Thermal-mechanical Process Research Articles

Simulation of thermal-mechanical coupling in Al alloy/steel inertia friction welding

Inertia friction welding (IFW) is a complicated thermal-mechanical coupling process. In-depth understanding of the thermal-mechanical process can improve the joint quality. In this study, a three-dimensional (3D) thermal-mechanical coupling model was successfully established to simulate the evolution of temperature, energy density, stress, strain, and flow behavior of the IFW process between 2219-O Al alloy and 304 stainless steel. The developed model was validated by comparing it with the measured temperature and deformation. Results show a transforming distribution of interface temperature from a M-shape to a squished V-shape during the IFW process. The characterization is attributed to the transformation of the accumulated friction energy density at the interface from a M-shape to an approximate V-shape due to the combined effect of varying sliding velocity and contact pressure. The Mises stress at the region near the interface is high and low at the steel and Al alloy sides, respectively. A significant portion of the kinetic energy stored in the flywheel is dissipated in the form of plastic deformation energy and cannot be neglected. The tracer particles show that most Al alloy at the initial interface is extruded to form flash, while a small amount of Al alloy is sticky and remains at the interface center. Increasing friction pressure and initial rotation speed result in higher interface temperature, axial shortening distance, and proportion of plastic work in total energy input.

High strength, high conductivity and excellent softening resistance Cu-Ni-Fe-P alloy

This article provided a detailed study on the evolution of microstructure and properties of Cu-Ni-Fe-P alloy during thermal mechanical process. The experimental results showed that the Cu-0.3Ni-0.3Fe-0.16P alloy had an optimal comprehensive properties of tensile strength (672 MPa) and electrical conductivity (69.2 %IACS). Through charactering microstructure by the TEM, the results showed that the ultrafine precipitates were (Ni, Fe)2P. The pre-aging sample had a higher tensile strength and electrical conductivity in the initial stage of the final aging because the pre-aging reduced the concentration of the dissolved Ni, Fe and P solutes in the Cu matrix and increased dislocations density during the subsequent rolling. The precipitation strengthening and dislocation strengthening were two main strengthening contributions to the Cu-Ni-Fe-P alloy. The high temperature softening resistance of Cu-Ni-Fe-P alloy was above 500 °C. And the softening mechanism of Cu-Ni-Fe-P alloy was significantly associated with the coarsen precipitates and the recrystallization during the high-temperature annealing. This work provided a guiding significance for the development of high-strength and high-conductivity Cu alloys.

Interaction between dynamic recrystallization and phase transformation of Ti-43Al-4Nb-1Mo-0.2B alloy during hot deformation

The β solidified γ-TiAl alloy holds important application value in the aerospace industry, while its complex phase compositions and geometric structures pose challenges to its microstructure control during the thermal-mechanical process. The microstructure evolution of Ti-43Al-4Nb-1Mo-0.2B alloy at 1200 °C/0.01 s−1 was investigated to clarify the coupling role of dynamic recrystallization (DRX) and phase transformation. The results revealed that the rate of DRX in α2+γ lamellar colonies was comparatively slower than that in βo+γ mixed structure, instead being accompanied by intense lamellar kinking and rotation. The initiation and development rates of DRX in α2, βo, and γ phases decreased sequentially. The asynchronous DRX of the various geometric structures and phase compositions resulted in the uneven deformed microstructure, and the dynamic softening induced by lamellar kinking and rotation was replaced by strengthened DRX as strain increased. Additionally, the blocky α2 phase and the terminals of α2 lamellae were the preferential DRX sites owing to the abundant activated slip systems. The α2→βo transformation within lamellar colonies facilitated DRX and fragment of α2 lamellae, while the α2→γ transformation promoted the decomposition of α2 lamellae and DRX of γ lamellae. Moreover, the varied βo+γ mixed structures underwent complicated evolution: (1) The γ→βo transformation occurred at boundaries of lamellar colonies, followed by simultaneous DRX of γ lamellar terminals and neighboring βo phase; (2) DRX occurred earlier within the band-like βo phase, with the delayed DRX in enclosed γ phase; (3) DRX within the βo synapses and neighboring γ phase was accelerated owing to generation of elastic stress field; (4) Dispersed βo particles triggered particle stimulated nucleation (PSN) of γ phase. Eventually, atomic diffusion along crystal defects in βo and γ phases caused fracture of band-like βo phase and formation of massive βo particles, impeding grain boundary migration and hindering DRXed grain growth of γ phase.

