Subsequent Hot Deformation Research Articles

Towards optimizing the microstructure of magnesium alloy during hot forming: Pre-deformation at cryogenic temperature

The preparation of massive twins by pre-deformation at cryogenic temperature (−196 °C) is expected to induce dynamic recrystallization (DRX) during hot forming, thereby optimizing the microstructure of hot-deformed alloys. In this study, Mg-5.5Gd-3Y-0.5Zr alloy was pre-deformed at room and cryogenic temperatures, respectively. Then, hot compression was carried out on the pre-deformed samples. The results indicate that the alloy fabricated by room-temperature pre-deformation (RT-PD) and then hot deformation (RT-HD) contains many initial grains which have not been refined, while the microstructure and mechanical properties of the alloy prepared through cryogenic-temperature pre-deformation (CT-PD) and then hot deformation (CT-HD) are improved significantly. There are only a few 101‾2 tension twins (TTWs) formed in RT-PD sample, so the sites for twin-induced dynamic recrystallization (TDRX) grains during subsequent hot deformation are limited. However, the number of twins in the alloy after CT-PD increases significantly, including 101‾1-101‾2 double twins (DTWs) and 101‾2-011‾2 twin-twin interactions, which facilitates the promotion of the TDRX and reduction of average grain size in the microstructure. In addition, a weak texture can be generated in CT-HD sample, primarily owing to the orientation inheritance of TDRX grains from various twins in CT-PD sample.

Design of High-Remanence Nd-Fe-B Hot-Pressed Magnets by Manipulating Coercivity of Hydrogenation-Disproportionation-Desorption-Recombination Treated Anisotropic Precursors.

We propose a method of manipulating the coercivity of anisotropic hydrogenation-disproportionation-desorption-recombination (HDDR) powders to fabricate high-remanence and fine-grained Nd-Fe-B magnets using only hot-pressing without a subsequent hot-deformation process. By reducing the Nd content of anisotropic HDDR precursors such that their coercivity (Hcj) is lowered, the c-axis of each HDDR particle is well-aligned parallel to the direction of the applied magnetic field during the magnetic alignment step. This is because the magnetic repulsive force between adjacent particles, determined by their remanent magnetization, decreases as a result of the low coercivity of each particle. Therefore, after hot-pressing the low-Hcj HDDR powders, a significantly higher remanence (11.2 kG) is achieved in the bulk than that achieved by hot-pressing the high-Hcj HDDR powders (8.2 kG). It is clearly confirmed by the large-scale electron backscatter diffraction (EBSD) analysis that the alignment of the c-axis of each anisotropic HDDR particle in the bulk is improved when low-Hcj HDDR powders are used to fabricate hot-pressed magnets. This coercivity manipulation of HDDR powders can be a helpful method to expand the use of HDDR powders in fabricating anisotropic Nd-Fe-B bulk magnets.

Formation and elimination mechanisms of prior particle boundaries in a new powder metallurgy superalloy

The formation of prior particle boundaries (PPBs) in a new powder metallurgy superalloy during hot isostatic pressing (HIP) is investigated. Also, the impact of subsequent hot deformation on the elimination of PPBs is systematically studied. The PPBs involves large γ′ phases, carbides (MC and M6C) and oxides (Al2O3, SiO2 and ZrO2). The reactions among the superficial oxides, the adsorbed C and O elements on the particle surface and internally-migrated alloying elements are the dominant reasons for the formation of PPBs during HIP. Besides, the existence of surface tension and surface energy induces the carbon segregation, as well as the formation of PPBs. Thus, the density of PPBs decreases when the spacing between powders is shrunk by a rapid deformation. In hot forming process, the PPBs can be efficiently eliminated via raising deformation temperature/true strain or reducing strain rate. The interactions between dislocations and PPBs, as well as the occurrence of dynamic recrystallization (DRX) induced by the strain/orientation gradient, lead to the elimination of PPBs. The thermodynamic stability of carbides declines with the raised temperature. It indicates that the dissolution of carbides and the elimination of PPBs can be accelerated via raising the deformation temperature. The breakage or deformation behavior of PPBs during hot extrusion is investigated by numerical simulation. The deformation degree of PPBs increases with the raised extrusion ratio. The above important findings are useful for developing hot extrusion processing to effectively eliminate PPBs.

