Hardness Gradient Research Articles

Effect of hardness gradient of laser bionic coupling unit on wear resistance of 6082 aluminum alloy

Different hardness units were processed by laser melting according to the study and observation of wear-resistant surfaces of shells. Then a biomimetic 6082 aluminum alloy surface consisting of various hardness gradient units was formed to simulate the non-smooth surface of natural organisms. The influence of unit hardness gradient on wear resistance was discussed. Wear test showed the bionic units positively affected the wear resistance of samples. Experiment showed the wear resistance of bionic samples with units of three hardness gradients was the highest.At the same time, The bionic wear-resistant mechanism of “soft and hard phases” was deeply analyzed and discussed.

Loaded contact fatigue-oriented carburizing surface optimization model of spiral bevel gears

In order to improve geometric accuracy and loaded contact fatigue life on tooth surface carburizing, an innovative optimization model is established for spiral bevel gears. First, finite element method (FEM)-based tooth surface carburizing simulation is performed. Where, the output carbon content and hardness gradient are used to determine the S-N curve of the given material 20MnCr5 steel. The output residual stress is integrated into the bending stress solution. The output carburizing surface deformation is introduced into the composite surface geometric accuracy. Second, loaded contact fatigue assessment is performed by FEM-based loaded contact stress and S-N curve. Where, loaded contact stress considering residual stress is determined by simulated loaded tooth contact analysis (SLTCA). Third, loaded contact fatigue life-oriented carburizing surface optimization model is established. In this modeling, the composite surface geometric error including carburizing deformation, loaded contact deformation and the prescribed ease-off is used to reflect manufacturing geometric accuracy. Moreover, the limit fatigue life at dangerous point is used a standard to determine the target surface. A sensitivity analysis strategy-based optimization strategy is developed. The proposed method can effectively improve the process parameter modification strategy by considering the collaborative optimization on geometric accuracy and loaded contact fatigue life. Moreover, it also can provide new technical for the current co-design and manufacture of spiral bevel gears.

Fabrication of functionally graded W[sbnd]Cu composite via variable speed induction sintering and subsequent infiltration

In the current study, a novel approach is proposed as a method for producing Tungsten Copper composite with functionally gradient structure. In this process, a green tungsten powder compact is sintered using a special designed induction sintering set up with variable velocities to create a gradient structure. Then, the copper is infiltrated into the sintered graded tungsten scaffold at 1200 °C at hydrogen atmosphere. To investigate the applicability of the process, tungsten powder is pressed and the compact is sintered using induction selective sintering with variable speeds (V1-V4). Tungsten skeleton was infiltrated with copper and WCu composite is produced. The mechanical properties, microstructure, and electrical conductivity at different zones were examined. Corresponding to the gradient structure, the composite showed gradients in hardness and electrical conductivity. The microhardness is varied from 219 Hv to 330 Hv, and electrical conductivities changed from 15 to 33%IACS. This method seems to be very interesting and promising for future industrial applications.

Effect of spatial hardness distribution in rolling contact fatigue performance of bearing contacts

In this investigation the effect of various spatial hardness gradients (e.g. linear and nonlinear) on rolling contact fatigue (RCF) life are examined through a Mises-based plasticity framework. The plasticity framework, which utilizes kinematic hardening was embedded into a finite element (FE) model, which utilized continuum damage mechanics (CDM) to facilitate life calculations by simulating material deterioration as a function of cycle. CDM critical parameters – elastic damage law resistance stress (σr), elastic damage rate exponent (m), plastic damage law resistance stress (S0), plastic damage rate exponent (q) – were established from open literature experimental torsion stress-life data. A parametric study was then performed to quantify the relation between RCF life and (1) type of spatial gradient (e.g. linear, polynomial degree 4, polynomial degree 0.25), (2) depth of spatial gradient normalized with respect to half-contact width (e.g. 1b, 2.5b, 5b, 10b), (3) magnitude of residual stress, and (4) contact pressure. Additionally, FE simulations were conducted on non-graded, or uniform hardness microstructures and used to compare to the results from the parametric study. The RCF results indicated that polynomial gradients with degree greater than 1 outperform linear and polynomial gradients with degree less than 1, with differences between gradient types more apparent at shallower gradient depths. RCF resistance is improved with increasing gradient depth and increasing compressive residual stress. Regression analysis of the results from this investigation yielded a fatigue life expression that was formed as a function of the 4 input variables, with life improvement factors comparing well with experimental data from the open literature.