Modeling of laser-assisted micro-milling Inconel718

Nickel-based superalloy Inconel718 is a typical difficult-to-machine material with the characteristic of high strength and hardness. Demands for micro parts of nickel-based superalloy Inconel718 are growing in aerospace, biomedical and other fields. Laser-assisted micro-milling (LAMM) can realize precision machining of the workpiece because of its lower cutting force. However, the machining mechanism and the change law of cutting force during LAMM of nickel-based superalloy Inconel718 are still unclear. There is a crucial need for modeling of LAMM Inconel718. This paper explores LAMM of nickel-based superalloy Inconel718 and realizes the simulation of LAMM process. A three-dimensional transient heat transfer finite element (FE) model is established to predict the temperature distribution of a Gaussian heat source in LAMM. Multiple sets of simulations of laser pre-heating with various laser power are conducted to analyze the temperature fields of workpiece and determine the appropriate laser parameters. A novel 3D model of the full machining process of LAMM Inconel 718 is constructed by ABAQUS with multi-subroutine, a fully coupled thermal-mechanical process is achieved. A modified Johnson-Cook constitutive model is used to reflect the size effect and flow stress of LAMM Inconel 718. Finally, the validity of the models is verified by experiments. Based on the verified model, the appropriate laser pre-heating parameters are obtained, and the prediction of cutting forces of LAMM Inconel718 is achieved.

A novel surface grain boundary engineering approach to improving corrosion resistance of a high-N and Ni-free austenitic stainless steel

Grain boundary engineering (GBE) can effectively improve the corrosion resistance of materials, but the traditional thermal-mechanical process still exhibits some limitations. In the present work, a novel surface spinning strengthening (3 S) technology followed by heat treatment was applied to modify the microstructure near the surface (∼ 400 μm) of a high-N Ni-free austenitic stainless steel. Under an optimal condition of annealing at 1323 K for 60 min after 3 S, the fraction of special boundaries mainly involving Σ3 boundaries in GBE layer was significantly improved from 48.3 % to 69.1 %, thus achieving excellent resistances to intergranular corrosion and stress corrosion cracking.

열가공제어한 저탄소강의 용접부 기계적성질에 미치는 용접기법의 영향

This study investigated the mechanical properties of thermal mechanical control process steel and obtained the following results. In a welded metal (WM) subjected to both single submerged arc welding (SSAW) and tandem SAW (TSAW), grain boundary ferrite (GBF) was generated along with a prior austenite grain boundary and ferrite side plate (FSP). In particular, TSAW with low basicity and a high heat input yielded coarser results than those of SSAW. In the heat affected zone (HAZ), as the heat input increased, the fraction of low-temperature transformation products such as bainite and acicular ferrite decreased, whereas the fraction of primary ferrites such as GBF, FSP, and polygonal ferrite (PF) tended to increase, and the grain size tended to coarsen. The hardness value of the WM increased in the order of electro gas welding, SSAW, and TSAW regardless of the heat input, and the hardness value of the HAZ tended to increase the softening width and decrease the minimum value with an increase in the heat input. Finally, the joint strength of the weld zone tended to decrease as the heat input increased. However, the joint strength of the weld zone under all the applied welding techniques met the value required by the base metal.

Simulation and Experimental Study of Nanosecond Pulse Laser Removal of Epoxy Paint on 6061 Aluminum Alloy Surface

Laser paint removal is a new cleaning technology that mainly removes paint through thermal ablation and mechanical stripping mechanisms. This paper established a thermal-mechanical coupling laser removal model of paint based on the heat conduction equation, Newton’s second law, and Fabbro’s theory. The removal process of epoxy resin paint film on an aluminum alloy surface via a nanosecond pulsed laser was studied using finite element simulations and experimental measurements. The simulation and experimental results show that the nanosecond pulse laser’s primary paint removal mechanism is the mechanical stripping caused by thermal stress and plasma shock. The laser paint removal threshold is 1.4 J/cm2. In addition, due to the different generation times of plasma shock and thermal stress, the mutual superposition of stress waves occurs in the material. This results in a discrepancy between the actual and thermal stress differences. Moreover, the thermal stress difference causes the maximum actual stress difference to fluctuate. The simulation model established in this paper can provide a reference for studying the thermal-mechanical coupling process of laser paint removal.

