Zone Hardness Research Articles

Influence of post-weld rolling and artificial aging on microstructure and mechanical properties of friction stir welded 2195-T4 Al-Li alloy joints

Fiction stir welding (FSW) of a naturally aged (T4) 2195 Al-Li alloy was conducted to study the effect of post-weld artificial aging (AA) and rolling+artificial aging (R+AA) on microstructure and mechanical properties of the FSW joints. Grain features were analyzed by electron back scattered diffraction (EBSD) technology, and precipitates were characterized using transmission electron microscopy (TEM) along [011]Al and [001]Al zone axes. Mechanical properties of the joints were evaluated through micro-hardness and tensile testing. The results indicate that grain development, including grain orientation, grain boundary components and dislocation density, varies within different regions of the As-welded joints due to complicated thermal-mechanical history. The alloying elements mostly exist as Guinier-Preston zones in the base material (BM) with a few formations of δ'/β', and slightly precipitate into T1 in the heat affected zone (HAZ), form σ in the thermal-mechanical affected zone (TMAZ), as well as develop T1 and S' in the nugget zone (NZ) after FSW. The lowest hardness zone (LHZ) exhibits elevated precipitation levels, generating coarse T1 and grain boundary precipitates with evident σ and precipitate free zone. The overall precipitation level of the As-welded joints remains minor, resulting in a slight decrease in hardness at the LHZ. Post-weld AA promotes many T1 formations, enhancing the joint strength and hardness. As a result, the ultimate tensile strength (UTS) of the joints increases from 440 MPa to 506.5 MPa, while the elongation decreases from 13.5% to 3.4%. Furthermore, hardness profiles transform from a low-amplitude wave curve to a high-amplitude W-shaped curve, generating a significant LHZ due to the minimal precipitation of T1. Pre-rolling deformation can enhance dislocation density and low angle grain boundaries (LAGBs), promoting the nucleation of high-density, fine T1 during artificial aging, and especially improves precipitation ability of the LHZ, leading to an elevated hardness increment in the region after AA. Compared to the Joint-AA, both strength and elongation of the Joint-R+AA increase, obtaining 530.5 MPa and 3.9%, respectively.

Understanding interfacial failure mechanisms of advanced high strength automotive steels resistance spot welds under opening-mode loading

This paper addresses interfacial failure (IF) of automotive steel resistance spot welds under opening mode loading (i.e., Mode I), simulated through the cross-tension test. Two modeling approaches were developed to predict cross-tension strength (CTS) during IF: one based on structural stress and the other on fracture mechanics. The structural stress model correlates CTS with the hardness of the fusion zone, while the fracture mechanics model links CTS to the fusion zone’s toughness. Our evaluation of these models across various automotive steels reveals that the hardness-based model is suitable for steels with fusion zone hardness below 350 HV, where IF typically results in ductile fracture. In contrast, for higher-grade steels with fusion zone hardness exceeding 400 HV, the toughness-based model is more appropriate, as IF occurs through brittle crack propagation. The transition in the interfacial failure mechanism of spot welds under Mode I loading—from ductile tensile failure to brittle crack propagation—is governed by the ratio of toughness and hardness of the fusion zone. These insights not only refine the prediction of CTS but also inform the design of effective post-weld heat treatments aimed at enhancing CTS without sacrificing strength under shear loading mode particularly through martensite refining strategy.

Evading strength and ductility trade-off in an inverse harmonic structured metal

In this study, we demonstrated an inverse harmonic structured pure nickel fabricated through a powder metallurgy method, characterized by hard fine-grained zones three-dimensionally enclosed within a connected network of soft coarse-grained zones. This unique heterogeneous structured nickel exhibits excellent strength-ductility synergy, achieving a 100% increase in strength while maintaining ductility comparable to that of the coarse-grained one.

Effect mechanism of softening zone constraint on tensile failure of high-strength steel laser-welded joints

Softening of the heat-affected zone degrades the mechanical properties of high-strength steel-welded joints. This work revealed the key mechanism to suppress the adverse effects of the soft zone on joint performance by analyzing the constraint between different zones. The base metal (BM) can compete with the soft zone for plastic deformation when the soft zone is strongly constrained by the hard zone, which helps to disperse local strain and prevent premature necking. The constraint mode of the soft zone tends to evolve asymmetrically from bilateral to unilateral during the deformation. Reducing the width of the soft zone helps to achieve unilateral constraint, thus shifting the fracture location from the soft zone to the BM.