A comprehensive quantitative characterisation of the multiphase microstructure of a thick-section high strength steel

The through-thickness heterogeneous microstructure of thick-section high strength steels is responsible for the significant scatter of properties along the thickness. In this study, in order to identify the critical microstructural features in the fracture behaviour and allow for design optimisation and prediction of structural failure, the through-thickness microstructure of thick-section steels was extensively characterised and quantified. For this purpose, samples were extracted from the top quarter and middle thickness positions, and a combination of techniques including chemical composition analysis, dilatometry, and microscopy was used. The hardness variation through the thickness was analysed via micro-Vickers measurements and the local hardness variation in the middle section was studied via nanoindentation. The middle section presented larger prior austenite grain (PAG) sizes and larger sizes and area fraction of inclusions than the top section. Additionally, cubic inclusions were observed distributed as clusters in the middle, sometimes decorating PAG boundaries. Defects associated with the cubic inclusions or the interface between the matrix and the circular and cubic inclusions were observed in the mid-thickness. Moreover, the middle section presented long interfaces with the most significant hardness gradients due to the presence of hard centreline segregation bands. Hence, the microstructural and nanoindentation analyses indicated the middle section as the most likely area to have the lowest fracture toughness and, therefore, the most unfavourable section for fracture performance of the investigated S690QL high strength steel. The detrimental effect of the middle section was confirmed via CTOD tests where the middle presents lower fracture toughness than the top section.

Grain Orientation and Hardness in the Graded Interlayer of Plasma Sprayed W on CuCrZr

In this work a W coating was deposited through PS in Ar-H2 atmosphere on a substrate of CuCrZr with an interlayer consisting of gradually changing fractions of Cu and W. The W coating and the interlayer showed good adhesion and no cracks were observed. The hardness of W and Cu along the interlayer was determined by nano-indentation tests and the results showed that a hardness gradient does exist in both the metals. Microstructural examinations suggest that the hardness gradient depends on the texture, which exhibits significant differences along the interlayer: multiplication and movement of dislocations, and localized plasticity under the indenting tip are strongly affected by grain orientation.

Functionally graded laminated composites fabricated from MAX-phase filled preceramic papers: Microstructure, mechanical properties and oxidation resistance

This paper describes the fabrication and characterization of novel preceramic paper-derived functionally graded materials (FGMs) based on Ti3(Si,Al)C2 MAX phase. The FGMs with different architecture were fabricated via spark plasma sintering of stacked preceramic papers at 1250 °C for 5 min. Microstructure, phase composition and elemental distribution were analyzed by scanning electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy, respectively. Oxidation tests were performed in air at 1300 °C for 5 h. FGMs containing Al- and Si-enriched MAX-phase layers were formed. The fabricated materials exhibit high flexural strength (over 600 MPa), which are dependent on microstructure and composition of individual layers as well as the architecture of composites. It was found that texturing of MAX phase grains during SPS results in anisotropic hardness of the composite. The difference in the composition of the individual layers also provides a hardness gradient in the composite. It was shown that the formation of the outer layer from the Al-enriched Ti3Al(Si)C2 MAX phase increases the corrosion resistance of Ti3SiC2-based composites. The high corrosion resistance of FGMs is due to the growth of a continuous and dense Al2O3 oxide layer.