Variations in microstructure and mechanical properties along thickness direction in a heavy high strength low alloy steel plate

A thermal-mechanical control process (TMCP) followed by tempering at 600 °C was applied to produce 420 MPa grade high strength low alloy (HSLA) steel heavy plates. The microstructure and mechanical properties through the thickness were analyzed using various techniques including optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD), tensile and impact tests. Microscopy analyses showed that a fine-grained structure with an average grain size of 6–10 μm could be obtained by TMCP. After subsequent tempering, the microstructure near the surface of the plate was dominated by bainite, while the microstructure of the core was mainly characterized by aggregated carbides and ferrite. In addition, the density of high-angle grain boundaries (HAGB) on the surface was significantly higher than that in the core. Tensile tests showed a decrease in both yield and tensile strengths from 530.1 and 626.9 MPa on the surface to 384.3 and 496.7 MPa in the core, respectively. The contribution of different strengthening mechanisms to the variations in strengths along the thickness was clarified. In addition, the deformation of ferrite and the initiation of micro-voids during tensile tests were also analyzed.

Effects of metalloid Si on the microstructure and mechanical properties of Fe36Ni36Cr10Mo1Al17−XSiX alloys

Multi-component alloys with good mechanical properties and casting liquidity have become a hotspot in the materials field. Typically, the addition of minor metal elements plays a vital role in strengthening the mechanical properties. However, doping metalloid elements such as Si into multi-component alloys is also an effective method to enhance the mechanical properties. In the present study, the effect of Si (0 ∼ 4 at%) on the microstructure and mechanical properties was investigated within both as-cast and thermal mechanical treatment states. In as-cast state, all 4 alloys exhibited dendrites and interdendrites structure. The fine structure enhanced tensile strength for Si1 and improved ductility for Si2. Once the Si content reached 4 at%, both the strength and ductility dropped dramatically. Phase selective recrystallization and aging greatly increased the tensile strength and ductility simultaneously. The alloys with addition of Si had similar mechanical properties after thermal mechanical treatment process. Si2 after recrystallization and aging was considered a promising candidate for further application with a yield strength of 960 MPa, tensile strength of 1463 MPa and fracture strain of 20.5%. This work provided a new prospect to design multi-component alloys with nonmetallic elements, which may be applied to wear, corrosion and oxidation resistance fields.

Imparted benefits on mechanical properties by achieving grain boundary migration across voids

Understanding the interaction of micro-voids and grain boundaries is critical to achieving superior mechanical properties for safety-critical parts. Micro-voids and grain boundaries may interact during advanced manufacturing processes such as sintering, additive manufacturing and diffusion bonding. Here, we show imparted benefits on mechanical properties by achieving grain boundary migration across voids. The micro-mechanisms and quantitative analysis of grain boundary migration on local deformation were studied by integrated in-situ EBSD/FSE and crystal plasticity finite element modelling. It is revealed that a migrated grain boundary does not alter the activated slip systems but precludes grain boundary-multislip interaction around interfacial voids to alleviate stress concentrations. The stress mitigation caused by grain boundary migration is almost the same as that caused by void closure under the example diffusion bonding thermal-mechanical process used in this study. This new understanding sheds light on the mechanistic link between GND hardening, grain boundary migration and the corresponding material tensile behaviour. It opens a new avenue for achieving superior mechanical properties for metallic parts with micro-defects such as those generated in diffusion-bonded, sintered and additive manufactured components.

