Papers by Author: Liang Zhu

Abstract: U71Mn rails were welded by ultra-narrow gap welding with constrained arc by flux strips，then normalizing treatment and stress relief annealing were performed for the joints. Another sample with no heat treatment, was studied in comparison. The effect of post weld heat treatment on the hardness and microstructure of rail joint were studied by scanning electron microscope (SEM) and microhardness test. The test results showed that normalizing treatment can improve the hardness of weld seam and base metal, and stress relief annealing couldn’t improve the hardness of joints obviously.

Abstract: Static three-point bending test of the laser welded Web-core steel sandwich panel was performed. The deformation and failure of the sandwich panel with narrow weld width T-Joints were investigated. The results indicate that the deformation undertakes the following three stages: elastic deformation, plasticity deformation and T-joints cracking. The initial yield load is 25 kN, The maximum bending load is 54 kN. The high strength rate characteristic not be fully reflected. The finite simulation result shows the whole structure has no chance to reach the designed maximum value when the T-joints formed plasticity rings. Then the plastic region was developing till the weld cracked. We considered that the T-joint’s cracking is a new failure mode for the web-core steel sandwich panel. Thus, the laser welded T-joints with narrow weld width are the weakest location for the total structure. It is very necessary to consider the T-joint’s geometrical and mechanical properties for the total structural design.

Abstract: This paper presents a detailed discussion of the bending properties of laser welded web-core steel sandwich plates and the influence of weld width on stiffness and strength. The over-hanging three point bending test was conducted on the laser welded web-core steel sandwich plates with various welds width by self-designed device, together with the finite element simulations. A good agreement is obtained between the 2D FE analyses and experiment results. The stiffness and strength of the sandwich plates increased with the increasing of weld width, especially for the weld width lower than 60% thickness of the core plate. It is necessary to consider the weld width in the design and evaluation of the stiffness and strength of the laser welded web-core sandwich plate. Base on the results, the FEM is the priority to analysis bending properties of laser welded web-core steel sandwich plates.

Abstract: All steel laser welded sandwich plates have found their position in the marine and land vehicles. To implement such design accurately, knowledge about its strength properties and the influence of laser weld dimensions on strength is necessary. In this paper, the over-hanging three point bending tests were conducted on the laser welded web-core steel sandwich plate with various weld dimensions by self-designed device, together with the finite element simulations. The results show that the joint is formed on the first plastic hinge when the load is no longer a linear relationship with the deflection, and the joint cracking when the load reaches the maximum bearing capacity of sandwich plate. The joint yield load and the maximum bearing capacity of the sandwich plates increased with the increasing of weld width. Meanwhile, the mode of failure of sandwich plates also changed: When the weld width is smaller, the mode of failure is joint yield, When the weld width is greater than 3mm, the mode of failure is the combination of partial joint yield, face plate and core plate local yield, while the face plate and core plate yield exists in the whole weld joint. It is necessary to consider the weld width in the design and evaluation of the strength of the laser welded web-core sandwich plate.

Abstract: The hot rolling processes of slabs have been simulated by 3-D explicit dynamics Thermal-mechanical Finite Element Model (FEM) to solve the problems of edge defects. It can analyze the deformation characteristics of rolling slabs, calculate Lode parameter values in rolling deformation zone and analyze the stress-strain state in the entrance and exit of the slabs edge. Because there are the relationship between plastic deformation types and Lode parameter, and the quantitative relation metal flow and various Lode parameter. Results showed that there are complicated metal flow and various Lode parameter values in the edge of rolling deformation zone. Although there stress-strain states are various in the entrance and exit of the slab edges the μdε<0, the tensile strain increment in the direction of max principle strain was bigger than another two directions and the edges had the character of tensile deformation. This is the major cause that edge of slab is more defects than other zone.

Abstract: The use of electrostatic probe is available and simple in atmospheric TIG arc plasma diagnostic. Usually electrostatic probe takes disturbance in arc plasma due to sweeping motion. A low disturbance electrostatic probe was developed. This probe consisted of aluminum wire and moved through arc plasma along probe’s longitudinal direction to avoid sweeping motion. And the signal collected by this probe required Abel inversion to convert into the value per 1 mm probe length. With the application of the low disturbance probe in biased condition, ion saturation current density in various sections along the axial direction of TIG arc was obtained. The result shows that half width and peak value of ion saturation current density increase with the enhance of arc current; the radius of current-carrying area can be estimated by the half width of ion saturation current density; along the axial direction of TIG arc, the radius of current-carrying area change to the minimum near cathode.

Abstract: Hot ductility in slab shell of two types of low Ni austenitic stainless steels was investigated via tensile test. Results showed that, reduction of area (RA) in Cr17Mn6Ni4Cu2N decreased gradually from surface to inside. However, RA in Cr15Mn9Cu2NiN shell surface was low, and increased from surface to inside. Analysis suggested that, the difference of RA between the two steels due to the different solidification process. In the shell of former steel, δ ferrite solidifies as the primary phase, then transforms into austenite. The space between primary arms increases from surface to inside, leading to the decrease of hot ductility. In the latter steel, δ ferrite does not solidify entirely as the primary phase because of the high cooling rate, but austenite solidifies directly from the retained liquid between δ ferrite dendrites. The change of solidification mode is most obviously in slab surface, which decreases its hot ductility.

Abstract: Ultranarrow gap welding with the gap of less than 5mm was executed successfully. In order to prevent the arc from being attracted by sidewall and going up along the sidewall, the flux strips consisting of the specified aggregates are placed onto the sidewalls to constrain the arc. This technique was tested on the welding experiment of pipeline steel with thickness of 30mm. The involved welding parameters were obtained. The width of gap is 4mm, the welding voltage 20~24V, and the heat input 0.6 KJ/mm. The cross sections of weld profile were analyzed. The width of heat affected zone is relatively narrow and about 1.3mm, and the weld hardness is much higher than that of HAZ and base metal.

Abstract: Electrical explosion of wire has a prosperous future in fine powder producing. In the process of electrical explosion of segmented metal wire (EESW),energy deposited in the wire was influenced by process variables such as the initial charging voltage of the capacitors, the length and the diameter of the segmented wire,and the electrode spacing. For understanding their relation completely, a series of experiments of electrical explosion was carried out with variations of the initial charging voltage and the segmented wire lengths and diameter. Results show that, energy deposition efficiency was weakly dependent on the wire length , whereas it has a strong dependence on the wire diameter, the initial charging voltage of the capacitors have an important influence on the energy deposition.

Abstract: In order to study the damage mechanism under different stress states of aluminum alloy components, two kinds of representative triaxial stress states were adopted, namely notch tensile and pure shear. The results of study showed: During the notch tensile test, stress triaxiality in the least transverse-section was relatively higher. With increasing applied stress, the volume fraction of the microvoid in notch root was increasing constantly. When microvoid volume fraction reached the critical value, the specimen fractured. During the pure shear test, stress triaxiality almost came up to zero, and there was almost no micro-void but localized shear bands within the specimen. The shear bands resulted from non-uniform deformation constantly under the shear stress. With stress concentrating, the cracks were produced in the shear bands and later coalesced. When the equivalent plastic strain reached the critical value, the specimen fractured. The modified Gurson damage model and the Johnson-Cook model were used to simulate the notch tensile and shear test respectively. Simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well. In addition, the empirical damage evolution equation for the notch specimen was obtained from the experiment data and FEM simulations.