•Pre-deformation accelerated the formation/refinement of deformation induced precipitates (DIPs).•Low angle grain boundaries transition model has been proposed for grain refinement.•DR treated alloy displays superior elongation than CR treated alloy in T6 state.An improved TMT (thermomechanical treatment) double step hot rolling (DR) including low-temperature pre-deformation, intermediate short-term annealing and final hot rolling, was proposed to manufacture fine grained AA7055 sheets. Microstructural analysis results show that the low temperature pre-deformation can accelerate the formation and spheroidization of fine precipitates which can exert great drag force to the migration of grain boundaries and dislocations so as to promote the formation of high density dislocation cells and accommodate sufficient deformation storage energy subsequently. Then, these dislocation cells may turn into polygon sub-grains by short-period intermediate annealing. With the final hot rolling, low angle grain boundaries (LAGBs like sub-grain boundaries) were gradually transferred into high angle grain boundaries (HAGBs) and fine-grained structures were obtained. Tensile testing results reveal that samples processed by the optimal DR processing (300 °C/60% + 430 °C/5 min + 430 °C/50%) display superior tensile plasticity than the conventional hot rolling (CR) samples without strength loss, which could be correlated with the fine grained structure of the DR sample. It indicates that the present DR processing may be a good alternative to efficiently produce fine grain structured heat-treatable aluminum alloys.

•The hyperbolic-sine constitutive analyses were conducted to determine the true stress exponent and activation energy in hot working region.•The origins of threshold stress could be related to the bypassing mechanism of dislocations over a particle.•The evolution of deformation mechanisms during hot working could be reflected by the true stress exponent and activation energy.•The effects of temperature and strain on the true stress exponent and activation energy were discussed.For characterizing the origins of threshold stress and the evolution of deformation mechanisms in hot working region, the hyperbolic-sine constitutive analyses were conducted to determine the threshold stress σ0, the true stress exponent nt and the true activation energy Qt in AA7050 Al alloys. The origins of σ0 could be related to the four bypassing mechanisms of dislocations overcoming an obstacle. The origins of σ0 are the local climb at 603–633 K, and it may change from the local climb to the detach stress with increasing strains at 663–693 K. The nt and Qt after Young's modulus and threshold stress correction can be associated with the specific deformation mechanism. The deformation mechanisms at peak strains may change from the climb-controlled dislocation creep (lattice diffusion) at 603 K to the Zn diffusion controlled solute drag creep which is controlled by (sub)grain boundary diffusion at 693 K. The nt and Qt decrease with increasing strains may be attributed to the grain boundary self-diffusion accelerated by the fluxes of vacancies for lower temperatures (603–663 K) and the atom diffusion controlled solute drag creep accelerated by the vacancies, (sub)grain boundaries, and the motion of high angle grain boundaries for higher temperatures (663–693 K), respectively.Download high-res image (140KB)Download full-size image

•The improved thermomechanical processing DHR can produce high-quality sheets with finer grains.•Grain refinement is preceeded via dislocation rearrangement and low angle grain boundaries transition.•Fine dispersed matrix precipitates and discontinuous grain boundary precipitates can be obtained by DHR.•DHR can improve tensile plasticity and corrosion resistance.An improved thermomechanical processing double step hot rolling (DHR) was proposed to manufacture fine-grained Al-Zn-Mg-Cu alloys based on pre-deformation, short time intermediate annealing and final hot rolling. The corresponding microstructure evolution, mechanical properties and corrosion resistance were investigated. The DHR processing can produce high-quality sheets with finer grains than the conventional hot rolling (CHR). The grain refinement is mainly proceeded via dislocation rearrangement and low angle grain boundary transition. The grain boundary area (with coarse grains) is small in the CHR alloy and enormous solute atoms could move from intra-granular area to grain boundaries. Therefore, coarse particles precipitated continuously along grain boundaries. However, larger grain boundary area is obtained in DHR alloy by finer grain structures leading to the formation of discontinuous grain boundary precipitates. The results reveal that the present DHR treated alloy possesses improved tensile plasticity and corrosion resistance than the CHR alloy because of refined grains and discontinuous grain boundary precipitates. Thus, the present DHR processing is a promising manufacturing process for obtaining fine grained heat-treatable Al alloy sheets.

Metastable 304 austenitic stainless steel was subjected to rolling at cryogenic and room temperatures, followed by annealing at different temperatures from 500 to 950°C. Phase transition during annealing was studied using X-ray diffractometry. Transmission electron microscopy and electron backscattered diffraction were used to characterize the martensite transformation and the distribution of austenite grain size after annealing. The recrystallization mechanism during cryogenic rolling was a reversal of martensite into austenite and austenite growth. Cryogenic rolling followed by annealing refined grains to 4.7 μm compared with 8.7 μm achieved under room-temperature rolling, as shown by the electron backscattered diffraction images. Tensile tests showed significantly improved mechanical properties after cryogenic rolling as the yield strength was enhanced by 47% compared with room-temperature rolling.