Salt Cavern Thermal Damage Evolution Investigation Based on a Hybrid Continuum-Discrete Coupled Modeling

The integrity and stability of salt caverns for natural gas storage are subjected to a gas cycling loading operation. The coupled effect of confining pressure and temperature on the response of the salt cavity surrounding the wall is essential to stability analysis. In this study, a hybrid continuum-discrete model accounting for the thermal-mechanical process is proposed to investigate the thermal-damage evolution mechanism towards a field case with blocks falling off the salt cavity. The salt cavity is modeled by continuum zones, and the potential damage zones are simulated by discrete particles. Three specimens at different locations around the surrounding wall are compared in the context of severe depressurization. The dynamic responses of rock salt, including temperature spatiotemporal variation, microscopic cracking patterns, and energy evolution exhibit spatial and confinement dependence. A series of numerical simulations were conducted to study the influence of microproperties and thermal properties. It is shown that the evolution of cracks is controlled by (1) the thermal-mechanical process (i.e., depressurization and retention at low pressure) and (2) the anomalous zone close to the brim of the salt cavity surrounding the wall. The zone far away from the marginal surrounding wall is less affected by temperature, and only the mechanical conditions control the development of cracks. This continuum/discontinuum approach provides an alternative method to investigate the progressive thermal damage and its microscopic mechanism.

Hardness Distribution of Al2050 Parts Fabricated Using Additive Friction Stir Deposition.

The solid-state additive friction stir deposition (AFSD) process is a layer-by-layer metal 3D-printing technology. In this study, AFSD is used to fabricate Al-Cu-Li 2050 alloy parts. The hardness values for various regions of the as-deposited built parts are measured, and the results are contrasted with those of the feedstock material. The as-fabricated Al2050 parts are found to have a unique hardness distribution due to the location-specific variations in the processing temperature profile. The XRD results indicate the presence of the secondary phases in the deposited parts, and EDS mapping confirms the formation of detectable alloying particles in the as-deposited Al2050 matrix. The AFSD thermal-mechanical process causes the unique hardness distribution and the reduced microhardness level in the AFSD components, in contrast to those of the feedstock material.

Twin-Related Grain Boundary Engineering and Its Influence on Mechanical Properties of Face-Centered Cubic Metals: A Review

On the basis of reiterating the concept of grain boundary engineering (GBE), the recent progress in the theoretical models and mechanisms of twin-related GBE optimization and its effect on the mechanical properties is systematically summarized in this review. First, several important GBE-quantifying parameters are introduced, e.g., the fraction of special grain boundaries (GBs), the distribution of triple-junctions, and the ratio of twin-related domain size to grain size. Subsequently, some theoretical models for the GBE optimization in face-centered cubic (FCC) metals are sketched, with a focus on the model of “twin cluster growth” by summarizing the in-situ and quasi-in-situ observations on the evolution of grain boundary character distribution during the thermal-mechanical process. Finally, some case studies are presented on the applications of twin-related GBE in improving the various mechanical properties of FCC metals, involving room-temperature tensile ductility, high-temperature strength-ductility match, creep resistance, and fatigue properties. It has been well recognized that the mechanical properties of FCC materials could be obviously improved by a GBE treatment, especially at high temperatures or under high cyclic loads; under these circumstances, the materials are prone to intergranular cracking. In short, GBE has tremendous potential for improving the mechanical properties of FCC metallic materials, and it is a feasible method for designing high-performance metallic materials.

Towards relieving center segregation in twin-roll cast Al-Mg-Si-Cu strips by controlling the thermal-mechanical process