The effect of Sn-rich phases on dynamic recrystallization behavior of Cu-9Ni-6Sn-(0.05 Nb) alloys

In this work, the number and size of Sn-rich phases were found to be different in Cu-9Ni-6Sn alloys with the different Nb addition. A low fraction of Sn-rich phases, located between secondary dendrite arms, was achieved in the Cu-9Ni-6Sn-0.05 Nb alloy. The subsequent hot deformation behavior of Cu-9Ni-6Sn-(0.05 Nb) alloys was investigated at a temperature range of 750 ℃ ∼ 850 ℃ and a strain rate of 0.01 s−1. The results revealed that the different distribution of Sn-rich phases changed the dynamic softening mechanism in two alloys during hot compression. Due to the distribution of a large number of Sn-rich phases in the grain interior, the dynamic softening mechanism observed during hot compression in Cu-9Ni-6Sn alloy was particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX). In contrast, the major dynamic softening mechanism in Cu-9Ni-6Sn-0.05 Nb alloy were discontinuous dynamic recrystallization (DDRX) and CDRX due to a different distribution of Sn-rich phases. This study provides a fundamental guide for further research on Cu-Ni-Sn alloy fabrication based on the results of dynamic softening mechanisms obtained from the hot compression experiment.

Dynamic precipitation and strengthening in a Mg-Zn-Gd alloy during hot deformation

Dynamic precipitation and microstructural evolution during hot deformation in a Mg-Zn-Gd alloy were investigated through systematic microstructural characterizations. Fast dynamic precipitation of β’-Mg7Gd rods was activated along dislocations during plastic deformation in Mg97Zn1Gd2 samples annealed at 773 K for 10 h. Transformation of Mg3GdZn to 14H long-period stacking ordered (LPSO) phase increased solute concentration in Mg matrix by ~ 50% after annealing, which perhaps play an important role in activating dynamic precipitation during subsequent hot deformation, besides assistance of heterogeneous nucleation along dislocations. However, dislocations in cold deformed samples would dissociate upon subsequent annealing at 573 K, producing stacking faults with Suzuki segregation of both Zn and Gd, instead of activating fast precipitation of β’-Mg7Gd rods. Moving dislocations may attract Gd atoms near their slip planes during hot deformation, which is critical for producing local compositional fluctuation high enough to facilitate dynamic precipitation of β’-Mg7Gd. Both the formation of 14H LPSO and dynamic precipitation of β’-Mg7Gd should play a role in strengthening the Mg97Zn1Gd2 alloy.

Evolution of Annealing Twins in a Hot Deformed Nickel-Based Superalloy.

The hot deformation characteristics of a GH4169 superalloy are investigated at the temperature and strain rate ranges of 1193–1313 K and 0.01–1 s−1, respectively, through Gleeble-3500 simulator. The hot deformed microstructures are analyzed by optical microscopy (OM), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD) technology. The effects of deformation parameters on the features of flow curves and annealing twins are discussed in detail. It is found that the shapes of flow curves are greatly affected by the deformation temperature. Broad peaks appear at low deformation temperatures or high strain rates. In addition, the evolution of annealing twins is significantly sensitive to the deformation degree, temperature, and strain rate. The fraction of annealing twins first decreases and then rises with the added deformation degree. This is because the initial annealing twin characters disappear at the relatively small strains, while the annealing twins rapidly generate with the growth of dynamic recrystallized grains during the subsequent hot deformation. The fraction of annealing twins is relatively high when the deformation temperature is high or the strain rate is low. In addition, the important role of annealing twins on dynamic recrystallization (DRX) behaviors are elucidated. The obvious bulging at initial twin boundaries, and the coherency of annealing twin boundaries with dynamic recrystallized grain boundaries, indicates that annealing twins can motivate the DRX nucleation during the hot deformation.

The study of hot deformation on laser cladding remanufactured 316L stainless steel

Laser cladding deposition (LCD) is widely used to remanufacture/repair workpieces because of its high design freedom to rebuild areas of damage. However, the process often introduces a columnar grain structure in the cladding layer, resulting in a large variation of microstructure and hardness across the cladding layer, welding interface, and base metal. Under fatigue and tensile loading, fractures can initiate in the lower hardness cladding layer. This study explores the feasibility of a new hybrid remanufacturing method integrating the LCD with a subsequent hot deformation process to refine grain structures, reduce hardness variations, and enhance mechanical properties. The effects of deformation temperatures and imposed plastic strains were studied by examining the microstructural and stress–strain behaviour of laser cladded 316L stainless steel. After LCD, compressive deformation was imposed at temperatures of 900 and 1100 °C, with engineering strain levels of 0.1 and 0.5. A high-quality metallurgical joint was achieved, with the optimal ultimate tensile strength and yield strength under process conditions of an engineering strain level of 0.5 imposed at 900 °C (35% improvement compared to the directly laser cladding remanufacturing process). Dynamic recrystallization process was observed by the electron back scatter diffraction technique to reveal the underlying mechanism.