Microstructure and mechanical properties of TiB2/Al-5Cu composites fabricated by multi-material laser powder bed fusion

This study compares the fabrication of aluminum matrix composites (AMCs) using multi-material laser powder bed fusion (MM-LPBF) with that of conventional methods. The MM-LPBF AMCs exhibit a lamellar structure with a microhardness of 90±2 HV0.05 in the soft zone and 126±2 HV0.05 in the hard zone. The microhardness of the LPBF AMCs is 97±1 HV0.05. Compression tests show that the MM-LPBF AMCs have anisotropic properties with a compressive yield strength of 155±2 MPa along the building direction and 186±4 MPa along the scanning direction. In comparison, the LPBF AMCs have a compressive yield strength of 171±4 MPa. The fracture morphology shows that a single crack in the lamellar composite is accompanied by a deflection of the crack angle. In summary, this work showcases the potential of the MM-LPBF technique for tailoring material properties on demand.

Coupled effect of microstructure heterogeneity and hydrogen on local embrittlement of CGHAZ and IC-CGHAZ in X65 pipeline steel

The local embrittlement of the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated CGHAZ (IC-CGHAZ) in X65 pipeline steel was investigated using an in situ crack-tip opening displacement test in air and H2S, combined with microstructure-based simulation. The IC-CGHAZ exhibited significantly lower cracking resistance, with the fracture toughness reduced by 50% compared to CGHAZ in both environments (0.160 mm in air and 0.017 mm in H₂S for IC-CGHAZ, compared to 0.307 mm in air and 0.034 mm in H₂S for CGHAZ). The martensite/austenite (M/A) constituents showed exceptionally higher intrinsic hardness than the ferrite matrix. The continuous distribution of M/As along the prior austenite grain boundaries resulted in local hardening and the formation of heterogeneous hard zones in IC-CGHAZ. Such microstructure heterogeneity resulted in the unique partitioning of plastic strain localization and stress triaxiality that promotes premature fracture, particularly with the coupled effect of hydrogen. This study provides a novel perspective on the fracture mechanisms and the typical fracture morphology of IC-CGHAZs in pipeline steel under the coupled effect of microstructure heterogeneity and hydrogen.

Structural mechanism of glass transition uncovered by unsupervised machine learning

Uncovering the structural origins of the ubiquitous dynamic arrest phenomenon at the glass transition has long been a challenge due to the difficulty in identifying a rational structural representation from a disordered medium. To address this challenge, we propose a novel approach based on unsupervised learning to define a set of structural fingerprints. In this approach, complex local atomic environments, ranging from short to medium range, are captured by the discretized radial distribution function and projected onto a simple two-dimensional space using a neural network-based autoencoder. This two-dimensional space is characterized by two static structural indicators, P1 and P2, providing a comprehensive and user-friendly representation of the mysterious “glassy structure”. By employing Gaussian mixture modeling, the structural space is autonomously divided into three sections, each representing a unique cluster with similar environments. These indicators not only elucidate the glass transition but also allow for the quantitative prediction of activation barriers for local structural excitations. Furthermore, the unsupervised clustering technique can distinguish between the structural features of “hard zones” and “soft zones”, as well as recently proposed superfast “liquid-like” atoms in glass. This unsupervised machine learning approach demonstrates the utility of seemingly agnostic local structure in amorphous materials, offering insights into the long-sought structural origins of the glass transition.

Influence of strength difference between soft and hard zones on hetero-deformation induced strengthening effect: Taking heterostructured β Ti alloy as an example

The large strength difference between soft and hard zones in heterostructured materials is a double-edged sword: on the one hand, the effective back (τb) and forward stress (τf) are generated to produce hetero-deformation induced (HDI) strengthening and strain hardening effect. On the other hand, the micro-cracks tend to initiate and propagate along the interface to cause premature failure. Here we prepared three different heterostructured Ti-4.8Al-5.6Mo–3V-1.5Cr-1.6Fe alloys to reveal the influence of magnitude of strength difference between soft primary α phase (αp) and hard β matrix on its tensile strength and ductility. It demonstrates that only when the sum of applied stress (τa) and τf is greater than the yield strength of hard β matrix (σyhard), i.e., τa+τf>σyhard, the alloy can obtain both high strength and ductility. Contrary, when τa+τf<σyhard, the alloy exhibits a highest strength but a lowest ductility. These findings are expected to help with designing heterostructured materials for superior mechanical properties.