Gradient nanostructure, phase transformation, amorphization and enhanced strength-plasticity synergy of pure titanium manufactured by ultrasonic surface rolling

In this study, ultrasonic surface rolling process (USRP) was used to fabricate a gradient nanostructured commercial pure titanium. The site-specific microstructure, refining mechanisms and mechanical properties were investigated by high-resolution transmission electron microscopy, electron backscatter diffraction and tensile test. The results show that the surface layer exhibited multi-gradient features along the depth, including grain size gradient, deformation twin gradient, strain gradient, orientation gradient and hardness gradient. The gradient structure consisting of equiaxed nanograin layer, elongated lamellar layer, deformation twinning profuse layer, and coarse-grained layer along the depth direction. The formation mechanisms of gradient nanostructure are thoroughly elucidated, and the synergistic effect of twin and dislocation plays an important role in grain refinement. In addition, amorphous bands (generated by localized dislocations or phase transformation triggered crystalline to amorphous transition) and nano-thick lamellar with face-centered cubic (FCC) structure (<0001> HCP //<001> FCC , {1 1 ¯ 00} HCP //{2 2 ¯ 0} FCC and {11 2 ¯ 0} HCP //{220} FCC ) were formed in the nanograin layer, and the phase transformation mechanisms were elucidated. The HCP to FCC phase transformation and amorphization play a role in refining grain. Tensile test results suggested that the gradient nanostructure improved the strength (yield strength and ultimate tensile strength improved from 342 MPa and 526 MPa to 412 MPa and 598 MPa, respectively) while maintaining enough plasticity. The strength-plasticity synergy was attributed to the coordinated deformation of the gradient nanostructure and the coarse-grained core.

Hydrogen nitrogening in great discharge with AC power

The paper substantiates and experimentally shows the possibility of anhydrous nitriding in a glow discharge (BATR) with AC power supply (AC), as well as the possibility of switching the shape of pulses, changing their polarity, which is solved by introducing a glow discharge nitriding device with a power source alternating current of a given frequency. The analysis of the results of metallographic studies of the modified layer indicated the presence of a more uniform gradient of hardness in depth (gradient decreased by 1.7 - 3.5 times), which increases the wear resistance of parts and quality indicators of strengthening parts while reducing processing costs and installation costs. At BATR in CCR with supply of current of industrial frequency the installation for realization of process becomes cheaper. The latter factor will contribute, among other aspects, to the wider introduction of BATR technologies. The use of alternating current affects the course and results of nitriding, as periodic changes in the polarity of the electrodes of the chamber contributes to the cleaning of the surface from the adsorption layer, which positively affects the nature of nitrogen saturation (gradient of microhardness times), contributes to a significant change in the phase composition of the surface modified layer and allows to expand the technological capabilities of nitriding in the direction of obtaining the required performance characteristics of working surfaces, in particular tribological characteristics

Combined plasma treatment of AISI-1045 steel by hastelloy deposition and plasma nitriding

AISI-1045 steel is medium strength steel, and it needs surface modification to use in machine components. The surface properties of steel can be improved by plasma nitriding. The nitriding performance can be upgraded by introducing special alloying elements (nitride forming elements such as chromium, molybdenum), which have a strong affinity with nitrogen. Here, the combination of cathodic cage plasma nitriding (equipped with hastelloy cathodic cage, containing nitride forming elements) and conventional plasma nitriding is used to improve the surface properties. Thus, it is used to deposit such alloying elements, and subsequent plasma nitriding is performed to cause nitrogen diffusion to improve the coating adhesion and hardness gradient formation. The substrate hardness (346 HV) is increased up to 982 HV, 1426 HV, and 1692 HV by hastelloy deposition and post-nitriding at 400 and 450 °C, respectively. Additionally, the duplex treated samples show a favorable hardness gradient. The treated samples show the formation of iron nitrides Fe4N, Fe2−3N, chromium, and nickel phases. The voids and pores have appeared on the hastelloy deposited sample, removed by post-nitriding. The wear resistance is significantly improved by duplex treatment. The duplex treatment reported here can be used effectively to improve the surface hardness and wear resistance of AISI-1045 steel. Additionally, a single processing unit is required, needs low-processing temperature, surface uniformity, the whole processing time is relatively short, and the compatibility of this system with the industry proposed that this work would be helpful for the industrial requirement.

