Applied Mechanics and Materials Vols. 117-119

Abstract: Based on the hot compression tests of AZ31B magnesium alloy, the trend of flow stress was analyzed and constitutive equation was established. Using Deform-3D Microsoft, the process of magnesium alloy wheel was numerical simulated. The effect of fraction coefficient and heating temperature on extrusion load and radial load on die surface was studied. It was shown that variety of these loads was tardo at prophase back-extrusion while violent at anaphase stage. The fraction coefficient was an important factor in total compression load and radial load on die surface was affected obviously by forming temperature.

Abstract: Several tests including CTOD and Charpy impact have been performed on the weld joints of high strength low alloy steel S335G10+N to identify the toughness. The fracture surface and microstructure of samples extracted from the weld joints are characterized using scanning electron microscopy. Based on all data obtained some relationships between toughness and microstructure has been detected. (1) The tendency of Charpy impact energy distribution with the position of notch is similar to the CTOD values in WP and FL but different in FL+2.CTOD values of FL+2 are greater than those of FL, but the Charpy impact energy has an opposite tendency. (2) The appearance of Pearlite and upper bainite found in WP sample S3554 is regarded as one of the reasons for the lowest CTOD values. The cavity and quasi-cleavage in S35510 fracture surface indicate an inferior toughness compared with some dimples and ripple morphology of slip separation in S3552. (3)Grains close to the fracture surface in the 35511 (Stable crack growth) of FL +2 were elongated owing to plastic deformation caused by crack growth. (4)Hardness of the HAZ is equal to fusion zone and lower than base metal or weld position in S35511. Hardness of fusion zone increases with the depth. (5)A big difference of Average Grain Size is found in microstructure close to the fracture surface among S3557, S3556 and S3559.The greater the Average Grain Size is, the lower the CTOD value will appear.

Abstract: The CGHAZ Microstructure has great effect on the toughness of welding joints and CTOD. This paper discussed the relationship between the CGHAZ and CTOD .The samples, carried out on high-strength steel EQ56, were observed by scanning electron microscopy. The Microstructural analysis result shows CGHAZ microstructure of EQ56-5, presenting high CTOD with 0.272mm, is low-carbon lath martensite and little M-A constituent, which contribute to the better toughness of EQ56-5. CGHAZ microstructure of EQ56-4, presenting the low CTOD with 0.082 mm, contains M-A constituent. The M-A constituent makes the CGHAZ brittle. The crack would grow easily along the M-A grain boundaries.

Abstract: In temper bead welding, toughness is one of the key criteria to evaluate the tempering effect. A neural network-based method for toughness prediction in the coarse grained heat affected zone (CGHAZ) of low-alloy steel has been investigated in the present study to evaluate the tempering effect in temper bead welding. Based on the experimentally obtained toughness database, the prediction systems of the toughness of CGHAZ have been constructed using RBF-neural network. The predicted toughness of the synthetic CGHAZ subjected to arbitrary thermal cycles was in good accordance with the experimental results. It follows that our new prediction system is effective for estimating the tempering effect in CGHAZ during temper bead welding and hence enables us to assess the effectiveness of temper bead welding.

Abstract: A fuzzy neural network based system was developed to control the quality of resistance spot welding. A fuzzy neural controller (FNC) was designed to realize the memory of the fuzzy neural control bases on a back propagation (BP) network, accordingly achieve the mapping of fuzzy input and output. The experiment results show that this method can compensate the multi-crossfire’s influence on the welding currency, keep a high control precision, and fulfill the demand of high precision quality monitoring to the resistance spot welding.

Abstract: In this study, in order to investigate effects of welding conditions and alloy element Cu in aluminum alloy on growth of intermetallic compounds at weld interfacial zones and weld strength of steel/aluminum alloy joints, the pure aluminum A1050 and the aluminum alloy A2017 to cold-rolling steel SPCC were welded by resistance spot welding. Two intermetallic layers were observed, and the major phases are Fe₂Al₅ adhering to the steel and FeAl₃ adhering to the aluminum. The results of EPMA suggested that diffusion of alloy element Cu in the A2017 happened at the interface, which may be the reason for that the anisotropic growth of Fe₂Al₅ might be inhibited. The tensile-shear strength of A2017/SPCC joint decreased with an increasing of the thickness of r=0.5, considering the nugget radius as 1.

Abstract: The purpose of this work was to investigate the effect of activating fluxes and welding parameter to the penetration and depth-to-width ratio (DWR) of 6061 aluminum alloy welds. Bead-on-plate argon gas tungsten arc (GTA) welding process was made on 6061 aluminum alloy of plate without filling metals. The activating fluxes used in the experiment were CaO, ZnO, SiO₂, TiO₂, Al₂O₃, Cr₂O₃, CaF₂ and Na₂CO₃. Based on the higher penetration of weld bead and welding arc voltage, four fluxes were selected to create three new mixtures using 50% of each original fluxes. The mixed fluxes were used to investigate the effect to the welding penetration. Finally, The Taguchi method was employed to achieve optimal DWR of weld bead and investigate the welding parameters that significantly affect welding quality. The experimental procedure of GTA welding with activating flux via the Taguchi method produced full penetration of weld bead on a 6.35 mm thickness of 6061 aluminum alloy plate with single pass weld.

Abstract: Based on the CTOD test, metallographic test is done. The effect of microstructure on CTOD of welded joints for offshore high-strength steel has been analyzed by SEM. The microstructure of different samples in a welding procedure and two different welding procedures are compared. From the microscopic point of view, the reasons why the toughness of the second group welded joints is better than the first group’s and why the toughness of the same group welded joints is different have been determined. And the main microstructure found in EQ70 steel welded joints is the ferrite, tempered sorbite and a small amount of pearlite. Besides some carbide particles are on the matrix of tempered sorbite. The material toughness with a small amount of acicular ferrite is better. Inclusions and pores also seriously affect the toughness of welded joints. Reducing the welding heat input appropriately can refine the grain of CGHAZ and improve toughness. But hardened microstructure may occur owing to too low welding heat input.