Friction stir welding of 2507 super-duplex stainless steel: feasibility of butt joint groove filling at different process parameters

ABSTRACT The present work explores the feasibility of groove-filling welding of 6 mm-thick 2507 super-duplex stainless steel (SDSS) in a similar butt-joint configuration using friction stir welding (FSW). A 60° V-groove with a 3 mm root face was designed and machined. The FSW trials were performed at varying rotation speeds (400–600 rpm), downward forces (15–25 kN), and a constant travel speed of 25 mm/min using two WC tools with two different cobalt (Co) contents of 8 and 10 wt.%. The groove filling achievements of the welded joints were monitored using three non-destructive tests: visual, penetrant, and radiographic. All the welded joints were subjected to transverse tensile testing in addition to hardness measurements across the joint area. Microstructure and fracture surface characterisation were also investigated to gain insights into the impact of heat input on joint performance through a comparison with the as-received SDSS. Among the trial FSW processes, the results indicate that the FSW parameters of 25 mm/min, 15 kN, and 400 rpm using a WC-8 wt.% Co tool were successfully used to completely fill the groove and produce a defect-free SDSS butt-joint. This joint has the highest average stir zone (SZ) hardness (342 ± 5 HV) and tensile strength (1256 ± 21 MPa) compared to partially jointed specimens.

Microstructure and Mechanical Properties of Dissimilar Resistance Spot Welded Zn-Coated DP800–TBF1180 Automotive Steels Using MFDC Technology

The use of advanced high-strength steel in the automotive industry is increasing in last decade. This is due to the restrictions to reduce fuel consumption and thereby decrease harmful carbon dioxide emissions. This paper aims to investigate welding properties of dissimilar resistance spot welded hot dip galvanized DP800–TBF1180 automotive steels using MFDC (Mid Frequency Direct Current) technology. LME crack occurrence due to the zinc coating was determined by magnetic particle test. The mechanical properties of welded joints were determined by tensile-shear, cross-tension and hardness measurements. The micro structural characterization was also performed in the weld zones of the joints. The appropriate welding parameter range were selected for welding processes by MFDC technology. Therefore, LME crack formation were not observed according to magnetic particle test. The highest strength was obtained as 17.60 kN and 5.51 kN by tensile-shear and cross tension tests, respectively with a welding current of 8 kA. In addition, a soft zone was found in the HAZ for both base metals. The hardness decrease in HAZ is more pronounced on the TBF1180 side. The soft zone hardness values of the sample S4 were approximately 330 HV, and the hardness values of the base material were measured in the range of 376-386 HV.

Enhancing mechanical properties of high Cr dual-phase FeCrNi medium-entropy alloy through mutual phase transformation and grain refinement

A high Cr Fe40Cr40Ni20 (at.%) medium-entropy alloy was prepared through vacuum arc melting. The alloy exhibited a dual-phase structure comprising face-centered-cubic (FCC) and body-centered-cubic (BCC) phases in a solid solution state. After annealing and cold rolling, two different heterogeneous structures emerged from the original FCC and BCC phases through mutual phase transformation. Moreover, the alloy exhibited a yield strength, ultimate tensile strength, and total elongation of 1.18 GPa, 1.25 GPa, and 13 %, respectively. The high strength of the alloy was mainly attributed to boundary strengthening and heterogeneous deformation-induced (HDI) strengthening within the two heterogeneous structures. Notably, the effectiveness of HDI strengthening was enhanced owing to the complete encapsulation and constraint of soft zones by hard zones in the heterogeneous structure originating from the BCC phase. Additionally, the deformation ability of the alloy mainly depended on dislocation slips and deformation twinning in recrystallized FCC grains and FCC strip phases, thereby ensuring satisfactory ductility.