Effects of beam oscillation on microstructural characteristics and mechanical properties in laser welded steel-copper joints

In this project, we present experimental laser welding of pure copper to stainless steel in the presence of beam oscillation. The microstructure, element distribution and mechanical properties of joints welded with and without exposure to beam oscillation were contrastively investigated employing scanning electron microscopy, energy dispersive X-ray spectrometry and other methodologies. The results showed that joint microstructure subjected to beam oscillation changed from massive and columnar structures to granular and globular structures. The driven force of circular oscillating beam on the molten pool produced a vortex flow, which promoted heat and mass transfer during laser welding. Moreover, it also led to the refinement of grains and discrete distribution of precipitated phases (rich-Fe α phase and rich-Cu ε phase), thereby reducing microstructural segregation and improving the resistance to crack in welds. The hardness gradient in joints suffered from beam oscillation was significantly reduced because of improvement in microstructural homogenization. The tensile strength and elongation of welded joints subjected to beam oscillation were 282.82 MPa and 2.68 mm respectively, 110.41% of strength and 153.14% of elongation compared to that of joints welded without exposure to beam oscillation. Joint improvement in strength and plasticity was attributive to microstructural refinement and compositional homogeneity.

Microstructure and wear behavior of inductive nitriding layer in Ti–25Nb–3Zr–2Sn–3Mo alloys

In human implant materials, beta (β) titanium alloys have been extensively used because of their excellent biocompatibility and lower elasticity modulus; however, they have poor hardness and wear resistance. Herein, vacuum induction nitriding technology is used to strengthen the surface of a Ti–25Nb–3Zr–2Sn–3Mo (TLM) alloy. The nitriding layer microstructure is characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM)–energy dispersive spectrometry (EDS) and optical microscopy (OM). The mechanical properties of nitriding layer are tested using cross-sectional hardness gradient and nano-indentation. Furthermore, the wear resistance and mechanism of nitriding layer are examined using a self-made reciprocating wear tester, a three-dimensional (3D) profilometer and SEM, respectively. The results demonstrate that an approximately 30-μm thick nitriding layer is formed on the TLM alloy surface after induction nitriding treatment. Interestingly, it forms a gradient structure that can improve the hardness and wear resistance of samples. Moreover, at a maximum test load of 10 N, the abrasion loss of the raw sample is 463 times that of the nitriding sample. Furthermore, the friction coefficient of the raw sample significantly exceeds that of the nitriding sample at three test loads. The wear mechanism between the friction pair Al 2 O 3 ball and the raw sample is primarily a combination of abrasive and adhesive wear, whereas that of the nitriding sample is primarily abrasive wear. The results demonstrate that the nitriding layer significantly improves the wear performance of TLM alloys. • The surface of TLM alloy can be rapidly strengthened by induction nitriding. • The gradient strengthening layer can be formed by induction nitriding. • The formation of gradient structure has excellent performance in improving the wear resistance of TLM alloy.

