Chun-ming Liu

EFFECT OF NANOCRYSTALLINE AND TWIN BOUNDARIES ON CORROSION BEHAVIOR OF 316L STAINLESS STEEL USING SMAT

By means of surface mechanical attrition treatment (SMAT), the grain size with a diameter of about 60nm formed at about 20μm depth and numerous mechanical twins at about 50μm depth from the treated surface were synthesized in 316L stainless steel because of the different distributions of strain and strain rate along depth orientation. For instance the maximum strain rate reached 103–104s−1 on the top surface. The relationship between the microstructure and the corrosion property was studied in 0.05M H2SO4+ 0.25M Na2SO4 aqueous solution, and the results show an extreme improvement of corrosion resistance owing to the appearance of twin boundaries and the obvious reduction in corrosion resistance attributed to the presence of nanocrystaline boundaries.

Effects of different tempers on precipitation hardening of 6000 series aluminium alloys

Abstract By means of Vickers hardness, mechanical property and formability tests, the effects of different tempers on precipitation hardening of 6000 series aluminium alloys for automotive body sheets were investigated. The results indicate that the short-time pre-aging at 170 °C makes for subsequent artificial aging precipitation hardening. With the increase of pre-aging time, the artificial aging hardenability increases. The aging hardening rate reaches the maximum when pre-aging time is up to 10 min. and then it decreases. The short-time pre-aging at 170 °C benefits sheets to obtain lower strength under delivery condition and consequently to improve stamping formability of automotive body sheets. The effects of different tempers on precipitation hardening are much more obvious than those of the alloying elements. It is a good treatment schedule to perform pre-aging for 5 min at 170 °C right after solution treatment.

Aging precipitation and recrystallization in high-nitrogen austenitic stainless steel

Abstract The interaction between precipitation and recrystallization in cold deformed Fe-18Cr-12Mn-0.48N high-nitrogen austenitic stainless steel was investigated by means of hardness test, optical microscopy (OM) and transmission electron microscopy (TEM). The results show that the recrystallization of the steel begins at about 750°C. When aging at 750°C, the precipitation occurs prior to recrystallization. Large numbers of the second phases nucleate in dislocation, grain boundary and subgrain boundary. Precipitation of the second-phase particles hinders the formation of recrystallization nucleus.

Effects of technical parameters of continuous semisolid rolling on microstructure and mechanical properties of Mg-3Sn-1Mn alloy

Abstract A sloping semisolid rheo-rolling process of Mg–3Sn–1Mn alloy was developed, and the effects of process parameters on the microstructure and mechanical properties of Mg–3Sn–1Mn alloy strip were studied. The results show that the primary grain average diameter of the strip increases with the increase of the roll speed. The primary grain average diameter decreases firstly and then increases with the increase of the vibration frequency, and the tensile strength and elongation of the strip increase firstly and then decrease with the increase of the vibration frequency. The primary grain average diameter increases with the increase of casting temperature, and the tensile strength and elongation of the strip decrease correspondingly. When the casting temperature is 670 °C, the roll speed is 52 mm/s, and the vibration frequency is 60 Hz, Mg–3Sn–1Mn alloy strip with good properties is produced. The mechanical properties of the present product are higher than those of Mg–3Sn–1Mn alloy casting with the addition of 0.87% Ce (mass fraction).

A Novel Semisolid Rheo-Rolling Process of AZ31 Alloy with Vibrating Sloping Plate

A novel semisolid rheo-rolling process of AZ31 alloy was achieved by combining the shape rolling mill with the vibrating sloping plate device. The process is expected to be developed as a high-speed, semisolid roll-casting technique. During the process, due to the strong cooling rate by the sloping plate and stirrings caused by vibration and metal flow, three mechanisms—heterogenous nucleation, eruptive nucleation, and nucleus multiplication—exist, which lead to fine spherical or rosette grains formation. Two grain-growing styles, direct globular growth, dendrite growth, and fracture happen on the sloping plate surface, which also devotes to the fine non dendrite formation. During the rolling process, the solid grain of the slurry is elongated a little, and its original shape is basically maintained. If the casting temperature is too high, the liquid segregation occurs. At the casting temperature range of 650–690°C, AZ31 alloy strip with a cross section size of 4 × 160 mm was prepared by the proposed process. The product has good quality surface and homogenous microstructure. The mechanical properties of the product are higher than that produced by conventional roll casting.

