Papers by Author: Chun Ming Liu

Abstract: In this paper, effects of technical parameters on process stability and microstructure during continuous rheo-extrusion process of 6201 alloy wire were investigated. The optimal process parameters for producing 6201 alloy wire are obtained. The casting temperature is from 720°C to 740°C, and the roll rotating speed is 15r/min. The 6201 alloy wire with good performance has been successfully manufactured. After heat treatment, the tensile strength of the product reaches 325 MPa and the resistivity reaches 32.47 nΩm, which is better than LHA2 conducting wire of China.

Abstract: In this paper, effects of the parameters of sloping plate process on microstructures of A2017 alloy were investigated, and semisolid ingot of A2017 alloy was prepared. The grain size of the ingot prepared by the wavelike sloping plate is much smaller than that prepared by the flat sloping plate. The primary grain becomes rounder and smaller with the increment of the sloping plate length. High preheating temperature of the plate reduces the nucleation rate of the melt and leads to coarse microstructure. When the casting temperature is 720°C, the sloping angle is 45°, and the wavelike plate length is between 400mm and 500mm, semisolid ingot of A2017 alloy with fine and homogeneous microstructures can be obtained. The reheated microstructure of semisolid ingot is mainly composed of spherical solid grains and the melted liquids and is suitable for semisolid forming.

Abstract: AZ31 magnesium alloy sheets with a cross section of 5mm×50mm were prepared by semisolid metallic rheo-rolling device self-designed, and microstructure formation mechanism of AZ31 magnesium alloy during rheo-rolling was investigated. The results reveal that during the preparation of semisolid slurry, melt firstly nucleates heterogeneously on the surface of sloping plate. Under the vibration and shear of sloping plate, a relatively homogenous temperature field and composition filed are formed around some grains and are favorable for the direct growths of globular grains; meantime, dendrites formed under the vibration and shear are broken up and rounded gradually and better semisolid slurry is formed. During rheo-rolling, under the action of roll interfacial friction and rolling force, laminar flow shear inside the alloy is enhanced, and grains are further broken up and elongated. When the pouring temperature is 670°C, excellent AZ31 magnesium alloy sheet with good microstructure can be obtained.

Abstract: Based on the SCR process, semisolid shearing-rolling process was proposed. Effect of strip size on neutral angle during the process was analyzed. Neutral angle decreases gradually with the increment of the strip width. When the strip width is less than 40 mm, the neutral angle decreases more obviously with the increment of the strip width. When the strip width is greater than 40 mm, the neutral angle decreases slowly with the increment of the strip width. The neutral angle decreases gradually with the decrease of the strip thickness. Neutral angle is mainly affected by the strip thickness.

Abstract: The effect of cold deformation on the yield strength and precipitate behavior of Al-1.3Mg-1.2Si-0.5Cu-0.7Mn alloy sheet subjected to T4P temper for automotive body panels before and after paint-bake treatment were investigated by tensile test and differential scanning calorimetry (DSC) analysis. The results show that the T4P alloy sheet has significant work-hardening response (WHR), and the yield strength increment exceeds 190MPa as the tensile deformation strain approaches 15%. The unstrained T4P alloy sheet has obvious paint-baking response (PBR), and the yield strength increment is up to 80MPa, while the PBR of the cold stretched T4P alloy sheets decrease with increasing level of tensile deformation, and the minimal PBR is only 17.8MPa. The prior tensile deformation accelerates the precipitation of β″ and β′ phase from the T4P alloy matrix during paint-bake cycle but weakens the age-hardening response (AHR) and PBR of T4P alloy sheet. Moreover, it is evident that the T4P alloy sheet simulation paint-bake treated at 170°C for 30min can not fully exert the aging potential of the alloy sheet.

Abstract: The texture, microstructure, plastic strain ratio r value, elongation, strain hardening index n, the value of cupping test I_E, and the correlation between r value and the other formability indices of two automotive aluminum alloys sheet 6016 and 6181, and commercial pure aluminum sheet were investigated. The results showed that the recrystallization textures of three aluminum alloys sheet are similar to each other, which mainly contain cube component. However, the r and n value, elongation, and I_E of three aluminum alloys sheet are different from each other evidently, and there is no correlation between texture and r value, and the other formability indices except the n value. The large quantity of second-phase particles in the aluminum alloy matrix has very important effect on both the r value and the formability of aluminum alloy sheet.

Abstract: The effect of vanadium on the bainite transformation of medium carbon high silicon steel during air cooling was studied by using Optical Microscopy (OM) and Transmission Electron Microscopy (TEM). The mechanical properties of the test steels subjected to heat treatment were measured by tensile, hardness and impact tests. The results showed that, through the same heat treatment process, the microstructure of the V-alloyed steel was comprised of Carbide-Free Bainite (CFB, bainite + retained austenite) and martensite while the microstructure of the V-free steel was composed of ferrite/pearlite, which made the V-alloyed steels exhibit superior combination of strength, hardness and toughness to the V-free steel, but the elongation of the V-alloyed steel was worse than that of the V-free steel somewhat. Vanadium was helpful for the transformation of bainite in the tested medium carbon high silicon steel under air cooling condition. The carbon-enriched retained austenite films in the CFB enhanced the toughness of the V-alloyed steel.

Abstract: The microstructure evolution and precipitation behavior of a low carbon high nitrogen martensitic stainless steel containing 15%Cr-5%Ni-1%Mo-0.16%N (in mass percentage) after tempering at temperatures ranged from 300 °C~700 °C were investigated by hardness test, X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The steel exhibits second hardening behavior after tempering at temperatures below 500 °C. Tempering at temperatures above 500 °C promotes the formation of reversed austenite and the precipitation of Cr₂N. The coherent orientation relationship of Cr₂N with martensite was revealed as , . Retained austenite originated from reversed austenite at room temperature results in the softening of the steel, and the retransformation of reversed austenite to untempered martensite after tempering at temperatures above 600 °C restores the hardness of the steel. σ phase was also found in specimens tempered at temperatures ranged from 500 °C to 700 °C.

Abstract: The effect of microstructure and mechanical properties on abrasion resistance of the medium-carbon low-alloy steel has been investigated under two body abrasive wear conditions. The results show that the microstructure of the test steel is mastenite and bainite/mastenite when the specimen subjected to water quenching and blow cooling respectively. The hardness of the test steel was over 52HRC when the specimen subjected to water quenching and blow cooling, however, effect of tempering temperature on hardness is slightly. The strength of the test steel is increased with the tempering temperature increased and the impact toughness change slightly under the blow cooling condition. The tensile strength of the test steel is decreased and the yield strength is increased with the tempering temperature increased when the specimen subjected to water quenching and followed tempering. The wear rate is increased with load and the wear mechanism is micro-cutting and microploughing. The wear resistance of bainite/martensite is better than single-phase martensite. The hardness and impact toughness are important factor under two body abrasive wear condition.

Abstract: Microstructures and textures after annealing at 680°C in low carbon Al-killed steels with low finishing temperature and high coiling temperature were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM) and X-Ray Diffractometer (XRD). The results show that higher coiling temperature and lower finishing temperature can both cause the appearance of equiaxed grain and line cementite. The equiaxed grain in 2# steel with higher coiling temperature is the more obvious. Advantage textures are weak in the steels with higher coiling temperature and lower finishing temperature and volume fraction of {111}fiber in 2# steel with higher coiling temperature is only 7.17%, so the stamping property should be worse.