An In-Depth Investigation of the Effects of Tungsten Inert Gas Welding Process Parameters on Hardness and Corrosion Resistance of 2205 DSS Weldments: New Design of Experiment Parametric Studies and Optimization

The aim of this work is to examine and analyze, using response surface methodology, how the TIG welding process parameters of welding current (WC), welding speed (WS), and N2 with argon as shielding gas affect the hardness and corrosion resistance of 2205 DSS weldments. Due to the equal amounts of ferrite and austenite phases and alloying elements, duplex stainless steel DSS offers good mechanical properties and corrosion resistance. The mechanical characteristics and resistance to corrosion of the weld zone, as well as the heat-affected zone of the DSS, are, however, disturbed as a result of the welding process since it changes the distribution of these two phases and also the alloy is thermally disturbed. Therefore, in this work, an in-depth investigation of the effects of the above-mentioned parameters on the DSS quality has been performed. Results showed that increasing welding current while decreasing welding speed, which corresponds to the highest heat input, led to lower critical pitting potential and weld zone hardness but higher heat-affected zone hardness. The same results were obtained for decreasing welding current while increasing welding speed, which correspond to the lowest heat input. However, the addition of a small percent (%) of N2 to argon shielding gas resulted in increasing the critical pitting potential and decreasing the hardness in welds and heat-affected zones. Numerically, the RSM planned experimental results showed that an optimum welding current of 175A, welding speed of 170 mm/min, and 10% N2 with argon as shielding gas maximized the critical pitting potential up to 318 mV and optimized the hardness of the weld and heat-affected zone to about base metal hardness of 288 and 286 HV, respectively.

Grain size gradient effect on grain boundary mediated deformation of nano-grained metals

The extraordinary mechanical performance of gradient nano-grained (GNG) structures is characterized by the heterogeneity of dislocation-based and grain boundary (GB)-based plasticity, where the grain size gradient effect on these deformation mechanisms even fracture process still remains unclear. Herein, molecular dynamics simulation is conducted on the GNG α-Fe structures to investigate the grain size gradient effect on the GB-mediated hetero-deformation mechanisms. The plastic deformation mechanism changes from GB motion (grain coarsening and merging) to GB-dislocation interaction when the grain size distribution extends from the Hall-Petch softening region to the strengthening region. With increasing grain size gradient, the ultimate tensile stress and flow stress increase in the GB motion-controlled samples, and strain partitioning between hard and soft zones is found to increase in the dislocation-GB interaction- controlled samples from an atomistic perspective. The increased strain partitioning and its accommodation process further contribute to the fracture resistance enhancement, which is demonstrated by the uniform hetero-deformation between coarse and fine grains of the GNG structure, and further avoids localized plastic deformation.

Hardness Distribution and Growth Behavior of Micro-Arc Oxide Ceramic Film with Positive and Negative Pulse Coordination.

Micro-arc oxidation (MAO) is a promising technology for enhancing the wear resistance of engine cylinders by growing a high hardness alumina ceramic film on the surface of light aluminum engine cylinders. However, the positive and negative pulse coordination, voltage characteristic signal, hardness distribution characteristics of the ceramic film, and their internal mechanism during the growth process are still unclear. This paper investigates the synergistic effect mechanism of cathodic and anodic current on the growth behaviour of alumina, dynamic voltage signal, and hardness distribution of micro-arc oxidation film. Ceramic film samples were fabricated under various conditions, including current densities of 10, 12, 14, and 16 A/dm2, and current density ratios of cathode and anode of 1.1, 1.2, and 1.3, respectively. Based on the observed characteristics of the process voltage curve and the spark signal changes, the growth of the ceramic film can be divided into five stages. The influence of positive and negative current density parameters on the segmented growth process of the ceramic film is mainly reflected in the transition time, voltage variation rate, and the voltage value of different growth stages. Enhancing the cathode pulse effect or increasing the current density level can effectively shorten the transition time and accelerate the voltage drop rate. The microhardness of the ceramic film cross-section presents a discontinuous soft-hard-soft regional distribution. Multiple thermal cycles lead to a gradient differentiation of the Al2O3 crystal phase transition ratio along the thickness direction of the layer. The layer grown on the outer surface of the initial substrate exhibits the highest hardness, with a small gradient change in hardness, forming a high hardness zone approximately 20-30 μm wide. This high hardness zone extends to both sides, with hardness decreasing rapidly.