Analysis of strain-induced precipitates by delta-processing in Inconel 718 superalloy

Delta-Processing (DP) is a suitable heat treatment process aiming at controlling grain growth during hot deformation of the well-known Ni-base superalloy Inconel 718 (IN718). DP is often applied by heating up to 900 °C for 24 h in order to promote δ-phase saturation prior to deformation. In addition to the δ-phase, Mo and Cr-rich carbides in the form of M23C6 may also precipitate during the subsequent hot deformation of this alloy in the range of 950–1000 °C These carbides are induced by deformation and can affect the static and dynamic recrystallization of the matrix.The objective of the current work is to understand the precipitation of M23C6 carbides when the material has been saturated with δ-phase precipitation, as well as their interaction with recrystallization phenomena. For this purpose, a series of double-strain hot compression test (double-hit) were carried out. Several holding times were applied between the first and the second hit in order to derive the softening/hardening fraction progress with time. Microstructural characterization of the samples was performed through Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) in order to identify the precipitating/dissolving phases under each testing condition.

A new phase transformation route for the formation of metastable beta-Zr

In this paper, we observe for the first time that a metastable beta-Zr in the body-centered cubic (BCC) structure can form via a new phase transformation route. The as-received alpha-Zr in the hexagonal closed-packed (HCP) structure transforms partially to the gamma-Zr in the face-centered cubic (FCC) structure via Shockley partial dislocations during deformation, while the gamma-Zr can continuously transform to the beta-Zr by a uniform shear along the $$\left\langle {112} \right\rangle_{{{\text{FCC}}}}$$ direction on the $$\left\{ {111} \right\}_{{{\text{FCC}}}}$$ plane during subsequent hot deformation. The beta phase is in a Pitsch–Schrader relationship $$((110)_{BCC}||(0001)_{HCP}, [1\overline{1}0]_{BCC}||[10\overline{1}0]_{HCP})$$ with the matrix alpha phase and in a Nishiyama–Wassermann relationship $$((110)_{{{\text{BCC}}}} ||(1\overline{1}\overline{1})_{{{\text{FCC}}}} , [1\overline{1}0]_{{{\text{BCC}}}} ||[12\overline{1}]_{{{\text{FCC}}}} )$$ , with the gamma phase, both of which have not been reported in Zr previously. Combined with corresponding molecular dynamics simulations, the phase transition mechanism and stability of the beta phase are studied. The results show that the beta-Zr phase can be retained in the gamma phase when the cooling is fast enough.

Study of the Features of Obtaining Bimetallic Pipes and Rods by Explosion Welding with Subsequent Hot Deformation

In this paper, we study the features of obtaining bimetallic pipes and rods using explosion welding and subsequent hot deformation. Explosion welding allowed obtaining continuous cylindrical two-layer billets with a combination of steel 20 + 08Cr18Ni10Ti layers with almost 100% adhesion of the layers, without surface defects, and with a specified residual deformation in diameter. We determined that replacing the atmosphere with helium in the welding gap does not reduce or eliminate microdefects in the joint. Subsequent hot plastic deformation did not affect the ratio of the component layer thicknesses. A combined technology for the production of bimetallic pipes and rods with a combination of steel 20 + 08Cr18Ni10Ti layers, which allows obtaining pipes and rods with a corrosion-resistant coating and sizes that are included in the range of pipes used in the energy industry, was developed and tested.

The effect of rare earth elements on the work softening behavior of as-cast Mg-4Al-2Sn

This work explores the addition of 1% Rear Earth (RE) elements on the Mg–4Al–2Sn magnesium alloy, with an emphasis laid on the microstructural evolution during solidification and subsequent hot deformation behavior. The morphology of the α-Mg dendrites changes from the butterfly-like (growth at the non-basal planes) to the snow-flake like (growth at the basal planes) due to the addition of RE elements. Dendrite morphology transition (orientation transition) lead to the formation of the various macro-texture at the solidification interval. Subsequently, an appropriate correlation was established between the dendrite orientation selection, solidification texture and deformation behavior of the as-cast microstructure. The workability increases due to the addition of RE elements, which is related to the initial solidification texture and the morphology of the α-Mg dendrites. The results indicated that the dendrites which have snow-flake like morphology in the RE bearing alloys was more favorable for breaking of as-cast microstructure and occurrence of dynamic recrystallization.