Steel-copper functionally graded material produced by twin-wire and arc additive manufacturing (T-WAAM)

In this work, a functionally graded material (FGM) part was fabricated by depositing a Cu-based alloy on top of a high strength low alloy (HSLA) steel by twin-wire and arc additive manufacturing (T-WAAM). Copper and steel parts are of interest in many industries since they can combine high thermal/electrical conductivity, wear resistance with excellent mechanical properties. However, mixing copper with steel is difficult due to mismatches in the coefficient of thermal expansion, in the melting temperature, and crystal structure. Moreover, the existence of a miscibility gap during solidification, when the melt is undercooled, causes serious phase separation and segregation during solidification which greatly affects the mechanical properties. Copper and steel control samples and the functionally graded material specimen were fabricated and investigated using optical microscopy, scanning electron microscopy, and high energy synchrotron X-ray diffraction. Retained δ-ferrite was found in a Cu matrix at the interface region due to regions with mixed composition. A smooth gradient of hardness and electric conductivity along the FGM sample height was obtained. An ultimate tensile strength of 690 MPa and an elongation at fracture of 16.6% were measured in the FGM part.

Surface hardening of high- and medium-entropy alloys by mechanical attrition at room and cryogenic temperatures

Surface hardening by mechanical attrition at room and cryogenic temperatures has been studied on CrMnFeCoNi high- and CrCoNi medium-entropy alloys. The hardness gradient is produced by severe plastic deformation through dislocation slip and mechanical twinning, finally leading to an ultrafine or even nanocrystalline structure. The hardness of samples after surface attrition at cryogenic temperature is slightly less than that of samples deformed at room temperature. A small amount of deformation-induced martensitic transformation is only observed in a narrow surface layer of the medium-entropy alloy severely deformed under cryogenic conditions. Reasons for these observations are discussed.

Effect of Synthesis Procedure on Particle Dispersion and Hardness of Al- Sic Functionally Graded Metal Matrix Composite

Abstract Fabrication of Functionally Graded Metal Matrix Composites (FGMMC) especially with high ceramic reinforcement's volume fraction is highly challenging. Depending on the processing technique and process parameters various defects may arise. This research aims to find the best procedure to make FGMMCs with the highest quality and minimum cost. A new method is proposed that incorporates lost-foam and melt infiltration with semicentrifugal casting to produce FGMMC. Experiments were performed to in-situ fabricate 6061-Aluminum alloy reinforced with gradient distributed Silicon carbide particles (Al/SiC FGMMC). Effect of SiC %, Al pouring temperature and rotational speed on the fabricated specimens hardness and reinforcement gradient were investigated using design of experiments and regression analysis. Results reveal the optimum procedure and process settings based on desired properties/gradient required. Mathematical model formulated captures the effect of these process parameters on process cost, and cost of poor quality. Improper selection of those parameters may lead to extensive losses due cost of poor quality which is 12 times higher than the material cost. The proposed manufacturing process proved satisfactory in ensuring proper dispersion. A desirability function can by used to determine the process parameters and volume fraction that minimizes the defects and gives superior properties for a specific application.

Enhanced plasma nitriding efficiency and properties by severe plastic deformation pretreatment for 316L austenitic stainless steel

In order to enhance the plasma nitriding efficiency and improve the hardness and wear resistance of 316L austenitic stainless steel, severe plastic deformation was adopted prior to plasma nitriding. The microstructure, phase constituents, hardness and wear resistance samples after plasma nitriding were determined by metallographic microscope, X-ray diffractometer, microhardness tester and universal friction and wear tester. The results showed that severe plastic deformation had significant enhancement effect on plasma nitriding efficiency. Under the same plasma nitriding parameters of 420 °C and 4 h, the thickness of nitriding layer was increased from 6.36 μm to 14.10 μm, more than twice thicker enhanced by severe plastic deformation. XRD confirmed that the nitriding layer is consisted of S phase. Meanwhile, the surface hardness was improved from 990HV to 1110 HV and with moderated gradient of cross sectional hardness; more valuably, the wear resistance was greatly enhanced by severe plastic deformation.