Serious center segregation greatly limits the application of twin-roll casting (TRC) technology for producing 6xxx alloy strips. Herein, Al-0.9Mg-0.6Si-0.2Cu-0.1Fe (wt.%, 6061) strips with different thicknesses were fabricated by TRC, and we found that the center segregation was well relieved with the thickness increased from 3 mm to 4 mm. To reveal the mechanisms of mitigation of center segregation in the 4 mm strip, various techniques including solidification simulation, crystallographic calculation, electron backscatter diffraction (EBSD), and electron probe micro-analyzer (EPMA) were utilized. The results disclosed that the Fe-containing phase in the 3 mm strip was π-AlFeMgSi, while the counterpart in the 4 mm strip was α-AlFeSi. The α-AlFeSi could serve as nucleation substrates for Mg2Si and Q-AlCuMgSi phases, thus promoting the uniform distribution of elements and preventing the accumulation of phases in the center region. Three matching planes between the α-AlFeSi and Q/Mg2Si were examined as: (112¯0)α-AlFeSi // (0001)Q, (0001)α-AlFeSi // (110)Mg2Si, and (112¯0)α-AlFeSi // (11¯0)Mg2Si. Meanwhile, the smaller roll separating force during the TRC process in the 4 mm strip could weaken the force-induced liquid flow behavior in the semi-solid region, which is the other reason for the alleviation of center segregation. Owing to the elimination of the center segregation, a more excellent fracture elongation was achieved in the as-homogenized 4 mm strip (∼29%) compared with the counterpart of the 3 mm strip (∼20%). This work may provide a strategy to eliminate the center segregation, thus further promoting the application of TRC process and producing high-performance Al alloy strips efficiently.

Influence of Si Content on the Microstructure and Tensile Properties of Weathering Bridge Steel Produced via Thermal Mechanical Control Process

In this study, the effects of Si on the microstructure and tensile properties of weathering bridge steel were elucidated. The thermal mechanical control process (TMCP), containing two stages of controlled rolling and accelerated cooling process, was simulated using a thermo-mechanical simulator for four experimental steels with varying Si contents (0.15–0.77 wt.%). Micro-tensile tests were performed, and the microstructures were observed via optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and electron back-scattered diffraction (EBSD). Furthermore, the tensile properties and microstructures of these steels were analyzed. The results show that a mixed microstructure comprising granular bainitic ferrite (GBF), quasi-polygonal ferrite (QF), and martensite/austenite (M/A) constituent was formed in each sample. With an increase in Si content, the GBF content decreased, QF content increased, mean equivalent diameter (MED) of the QF+GBF matrix increased, and the fraction and average size of the M/A constituent increased. With a rise in Si content from 0.15 to 0.77 wt.%, the contributions of dislocation strengthening, grain boundary strengthening, and precipitation strengthening decreased from 149, 220, and 21 MPa to 126, 179, and 19 MPa, respectively. However, the combined contribution of solution strengthening, lattice strengthening, and M/A strengthening increased from 41 to 175 MPa, which augmented the final yield strength from 431 to 499 MPa. The decreasing yield ratio shows that strain hardening capacity is enhanced due to an increase in the fraction of the M/A constituent as well as in the MED of the QF+GBF matrix. Furthermore, the mechanisms by which Si content controls the microstructure and mechanical properties of weathering bridge steel were also discussed.

Simulation analysis of titanium alloy micro-drilling based on thermo-mechanical coupling

With the increasing demand for high precision micro parts in machining field, the development of micro hole drilling technology, especially the finite element simulation technology is rapidly promoted. However, there are many limitations in the simulation of micro-drilling, such as element size, computational efficiency, chip forming, etc. In the present work, a finite element software, Abaqus has been used to simulate the thermal-mechanical coupling micro-drilling process of titanium alloy material, and the key technologies, such as twist bit modeling, material constitutive model, chip separation criterion and element division, were investigated. Through simulation, it was found that with the increase of rotational speed and the decrease of feed speed, chip shape gradually fragmented, in addition, thrust force and torque diminished. Since chip shape, thrust force and torque are important factors affecting drilling quality and tool life, the work could offer important guiding significance for cutting parameter optimization.

Improving strength and ductility of Ti-4Al-0.005B titanium alloy through tailoring equiaxed and lamellar microstructure during thermal mechanical process

The Ti-4Al-0.005B alloy were rolled at 940 °C or 990 °C followed by annealing at 650 °C. In order to control the final microstructure, rolling at medium temperature of 550 °C was processed in two other plates after rolling at 940 °C or 990 °C. The rolling and tensile behaviors of Ti-4Al-0.005B alloy with equiaxed and lamellar microstructure are comparatively studied using electron backscatter diffraction (EBSD) technique. It is found that the grain refinement mechanism of 940 °C and 940 + 550 °C rolled samples is continuous dynamic recrystallization (CDRX), while the 990 + 550 °C samples combine CDRX and twinning. The strong basal/near basal textures (B/N textures) in 940 °C and 940 + 550 °C rolled samples was mainly related to basal slip and pyramidal <c+a> slip. Strong transverse textures (T-type texture) formed in 990 °C rolled sample due to variant selection during the β→α phase transformation and 990 + 550 °C rolled sample exhibits a mixed of T-type texture and B/N texture. The 990 + 550 °C rolled sample exhibited a best mechanical properties combined of strength and ductility due to its mixed texture and refined α grains.

