Longgang Hou

Hot Deformation Behavior and Processing Map of Spray Formed M3: 2 High Speed Steel

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 0.001–10 s−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, ε̇ = 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.

Microstructural evolution of Al-8.59Zn-2.00Mg-2.44Cu during homogenization

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.

Effect of cryogenic rolling and annealing on the microstructure evolution and mechanical properties of 304 stainless steel

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.

Strain Analysis during the Symmetric and Asymmetric Rolling of 7075 Al Alloy Sheets

The strain components (shear strain, strain in rolling direction), equivalent strain and deformation of elements through sheet thickness in symmetric / asymmetric rolled AA 7075 Al alloy sheets were quantified using finite element method (FEM) with taking into account the deformation histories. The FEM results showed that positive and negative shear strains can be formed after a single-pass rolling and the total shear strain in the middle thickness of asymmetric rolled sheet (asymmetric ratio: 1.25) can be greatly increased compared to that of symmetric rolled sheet for the introduction of severe plastic shear deformation, so as to improve the equivalent strain. Using the engraved-mark experimental method the FEM results were verified to be consistent. Moreover, it can be found that much more shear bands will be introduced through the thickness of asymmetric rolled sheet, especially in the middle layer, indicating that asymmetric rolling greatly favors to through-thickness deformation accumulation.

Hot Deformation of Spray-Formed Nb-Containing High Speed Steel——A Study Using Processing Map

The hot deformation behavior of as spray-formed Nb-containing AISI M3: 2 high speed steel has been investigated by compression tests at a temperature range of 950-1150℃ and a strain range of 0.001-10 s-1with 50% reduction. Processing maps were developed according to the principles of Dynamic Material Model. It was found that the flow curves assumed the classic shape of dynamic recrystallization(DRX)-rising to a peak, following a softening to a steady state. The hot working process of the steel can be carried out safely in the domain of(Td: 1050-1150℃, e: 0.01-0.1 s-1). To obtain microstructures of the steel with fine grains and uniform distribution of fine granular carbides, the hot working process should be carried out at 1150℃ and strain rate of 0.1 s-1. The flow instability took place when strain rates exceed 1 s-1. After a proper hot working and heat treatment, the hardness and bending strength of the spray-formed Nb-containing M3:2 high speed steel is 67 HRC and 3467 MPa, respectively.

Effect of Strain‐Induced Precipitation on Microstructures and Fatigue Properties of AA 7050 Alloy

A novel hot rolling process based on strain-induced precipitation was proposed for producing fine-grained structures and discrete grain boundary precipitates (GBPs) in AA 7050 alloy. Effects of this rolling process on microstructures and fatigue properties were studied presently. The fine spherical precipitates and high-density dislocations were formed by the accumulative warm rolling deformation. And the migrations of the grain boundaries and dislocations can be hindered during subsequent heating and hot rolling by the pinning effect of the strain-induced precipitates, which contributed to the fine-grained structures and discontinuous GBPs without the precipitation free zones (PFZs) in AA 7050-T6 alloy. The fatigue testing results showed that the fine grains can suppress the initiation and propagation of the micro-cracks much more effectively and the fatigue life was significantly improved.

Improving the Microstructures and Mechanical Properties of Hypereutectic Al-Si Alloys by Spray Forming Technique

The hypereutectic Al-Si alloys with different Cr additions have been prepared by spray forming and cast processes. With adding Cr into Al-25Si-5Fe-3Cu (wt.%, denoted as 3C) alloy, the long needle-like δ-Al4FeSi2 phases in the cast 3C alloy were almost substituted by skeletal α-Al(Fe,Cr)Si phase, which was refined into granular α-Al(Fe,Cr)Si phase (≤ 3-5 μm) in Cr-added deposits. By using hot extrusion almost full densification of the deposits can be made, but induces coarsening of Fe-bearing phases in as-deposited 3C and C10 (adding 1.0 wt.% Cr into 3C alloy) alloys. The size of the granular α-Al(Fe,Cr)Si phase is still less than 5 μm after hot extrusion of the as-deposited C20 alloy, while some plate-like β-Al5(Fe,Cr)Si phases (~20 μm) disappeared. The thermal stability, mechanical properties were also evaluated.