Influence of laser-hardening on microstructure and hardness of plasma-nitrided compacted graphite iron

Duplex surface treatments involving plasma nitriding (PN) followed by laser hardening (LH) were performed in this work on grade 450 compacted graphite iron (CGI) to investigate the effects of PN duration (1 and 3 h) and LH power (400, 449, and 514 W) on microstructure evolution, nitrogen profile, and hardening depth of CGI. X-ray diffraction (XRD) phase analysis indicated a reduction in and partial decomposition of ε and γ’ nitrides corresponding to an increase in laser power. The existence of a new nitrogen-rich austenitic phase—FeN0.076—was observed along with the identification of certain iron oxides. The proposed duplex treatment of CGI was found to facilitate the establishment of duplex layers with higher surface hardness (826–951 HK 0.05) and greater hardening depths (300–750 μm). Additionally, a reduction in the hardness gradient was observed between the surface and substrate. This affords a major advantage in applications involving operations on mechanical components.

Growth and mechanical properties of Nb[sbnd]Cr carbide coatings on graphite by TRD technique

Niobium‑chromium (NbCr) carbide coatings were deposited on graphite using the thermo-reactive deposition and diffusion (TRD) technique. The microstructure, hardness, and coefficient of thermal expansion were characterized to search for the optimal treatment parameters for hard and adherent coatings on graphite. The growth rate increased with treatment temperature for fixed treatment duration and remained constant at a constant temperature for about 6 h. The coatings were found to consist of alternating Nb and Cr layers, and this interesting multilayer structure might be explained by the thermodynamics of oxide reduction and carbide formation reactions. Compared to the single-carbides layer, the hardness of the complex coatings increased from about 2500 HV to above 3000 HV, and displayed a smaller gradient of the depth-dependent hardness from the coating to the substrate. The variation of coefficient of thermal expansion of the samples with temperature was similar to that of the graphite substrate at temperatures from 40 to 450 °C. These results showed the effectiveness of the TRD method for synthesizing hard and adherent carbide coatings on graphite, and for other coating/substrate systems with sharply dissimilar mechanical properties.

Effect of Heat Input on the Ballistic Performance of Armor Steel Weldments

The purpose of this study is to examine the projectile penetration resistance of the base metal and heat-affected zones of armor steel weldments. To ensure the proper quality of armor steel welded joints and associated ballistic protection, it is important to find the optimum heat input for armor steel welding. A total of two armor steel weldments made at heat inputs of 1.29 kJ/mm and 1.55 kJ/mm were tested for ballistic protection performance. The GMAW welding carried out employing a robot-controlled process. Owing to a higher ballistic limit, the heat-affected zone (HAZ) of the 1.29 kJ/mm weldment was found to be more resistant to projectile penetration than that of the 1.55 kJ/mm weldment. The ballistic performance of the weldments was determined by analyzing the microstructure of weldment heat-affected zones, the hardness gradients across the weldments and the thermal history of the welding heat inputs considered. The result showed that the ballistic resistance of heat affected zone exist as the heat input was decreased on 1.29 kJ/mm. It was found that 1.55 kJ/mm does not have ballistic resistance.

Evolution of a low-alloy steel/nickel superalloy dissimilar metal weld during post-weld heat treatment

The design of the dissimilar metal weld investigated here is aimed at applications in the steam generator of a sodium-cooled nuclear reactor, with a multi-decade lifespan in demanding operational conditions. It consists in a narrow-gap joint between 2.25Cr-1Mo low-alloy steel and an austenitic alloy using a nickel-based alloy 82 as filler material. This study focuses on understanding the microstructural and micromechanical evolution in the near fusion boundary region between the low-alloy steel and the nickel alloy filler metal during post-weld heat treatment, using notably electron probe micro-analysis and nano-indentation. The difference in matrix phase and chemical composition between the two alloys leads to a large difference in chemical potential for carbon, which is mobile at the post-weld heat treatment temperature. A number of fine-scale characterization techniques were used to assess the gradient of composition, hardness, and microstructures across the fusion boundary, both as-welded and after post-weld heat treatment. This complete analysis permits to highlight and understand the main microstructural and micromechanical changes occurring during post-weld heat treatment and opens the way to their long term study in service conditions.