Microstructural Evolution and Hardness Responses of 7050 Al Alloy during Processing.

The thermal-mechanical process (TMP) plays an important role in the industrial production of 7050 Al alloys for aircraft components. In this work, the microstructural evolution and influence on hardness of a 7050 alloy ingot, during the process from preheating, deformation and cooling to final heat treatment, have been investigated considering the effect of temperature and the post-deformation cooling path. The results showed that an increasing temperature and decreasing cooling rate during TMP can lead to enhanced hardness of the alloy after heat treatment. Moreover, the variation of recrystallization before and after heat treatment was strongly dependent on the cooling path after deformation. Finally, this study provided a general understanding on the relationship between microstructural evolution and harness of the 7050 alloy.

Tailoring Mechanical and Electrochemical Properties of the Cr15Fe20Co35Ni20Mo10 High-Entropy Alloy via the Competition between Recrystallization and Precipitation Processes

A strategy to improve the mechanical and electrochemical properties of Cr15Fe20Co35Ni20Mo10 (Mo10) high-entropy alloys (HEA) by regulating the thermal-mechanical process was investigated. Due to the mutual competition between recrystallization and μ-phase precipitation behavior, the microstructure after annealing consists of recrystallized fine face-centered cubic grains with numerous annealing twins, non-recrystallized deformed grains with high-density dislocations as well as high-density nanoscale μ-phase precipitates. The combination of grain boundary strengthening, precipitation strengthening, and hetero-deformation induced strengthening endowed an ultrahigh yield strength of 1189 MPa and a uniform elongation of 17.5%. The increased yield strength activated the formation of stacking faults and deformation twinning as the additional deformation modes, which enabled the Mo10 HEA to exhibit a high strain-hardening rate and thus maintained superior ductility and enhanced tensile strength. Most importantly, when high-density dislocations accumulate at the phase boundaries, the nanoscale μ-phase can plastically deform by dislocation slips and the formation of stacking faults, which can relieve the high stress concentrations and thus prevent the cracking. The electrochemical properties of the annealed Mo10 HEA are decreased (compared to the homogenized ones), but can be optimized by adjusting the content and size and fraction of the μ-phase. This work sheds light on developing high-performance HEAs.

Effects of a short-duration preprecipitation process prior to predeformation aging on the mechanical properties and corrosion resistance of Al-Cu-Mg-Ag alloy

PurposeThe effects of the thermo-mechanical treatment on the properties and microstructure of the Al–Cu–Mg–Ag alloy were investigated.Design/methodology/approachA short-duration preprecipitation process is designed prior to predeformation aging. The novel predeformation aging (solution treatment + holding at 185 °C for 15 min+ rolling deformation + aging at 185 °C, also named T8) was performed on a heat-resistant Al–Cu–Mg–Ag alloy.FindingsThe purpose of this study indicate that a short-duration heat treatment before predeformation is beneficial to the precipitation of O during the aging process. The precursors of O during this process might pin the dislocation and cause the grains to orient along some specific direction, which might be advantageous to the precipitation of O while disadvantageous to that of θ′. This novel thermal-mechanical process could result in an increase in the quantity and decrease in the size of the precipitation of O, which leads to a remarkable strength effect. The potential increases while the current density decreases with an increase in the deformation amount, which implies a smaller intergranular corrosion rate. The fine deformed structure leads to an opposite behavior in the exfoliation corrosion test compared with that for intergranular corrosion.Originality/valueThe intergranular corrosion resistance of the Al–Cu–Mg–Ag alloy is enhanced, whereas the exfoliation corrosion resistance is reduced by novel predeformation aging.