected zone (HAZ), and the hardness in the stir zone increased after 4 passes of FSW. SEM images show clusters of incoherent precipitates, fragmentation, precipitate dispersion, dissolution, and reprecipitation of precipitates during each subsequent pass. The identification of the precipitates was carried out using EDS and XRD analysis.

On the heterogeneous microstructure and strengthening mechanisms in a laser welding AZ80 magnesium alloy

This study investigates the microstructural characteristics and mechanical properties of a laser welded AZ80 magnesium alloy. The welding process led to the formation of coarse-grained fusion zone (FZ), where a secondary phase formed continuous network. Mg17Al12 precipitation and coarsening of grain boundaries occurred in the heat affected zone. The welded joint exhibited excellent mechanical properties with a yield strength of 202 MPa and a joint efficiency of 92%. The microstructure analysis via EPMA and EBSD in conjunction with synchrotron X-ray diffraction analysis reveals that precipitates and increased dislocation density in the fusion zone are primary strengthening mechanisms for the laser welded AZ80 Mg alloy.

Study of GTA-Welded Joints of ZW61 Magnesium Alloy — Effect of Welding Current on the Microstructure and Mechanical Properties

An attempt has been made to weld ZW61 magnesium alloy extruded plates by gas tungsten arc welding (GTAW). In this paper, the zone with the largest average grain size in the welded joint was the fusion zone (FZ) rather than the heat-affected zone (HAZ). The grains of the FZ were nearly equiaxed, and their average size increased gradually with the increase of welding current. Simultaneously, the mechanical properties of the joints decreased with the increase of welding current under the set conditions of this experiment. The optimum mechanical properties of the joints were obtained at a welding current of 100 A. Their ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) were 230 MPa, 103 MPa, and 19.2%, respectively, while the joint efficiency (σb / σb BM) was only 77.7%. The fracture locations of all the joints were found in the FZ. Excessive grain size was one of the significant factors contributing to the weakness of this zone. In addition, the semicontinuous second phase on the grain boundary induced high-stress concentrations, contributing to the fracture of the welded joint. The tensile properties of the joints can be improved by limiting heat input.

Femtosecond laser joining of Stellite and stainless steel

This research explores the practicality of fusing Stellite 6, a cobalt-chromium alloy known for its high performance, with stainless steel, utilizing various laser welding approaches. The primary challenge addressed is the joining of dissimilar materials, which presents obstacles such as divergent melting points and disparate coefficients of thermal expansion. The aim is to achieve a metallurgical bond between Stellite and stainless steel that retains desirable properties. The study employs both continuous wave and femtosecond laser welding techniques, subjecting the resultant joints to rigorous analysis to assess their impact on the properties of the bond. Initial tensile testing delineated the intrinsic mechanical characteristics of the materials, revealing that while Stellite exhibits a lower ultimate tensile strength, it compensates with greater elongation compared to stainless steel. The use of continuous wave laser welding proved to be capable of creating the bond; however, it also precipitated a considerable decline in the tensile strength of the Stellite component as a result of the thermal processing involved. In contrast, femtosecond laser welding emerged as a more effective method, enhancing the joint’s overall strength and ductility. This improvement is attributed to the femtosecond laser’s precise control over thermal exposure, which confines the heat to the intended weld zone, thereby safeguarding the adjacent material from damage. Further insights were gleaned from Scanning Electron Microscopy, which showed a preferable intergranular fracture in samples welded with the femtosecond laser—a feature typically associated with ductile failure modes. The femtosecond laser welding approach culminated in a joint efficiency of 53.7%, mirroring the innate yield strength of the Stellite wire. This outcome suggests that such welded joints possess the requisite robustness for practical deployment, thus underscoring the potential of femtosecond laser welding in applications requiring the joining of Stellite to stainless steel.

Achievement of superb-strength lap joint via opposite-directions flowing friction stir lap welding of 2024 aluminum alloys

It is common for the rotating pin largely plunging into lower sheet to break up lap interface of Friction Stir Lap Welding (FSLW) joint, but the unavoidable up-bending morphology of hook outside Nugget Zone (NZ) largely reduces the joint bearing ability. Based on the novel Opposite-directions Flowing FSLW (OF-FSLW) by the self-developed rotating tool with an X-shaped right-left thread pin, the 2024 aluminum alloys lap joint was successfully welded in this study. The migration law of lap interface during welding was investigated by the experimental and numerical methods, and then how the rotating pin and its rotating velocity affect the formation and strength of OF-FSLW joint was further analyzed. The results show that the Material Concentrated Zone (MCZ) which formed above the original lap interface made the hook bend downward, the NZ greatly enlarged and the beginning part of cold lap compressed and thickened, thereby heightening the joint bearing ability. For the OF-FSLW joint, its maximum tensile strength was 403 MPa, and the corresponding joint efficiency of 90.8% was an incredible and superb value for the 2000 series heat-treatment strengthened aluminum alloys friction stir welded joint. The OF-FSLW technology by the rotating tool with an X-shaped right-left thread pin is proven to be a greatly effective approach for manufacturing the aluminum alloys lap joint with superb strength.

Microstructure, Mechanical Properties and Corrosion Performance of Laser-Welded NiTi Shape Memory Alloy in Simulated Body Fluid.

Laser-welding is a promising technique for welding NiTi shape memory alloys with acceptable tensile strength and comparable corrosion performance for biomedical applications. The microstructural characteristics and localized corrosion behavior of NiTi alloys in a simulated body fluid (SBF) environment are evaluated. A microstructural examination indicated the presence of fine and equiaxed grains with a B2 austenite phase in the base metal (BM), while the weld metal (WM) had a coarse dendritic microstructure with intermetallic precipitates including Ti2Ni and Ni4Ti3. The hardness decreased from the BM to the WM, and the average hardness for the BM was 352 ± 5 HV, while it ranged between 275 and 307 HV and 265 and 287 HV for the HAZ and WM, respectively. Uni-axial tensile tests revealed a substantial decrease in the tensile strength of NiTi WM (481 ± 19 MPa), with a reduced joint efficiency of 34%. The localized corrosion performance of NiTi BM was superior to the WM, with electrochemical test responses indicating a pitting potential and low corrosion rate in SBF environments. The corrosion rate of the NiTi BM and WM was 0.048 ± 0.0018 mils per year (mpy) and 0.41 ± 0.019 mpy, respectively. During welding, NiTi's strength and biocompatibility properties changed due to the alteration in microstructure and formation of intermetallic phases as a result of Ti enrichment. The performance and safety of welded medical devices may be impacted during welding, and it is essential to preserve the biocompatibility of NiTi components for biomedical applications.