Study on GISSMO fracture model of hot-stamped Tailored-Tempering-Properties parts

To accommodate the varying mechanical demands in different areas of vehicle components, the Tailored-Tempering-Properties technology of hot-stamped parts of high-strength steel came into being. This research crafted a segregated hot and cold partition die system to fabricate U-shaped parts from 22MnB5 with Tailored-Tempering-Properties. The sides and bottom surfaces of the hard and soft zones of the U-shaped parts were sampled, and uniaxial tensile experiments were carried out to obtain the stress-strain curve, tensile strength, and elongation. In addition, notch, shear, and tensile shear sampling were carried out in the soft area at the bottom flange of the U-shaped part, and the force-displacement curve and fracture strain under different stress conditions were determined by tensile test. The GISSMO fracture model was selected to simulate the failure fracture of the material, and the parameters were obtained by Hypermesh pretreatment, LS-DYNA solution calculation, and LS-OPT optimization. Finally, the GISSMO fracture model suit for the soft zone of 22MnB5 U-shaped parts was obtained to characterize and predict its fracture behavior, which is an essential basis for guiding the design and optimization of parts.

Thermal cycling-induced evolution of structure and local mechanical properties in metallic glass

The influence of thermal treatment on the long-term mechanical properties of two different kinds of metallic glasses was investigated in this study. As model systems, bulk metallic glasses (BMGs) with the composition of Pd43Cu27Ni10P20 cycled 60, 200, and 500 times between 77 K and room temperature and Zr44Ti11Cu10Ni10Be25 cycled 300, 500, and 700 times have been employed. To ensure that meaningful steady-state properties have been reached, experiments have been performed three years after sample preparation. It was found that the thermal cycling of the Pd-based BMGs had induced rejuvenation when compared to the non-cycled sample, including a higher relaxation enthalpy, lower modulus, hardness, activation volume, and shear transformation zone volume. However, for Zr-BMGs, a rejuvenation to relaxation behavior is observed. Also, while Zr-BMGs showed significantly changes in yielding behavior upon thermal cycling, fluctuations in Pd-BMGs were negligible. We speculate that the observed substantial differences between these two samples might originate because Pd-BMG has a denser structure and higher fragility, making it more difficult to induce excess free volumes.

Failure of dissimilar QP980/DP600 advanced high strength steels resistance spot welds

This study investigates the microstructure, failure characteristics, and mechanical properties of dissimilar resistance spot welds between QP980 quenching and partitioning steel and DP600 dual phase steel. The dissimilar spot welds exhibited a heterogeneous microstructure, with a fusion zone predominantly composed of martensite, significantly overmatching both base materials. A fully martensitic structure with grain size gradient was observed in the upper-critical heat affected zone (UCHAZ) on both sides of the dissimilar joint. In the sub-critical heat affected zone (SCHAZ), no significant phase transformation occurred on both sides. However, at high welding currents, a slight tempering of pre-existing martensite was detected on the QP980 side resulting in a minor hardness drop in this zone. There was a critical welding current at which the failure mode was changed from interfacial mode to pullout mode. The dissimilar QP/DP joint showed a lower tendency to fail via interfacial failure mode compared to similar QP/QP and DP/DP joints, attributed to a higher fusion zone hardness overmatching factor and greater nugget rotation. Pullout failure of dissimilar QP/DP combination was assessed in detail through interrupted tensile shear test. It was found that failure at stronger QP980 side occurred by ductile cracking mechanism initiated from the notch tip, while failure at softer DP600 side occurred by through-thickness localized necking. The former was found to be the prior failure mechanism controlling the load bearing capacity of the dissimilar joint. The mechanical properties of dissimilar QP/DP joints were discussed in terms of peak load and energy absorption, and then were compared to those of similar DP/DP and QP/QP joints in the light of weld microstructure and failure mechanisms.

Recrystallized hard zone and resultant tri-modal microstructure produces superior mechanical properties in a single-phase heterostructured high-entropy alloy

Heterostructured high-entropy alloys (HS-HEAs) integrate two innovative concepts in metallic materials that encompass alloy design and microstructural engineering. HS-HEAs have demonstrated outstanding mechanical properties. However, it remains challenging to explore these materials due to the intricate nature of their composition and microstructure. In this study, we introduced heterostructures into a single-phase FCC HEA through cold rolling and partial recrystallization. This process resulted in an exceptionally high density of nanoscale recrystallization twins, leading to significant hardening and the formation of heterogeneous hard zones in recrystallized regions with small grain sizes. This tri-modal heterostructure results in improved balance between strength and ductility, high zone boundary density and strong hetero-deformation induced (HDI) effects, and a unique two-stage strain partitioning mechanism that postpones necking. As a result, the alloy exhibits enhanced strength and ductility, resulting in superior mechanical properties. The identification of the recrystallized hard zones is crucial for gaining a proper understanding of the hetero-zones in HEAs and provides valuable insights for the future design of HS-HEAs.