Effects of magnesium content on phase constituents of Al-Mg-Si-Cu alloys

Abstract By means of scanning electron microscopy(SEM), energy dispersive spectrum(EDS), X-ray diffractometry(XRD) and metallographic analysis, the effects of variation of magnesium content on phase constituents of Al-Mg-Si-Cu alloys were investigated. The results indicate that the constituents formed during casting alloys are main Al 1.9 CuMg 4.1 Si 3.3 , Al 4 (MnFe) 3 Si 2 and Mg 2 Si, while pure Si is only present in the alloy containing lower magnesium content. Increasing Mg content leads to increasing the amount of Mg 2 Si, but decreasing the amount of Al 1.9 CuMg 4.1 Si 3.3 and Al 4 (MnFe) 3 Si 2 . During the following homogenization process, Al 1.9 CuMg 4.1 Si 3.3 is completely dissolved, Al 4 (MnFe) 3 Si 2 and pure Si remain unchanged. After rolling and final heat treatment, the constituents in the alloys change no longer.

Microstructure evolution and solidification behaviors of A2017 alloy during cooling/stirring and rolling process

A novel semisolid rheo-rolling process of A2017 alloy was achieved by combining the shape rolling mill with the vibrating sloping plate device. The microstructure evolution and solidification behaviors during the process were investigated. The high cooling rate caused by the sloping plate and stirring action caused by the vibration and metal flow lead to a high nucleation rate as well as two primary grain growth patterns, direct globular growth as well as dendrite growth and subsequent breakage, which causes the formation of fine spherical or rosette primary grains. During the rolling process, the grains of the strip were elongated. The primary grain size of A2017 alloy strip increases with the increment of casting temperature. When the casting temperature was between 650 °C and 660 °C, A2017 alloy strip with good quality was produced by the proposed process. The microstructures of the strip are mainly composed of spherical or rosette grains.

Microstructure formation mechanism during a novel semisolid rheo-rolling process of AZ91 magnesium alloy

A novel semisolid rheo-rolling process of AZ91 alloy was proposed. The microstructure formation mechanism of AZ91 magnesium alloy during the process was studied. The results reveal that the eruptive nucleation and the heterogeneous nucleation exist. During the grain growth process, the grain breakage took place and transformed into fine spherical or rosette grains on the sloping plate gradually, the other grain growth style is direct globular growth. Due to the secondary crystallization of the remnant liquids in the roll gap, the microstructure of the strip becomes finer with the increment of the casting temperature from 650 °C to 690 °C. But when the casting temperature reached 710 °C, a part of the liquid alloy transformed into the eutectic phases, and the primary grains ripened to form coarse dendrites. In the casting temperature range from 650 °C to 690 °C, AZ91 alloy strip with fine spherical or rosette grains was prepared by the proposed process.

Process parameter optimizing and studies on microstructure and properties of AZ31 alloy prepared by semisolid rolling process

A novel continuous semisolid rolling process for producing AZ31 alloy strip was developed. The process parameters were optimized, and microstructure and properties of AZ31 alloy prepared by the process were studied. The results reveal that primary grains of the strip become coarse, and the grain structure transforms from round shape to dendrite with the increment of casting temperature gradually. Eutectic phase fraction and primary grain size increase with the increment of roll speed. The primary grain size decreases firstly and then increases with the increment of the vibration frequency correspondingly. When the casting temperature is from 650 °C to 690 °C, the roll speed is 0.069 m·s−1, and the vibration frequency is about 80 Hz, AZ31 alloy strip with a cross section size of 4 mm×160 mm was prepared by the proposed process. The ultimate tensile strength and elongation are improved 1% and 57%, respectively.

Boundary layer distributions and cooling rate of cooling sloping plate process

According to the principle of grain refining and slurry preparation by cooling sloping plate process, the distributions of boundary layers during melt treatment by cooling sloping plate were studied, and mathematic model of cooling rate was established. The calculation value approximately agrees with the experimental result. Laminar flow and turbulent flow exist on sloping plate surface commonly. The thickness of velocity boundary layer and the critical transfer distance from laminar flow to turbulent flow increase with the decrease of initial flow velocity. The thickness of temperature boundary layer increases with the increment of flow distance and the decrease of initial flow velocity. The melt cooling rate and melt thickness have an inverse proportion relationship. The melt cooling rate increases along the plate direction gradually when the initial flow velocity is lower than 1 m/s, the melt cooling rate keeps nearly a constant when the initial flow velocity is 1 m/s, when the initial flow velocity is higher than 1 m/s, the melt cooling rate decreases gradually. The melt cooling rate of cooling sloping plate process can reach 102–103 K/s and belongs to meta-rapid solidification scope.