Precipitation behaviors and orientation evolution mechanisms of α phases in Ti-55511 titanium alloy during heat treatment and subsequent hot deformation

The precipitation orientation of α phases in a Ti-55511 alloy during the heat treatment and the subsequent hot deformation is investigated. It is observed that the β grains prominently influence the precipitation process of α phases, i.e., for the neighbouring β grains without similar {110} pole, the precipitation orientation of grain boundary (GB) α phases (αGB) keep {0001}//{110}β1 or {0001}//{110}β2 to maintain the burgers orientation relationship (BOR) with the neighbouring β grains. For the neighbouring β grains with similar {110} pole, the {0001} poles of αGB phases precipitate parallel to this similar {110} pole and maintain BOR with the neighbouring β grains. For the neighbouring β grains with multiple similar {110} poles, the {0001} poles of αGB phases precipitate along these similar poles and maintain BOR with the neighbouring β grains. Besides, the interface instability nucleation and sympathetic nucleation are the main nucleation modes of the GB widmanstatten α phase (αWGB), which primarily precipitate within β grains with the similar orientation of αGB phases. The intracrystalline widmanstatten phase (αWI) precipitate in a single β grain with 12 precipitation orientations. The BOR between αWI phases and β grains is destroyed during the subsequent hot compressive deformation.

Study of the Surface Defect Nature of Hot-Rolled Products in the Edge Zone

The study results of the edge zone of more than 300 hot-rolled products of steels 09Г2С, 22ГЮ, 20, К56 produced according to the combined technique of smelting, casting, and subsequent rolling in the conditions of casting and rolling complex are presented. Simulation shows that location of imperfections and defects on the slab serves as a prerequisite for defect formation in coil stock. The most common defects include cracks and scabs. The formation of these defects is often associated with the disclosure of oscillation marks during subsequent hot deformation. In such cases, the defect formation nature is difficult to identify by metallographic methods, since signs of the steelmaking nature of their formation are not observed in the cross section of the discontinuity.

Исследование особенностей получения биметаллических труб и прутков сваркой взрывом c последующей горячей деформацией

Исследование особенностей получения биметаллических труб и прутков сваркой взрывом c последующей горячей деформацией

Obtaining two-layered pipes and rods using explosion energy and hot deformation

In this work, experimental studies were carried out to obtain bimetallic pipes and rods using explosion welding and hot deformation. The cladding layer was made of 08Cr18Ni10Ti stainless steel. It was found that explosion welding provides high-quality adhesion of the bimetal layers. Subsequent hot plastic deformation did not affect the ratio of the component layer thicknesses and the quality of the welded joint, which is confirmed by studies of the microstructure and mechanical tests of samples for flattening. Based on the results obtained, a combined technology for the production of bimetallic pipes and rods with a combination of layers of steel 20 + 08Cr18Ni10Ti was developed and tested.

Change in the Microstructure of Ni Alloy Disk Workpieces for Gas Turbine Engines Produced by the HIP + Deformation Method

Abstract—The changes in the microstructure during thermomechanical treatment under different regimes of a new granulated heat-resistant alloy based on VZh178P nickel obtained by the method of hot isostatic pressing and subsequent hot deformation (HIP + deformation) are studied. The change in the grain size of the γ matrix after the main operations of the technological conversion is shown; the dynamics of their growth in hardening below and above the temperature of total dissolution of the γ' phase and the microstructure after the total thermal treatment are shown.

Alloy Design and Processing Design of Magnesium Alloys Using 2nd Phases

Two phases dominate the performance of commercial Mg alloys: (1) β Mg17Al12 and (2) porosity. Alloy design and process design to optimize the morphology of the first and to minimize the second are discussed. Second phase β can be designed to improve tensile strength, fatigue strength, toughness, texture, formability and corrosion resistance of Mg alloys. Processing is applied to refine the grain size, to array this phase in micron sizes at grain boundaries, and to further precipitate this phase in nanometer arrays within the grains. Therein, the 2nd phase β plays the following roles: (1) retarding grain growth, (2) randomizing texture, (3) Hall–Petch hardening, (4) Orowan hardening, and (5) moderating corrosion. Porosity is a detrimental 2nd phase common to cast Mg alloys in the form of gas or voids; its control being essential to engineering applications. Porosity can be diminished by sub-liquidus molding with high-velocity/high-pressure injection and then eliminated by subsequent hot deformation.