The microstructure/mechanical properties, annealing behavior, and post-annealed microstructure/property relationship of 5052 Al alloy processed by cryogenic rolling (CR) and room-temperature rolling (RTR) have been investigated. It was observed that the CR-processed 5052 Al alloy exhibits a refined crystallite size along with higher-density dislocations than that of the RTR-processed sample. As a result, the tensile strength of the CR-processed alloy increased significantly to 345 MPa because of the enhanced dislocation strengthening and grain-boundary strengthening. The effective suppression of dynamic recovery and accumulation of high-density dislocations caused by CR help accelerate recrystallization kinetics and refine the grain size, gaining higher yield strength for the post-annealed CR 5052 Al alloy compared to that of other commercial 5052-O Al alloys.

An improved double step hot rolling (DTR) processing was proposed to manufacture fine grained Al–Zn–Mg–Cu alloys based on pre-deformation, short-period annealing and final hot rolling. The DTR processing can produce high-quality rolling sheets with much finer recrystallized structures and significantly improved mechanical properties when compared with the conventional hot rolling (CTR). The main differences between the two TMTs (thermomechanical treatments) were investigated and the specific grain refining procedure of DTR was carefully characterized. The results show that the grain refinement is mainly proceeded via dislocation rearrangement and low angle grain boundaries transition, which can be attributed to the pinning effect of deformation induced precipitates (DIPs). Pre-deformation can accelerate the formation and spheroidization of fine DIPs which prohibit the migration of grain boundaries and the movement of dislocations. As a result, high density dislocation cells are formed and turn into polygon sub-grains after short-period intermediate annealing. During the final hot rolling, low angle grain boundaries gradually transferred into high angle grain boundaries which contributes to the final fine grained structures. The initiation and propagation of cracks were also delayed by grain refinement.

•Spray forming can effectively refine the microstructure of M3:2 steel.•Niobium accelerates the decomposition of M2C carbides.•Niobium increases the hardness and bending strength of spray formed M3:2 steel.•Spray-formed niobium-containing M3:2 steel has the best tool performance.The microstructures and properties of spray formed (SF) high-speed steels (HSSs) with or without niobium (Nb) addition were studied. Particular emphasis was placed on the effect of Nb on the solidification microstructures, decomposition of M2C carbides, thermal stability and mechanical properties. The results show that spray forming can refine the cell size of eutectic carbides due to the rapid cooling effect during atomization. With Nb addition, further refinement of the eutectic carbides and primary austenite grains are obtained. Moreover, the Nb addition can accelerate the decomposition of M2C carbides and increase the thermal stability of high-speed steel, and also can improve the hardness and bending strength with slightly decrease the impact toughness. The high-speed steel made by spray forming and Nb alloying can give a better tool performance compared with powder metallurgy M3:2 and commercial AISI M2 high-speed steels.

Bending behaviors during multi-pass asymmetrical rolling (ASR) process were optimized by adjusting the thickness reduction per pass (ε). The bending curvature can be reduced significantly, almost closing to zero at a critical thickness reduction per pass (εc), by which the continuous multi-pass ASR process can be guaranteed. εcs were obtained by Finite element (FE) simulation firstly, then applied them to the multi-pass ASR-processing. The results show that the predicted εcs are consist well with the experimental values, which can make the ASR-processed plate exit without bending during multi-pass ASR processing. Microstructural evolution, mechanical properties and fracture toughness of symmetrical rolled (SR) and ASR-processed plates with bending behavior optimization were contrastively studied and it shows that the ASR processing can impose more severe deformation to the coarse constituent particles and matrix due to the introduction of additional shear strain, resulting in the improvement of microstructural homogeneity throughout the plate thickness. And mechanical properties and the fracture toughness of the ASR-processed flat plate are improved significantly due to less coarse constituent particles after annealing. In addition, effect of the recrystallization microstructure on the fracture toughness of T6-treated SR- and ASR-processed plates is also discussed in this study.