A comparison of different experimental approaches in the determination of dynamic fracture toughness (J1d) behavior for RHA steel cost sustainable MMAW weldments

In the present investigation, the low hydrogen ferrite consumable weldment (weldment-1) and austenitic stainless steel consumable weldment (weldment-2) of rolled and homogenous armor steel (RHA) have been prepared with manual metal arc welding (MMAW). Further, the influence of weldments (weldment-1 and weldment-2) microstructure on the dynamic fracture toughness (J1d), strength, ductility and hardness has been evaluated and compared. The three different approaches have been utilized to estimate J1d for RHA weldments. Further, all three approaches’ outcomes have been compared with the elastic-plastic fracture mechanics (EPFM) approach through Jp-R curve. The results revealed that weldment-1 (yield strength 504.24 MPa and ultimate tensile strength 763.03 MPa) exhibited 26.93 and 25.07% more yield and tensile strength respectively than weldment-2 (yield strength 384.55 MPa and ultimate tensile strength 593.05 MPa). Further, the elongation of weldment-2 (elongation 42%) is 26.72% more than weldment-1 (elongation 32.10%). The joint efficiency and fusion zone hardness of weldment-1 are 5.85 and 57.56%, respectively more than weldment-2. The EPFM approach has good agreement with the correction factor approach (approach 2) and the compliance change rate approach (approach 3) with less than 7% difference in the estimation of dynamic fracture toughness of base metal and weldments (weldment-1 and weldment-2). Further, the percentage difference between maximum load point approach (approach 1) and the EPFM approach is more than 15% difference. The fractographic investigation confirms the ductile fracture in all under-investigated materials.

Outcomes of Connective Tissue (CT) Graft on Periodontal Health in Relation to Dental Implants Placed in Esthetic Zone of Anterior Maxilla

Background: Since 1998, maxillary anterior single immediate implant insertion and provisionalization (IIPP) has been successful and practical. It preserves natural gingival profiles without extending treatment time or needing a removable interim prosthesis. The gingival profile includes a hard tissue zone with bone and a soft tissue zone without bone, extending from the free gingival border to the bony crest's apex. Objective: The purpose of this study was to examine the effect of connective tissue transplant on the thickness and appearance of soft tissue around single implants in the anterior maxilla's aesthetic zone. Study Design: A Prospective cross-sectional study. Settings: A Prospective cross-sectional study was conducted CMH Dental College Multan. Duration: This study was conducted from January 2021 to April 2022. Methods: A total of 64 individuals were included who had criteria for a single cosmetic implant implantation. Detailed demographic information was recorded for enrolled patients once informed written consent was obtained. Patients were equally divided in two groups. Group I (consisting of 32 patients) underwent implant insertion and connective tissue graft implantation (1.50 mm thick) while group II (consisting of 32 patients) simply had implant insertion. Six-month and twelve-month clinical assessments were conducted. Postoperative pain and discomfort were measured. Results: There were 37 (57.8%) males and 27 (42.2%) females among all cases. Buccal tissue thickness in group I significantly increased from 2.45 ±1.80 mm at baseline to 4.0±1.1 mm after 6 months and 3.9±3.46 mm after 12 months (P &lt;0.05), whereas no change was seen in the implant group. After a year, the buccal deficiencies in Group I were significantly less than those in the implant group (∆ -0.46±1.42 mm and ∆ -1.67 ±2.28 mm,). After 12 months, there was a statistically significant difference in proximal bone resorption between the two groups (0.59±0.37 mm and 1.0±0.19 mm), with group I exhibiting less bone resorption than the implant group. Conclusion: In this research, we observed that a single implant placed in the front maxilla in conjunction with a connective tissue graft may enhance peri-implant mucosa thickness and decrease proximal bone resorption.