Dynamic recrystallization of titanium: Effect of pre-activated twinning at cryogenic temperature

A pre-cold-deformation process was applied for commercially pure titanium at cryogenic temperature to activate high-density deformation twins, and subsequent hot-deformation was used to induce dynamic recrystallization (DRX). Three major types of twins were effectively activated during cryogenic deformation, including {112¯2} contraction twins, {112¯1} extension twins, and {101¯2} extension twins. A special type of slipped {112¯1} “twins” was also activated by a sequential effect of twinning and slip. Selection of twinning variants followed Schmid's law well, where only the twinning systems with a Schmid factor of m ≥ 0.4 could be activated. The pre-activated twinning led to a remarkably stable flow stress at 500 °C up to a true strain of 1.0, due to the attainment of dynamic equilibrium between strain hardening and high-temperature softening. Two DRX stages occurred: (1) twin-active DRX stage, and (2) discontinuous dynamic recrystallization (DDRX) stage. The DRX mechanisms identified were twinning-induced DRX (or TDRX) and DDRX. While the low-temperature slip alone had little influence on DRX, the pre-activated {112¯2} twins, {112¯1} twins, {101¯2} twins and slipped {112¯1} “twins” contributed effectively to DRX in the form of spheroidization of twin lamellae due to twin-dislocation interactions, leading to a substantial grain refinement from ∼41 to ∼1 μm during subsequent hot-deformation.

Achieving fine-grained equiaxed alpha via thermo-mechanical loading under off-equilibrium state in two-phase Ti-alloys

An off-equilibrium hot deformation scheme was proposed to achieve fine-grained equiaxed alpha (αs) for two-phase titanium alloys. Controlled cooling and deformation was carried out at the soaking temperatures of 950–990 ℃,cooling rates of 50–140 ℃/min and strain rates of 0.01–0.1 s−1 in TA15 alloy. The results showed that fully fine-grained equiaxed αs (∼500 nm) was obtained after heating to near β-transus temperature, fast cooling (i.e., >140 ℃/min) and subsequent hot deformation at strain rates above 0.1 s-1. The transformation of nucleation mode from complete wetting to incomplete wetting promoted by hot deformation mainly resulted in the formation of fine equiaxed αs at the β/β grain boundaries. For intragranular αs phase, selective heterogeneous nucleation at dislocations with reduced nucleation barrier and weak variant selection mainly led to the formation of equiaxed α precipitates. A comparison among different off-equilibrium cases revealed that flow stress was decreased greatly by increasing the soaking temperature due to more αp → β transformation. This would provide a novel path to achieve grain refinement of equiaxed structure with decreased forming load and low cost.

Microstructure and Mechanical Properties of the As-Cast and As-Homogenized Mg-Zn-Sn-Mn-Ca Alloy Fabricated by Semicontinuous Casting.

In this paper, a new type of low-cost Mg-3.36Zn-1.06Sn-0.33Mn-0.27Ca (wt %) alloy ingot with a diameter of 130 mm and a length of 4800 mm was fabricated by semicontinuous casting. The microstructure and mechanical properties at different areas of the ingot were investigated. The microstructure and mechanical properties of the alloy under different one-step and two-step homogenization conditions were studied. For the as-cast alloy, the average grain size and the second phase size decrease from the center to the surface of the ingot, while the area fraction of the second phase increases gradually. At one-half of the radius of the ingot, the alloy presents the optimum comprehensive mechanical properties along the axial direction, which is attributed to the combined effect of relatively small grain size, low second-phase fraction, and uniform microstructure. For the as-homogenized alloy, the optimum two-step homogenization process parameters were determined as 340 °C × 10 h + 520 °C × 16 h. After the optimum homogenization, the proper size and morphology of CaMgSn phase are conducive to improve the microstructure uniformity and the mechanical properties of the alloy. Besides, the yield strength of the alloy is reduced by 20.7% and the elongation is increased by 56.3%, which is more favorable for the subsequent hot deformation processing.