Formability of peak-aged AA 7075-T6 sheet across a temperature range from room temperature to 250 °C as well as post-forming microstructure/property were investigated in this paper. It showed that the optimized formability was obtained at 200 °C, due to the higher ductility and work hardening capacity at this temperature. The dominant phases in 7075-T6, 200 °C warm formed, and 250 °C warm formed samples were fine η′ and GP zones, well-developed η′, and coarser η phases, respectively. Post-forming and post-paint-baking mechanical property of 7075 samples was closely associated with those corresponding microstructures. Specifically, 200 °C warm forming caused limited coarsening of matrix precipitates (MPts) and generated a certain number of dislocations, due to the combined effect of the decreased precipitation strengthening and increased dislocation strengthening, the inherent high strength of 7075-T6 was perfectly preserved after 200 °C forming. However, 250 °C forming led to severely decreased hardness due to the sharply coarsened MPts. Furthermore, the paint-baking treatment exerted little influence to MPts, grain boundary precipitates (GBPs) and precipitate free zone (PFZ), thus the hardness loss caused by paint-baking was unobvious. Overall, 200 °C was the appropriate temperature under which the peak-aged 7000 alloys exhibited enhanced formability and maintained high post-forming strength, after paint baking, the drawn component possessed microstructure/property similar to those of retrogression and re-aged (RRA) samples.

The effects of systematic variation of Mg and Cu contents (Mg: ~1.5, 2.0, and 2.5, Cu: ~1.5, 2.0, 2.5, and 2.9 wt%) on the microstructures and mechanical properties of high-Zn (8.5 wt%) Al–Zn–Mg–Cu alloys are investigated. Fracture toughness is experimentally approached by the Kahn tear test. Results showed that, under same ageing condition, the conductivity, hardness, strength and toughness of the designed alloys are primarily determined by Mg content: the higher the Mg content, the higher the hardness and strength, but the lower the conductivity and toughness. Increasing Cu content can produce a similar phenomenon, but with weak effects compared with Mg. The experiments and thermodynamic/kinetic simulation indicate that, increasing Mg/Cu content can improve the volume fraction of matrix precipitates, so as to improve the strength and hardness, and the effects of Mg are stronger than Cu. Additionally, increasing Mg content can somewhat reduce the sizes of the matrix precipitates especially in overaged condition, which is also good for the strength and hardness. However, with increasing Mg content the area fraction of the grain boundary precipitates (GBPs) and the yield stress contrast between grain interiors and precipitate free zones (PFZs) at grain boundary can be increased greatly, consequently promoting intergranular fracture and decreasing toughness. For the alloys with low/middle Mg content (e.g., 1.5/2.0 wt%), increasing Cu content will improve the yield stress contrast between grain interiors and PFZs as well as the recrystallization degree, so that intergranular fracture will be promoted for toughness reduction. For the alloys with high Mg content (e.g., 2.5 wt%), the increased undissolved phases induced by high Cu content will promote fracture at/near coarse constituent particles, favoring further toughness reduction.

Hot deformation behavior of a new type of M3 : 2 high speed steel with niobium addition made by spray forming was investigated based on compression tests in the temperature range of 950–1150 °C and strain rate of 0.001–10s−1. A comprehensive constitutive equation was obtained, which could be used to predict the flow stress at different strains. Processing map was developed on the basis of the flow stress data using the principles of dynamic material model. The results showed that the flow curves were in fair agreement with the dynamic recrystallization model. The flow stresses, which were calculated by the comprehensive constitutive equation, agreed well with the test data at low strain rates (≤1 s−1). The material constant (α), stress exponent (n) and the hot deformation activation energy (QHW) of the new steel were 0. 00615 MPa−1, 4.81 and 546 kJ ·mol−1, respectively. Analysis of the processing map with an observation of microstructures revealed that hot working processes of the steel could be carried out safely in the domain (T=1050–1150 °C, ɛ = 0.01–0.1 s−1) with about 33% peak efficiency of power dissipation (η). Cracks was expected in two domains at either lower temperatures (<1000 °C) or low strain rates (0.001 s−1) with different cracking mechanisms. Flow localization occurred when the strain rates exceeded 1 s−1 at all testing temperatures.

The microstructural evolution and phase transformations of a high-alloyed Al-Zn-Mg-Cu alloy (Al-8.59Zn-2.00Mg-2.44Cu, wt%) during homogenization were investigated. The results show that the as-cast microstructure mainly contains dendritic α(Al), non-equilibrium eutectics (α(Al) + Mg(Zn,Al,Cu)2), and the θ (Al2Cu) phase. Neither the T (Al2Mg3Zn3) phase nor the S (Al2CuMg) phase was found in the as-cast alloy. The calculated phase components according to the Scheil model are in agreement with experimental results. During homogenization at 460°C, all of the θ phase and most of the Mg(Zn,Al,Cu)2 phase were dissolved, whereas a portion of the Mg(Zn,Al,Cu)2 phase was transformed into the S phase. The type and amount of residual phases remaining after homogenization at 460°C for 168 h and by a two-step homogenization process conducted at 460°C for 24 h and 475°C for 24 h (460°C/24 h + 475°C/24 h) are in good accord with the calculated phase diagrams. It is concluded that the Al-8.59Zn-2.00Mg-2.44Cu alloy can be homogenized adequately under the 460°C/24 h + 475°C/24 h treatment.