Microstructural Analysis of Ultrasonic Welded AA6061 by Electron Backscattered Diffraction

Ultrasonic welding can be used to join plastic and metal through high-frequency (more than 20 kHz) acoustic vibrations. Aluminium alloy is widely used in electronics, automotive and aerospace. The mechanical vibrations used during ultrasonic metal welding are introduced horizontally. During ultrasonic metal welding, a complex process is triggered involving static forces, oscillating shearing forces and a moderate temperature increase in the welding area. As the energy is introduced to the weld zone, there are three important phenomena occurring which are surface effect, volume effect and thermal effect. As it is known, microstructure is directly linked to the mechanical properties including fatigue resistance, fatigue life and fracture strength. So the microstructure analysis is important to evaluate the ultrasonically welded aluminium alloy’s properties. Also, mechanical properties of metal sheets, such as plastic anisotropy and formability, can be improved by a proper crystallographic texture control. That means the texture has great influence on the plastic anisotropy of the final recrystallized sheets. Optical microscopy (OM) can be used for showing the grain size and grain shape. However, to examine the relationship between mechanical behaviour and microstructure of the weldment, optical microscopy(OM) is not sufficient. OM cannot delineate a grain structure in the weld zone. The etched surface exhibited granularity at the micron level, but it was not clear if that reflected the grain structure. Electron backscatter diffraction (EBSD) in an SEM has become the most widely used technique for determination of microtexture. In this research, high-resolution electron backscatter diffraction is used to study the effects of the vibration on the evolution of microstructure in AA6061. The orientation image maps (OIM), polar figures (PF), and orientation distribution functions (ODF) of the samples are obtained. The characteristics of the crystallographic orientation, the grain structure and the grain boundary are analysed to find the effect of ultrasonic vibration on the microstructure and texture of the bond. The ultrasonic vibration will lead to exceptional refinement of grains to a micron level along the bond area and affect the crystallographic orientation. The grain size of upper foil is much smaller than that of the lower foil. And ultrasonic vibration results in a very weak texture with some characteristic intensity of a rotated cube-orientation and the Brass-orientation and S-orientation. In the lower foil, there is no much influence of the ultrasonic vibration. So there is very strong cube orientation in the lower foil.

加工条件对管线钢组织影响的实验研究

管线钢的最终组织和性能与控轧工艺以及加速冷却直接相关。采用一种新型的模拟轧制过程设备来研究X70级别管线钢在热变形冷却后的组织;设计了一系列的平面应变压缩实验来模拟板带轧制过程参数特别是精轧温度和冷却速率对最终组织的影响。采用恒定的变形速度70 s-1以及恒定的道次间隙时间,在850、900、950℃温度下的单道次和多道次压缩变形,随后研究2种不同冷却速率(3℃/s,6℃/s)获得的最终微观组织。观察发现,对于950、900、850℃的变温变形,出现了严重的混晶现象,而850℃下进行的3次压缩试样在6℃/s冷却到660℃后铁素体晶粒尺寸达到4.3μm。

Effect of Cooling Rate on the Room Temperature Microstructure Following the Intercritical Deformation of Steel S460

The effect of final hot rolling in the intercritical (α+γ) region on microstructure and properties is very specific to the individual processing conditions and the chemical composition of a steel.S460 is a plate steel processed in this way.To reproduce at the laboratory scale,a multi-stage simulation was developed which included a high temperature austenite deformation and an isothermal hold.The effect of the applied cooling rate following intercritical deformation was investigated.At 1K/s (typical industrial cooling) the microstructure was similar to the reference sample,but included an intragranular ferrite fraction.This was due to differences in processing history,and considered to be linked to a larger prior austenite grain size.At an accelerated cooling rate (15K/s),acicular ferrite formed on shear bands within the strained austenite phase.EBSD scans have been completed to provide further information about the microstructures,with band contrast able to identify the pearlite phase at the slowest cooling rate.This is a starting point from which to focus on the ferrite morphologies.

Microstructure Evolution as an Effect of Strain Reversal During the Hot Deformation of Microalloyed Steels

Susceptibility of the steel for further cold processing strongly depends on the whole prior history of deformation in the hot working regime.In the industrial hot working processes,e.g.at the roughing stage of hot rolling,or forging,material undergoes complex deformation modes that leads to gradients of both microstructure and properties across the deformed cross-section.Torsion and multiaxial compression tests with different amounts of strain reversals were conducted at elevated temperatures to study the effect of applied strain path on the phenomena occurring during microstructure evolution of Nb-microalloyed steels.Gathered results will provide data for the modification of existing constitutive equations that mostly do not account for the strain path changes.Better through-process modelling will then enable to achieve better properties and quality of the products for further cold processing.

Multi-Pass Simulation of Heavy Plate Rolling Including Intermediate Forced Cooling

Thermomechanical Controlled Processing (TMCP) including accelerated cooling after the final hot rolling pass is a well-established technology,widely applied in HSLA steel plate production.However,there are still certain limitations,especially for thicker plate.The rolling schedule includes a long holding period (HP) after the roughing stage to allow the temperature to fall sufficiently for optimised TMCP during finishing.Intermediate Forced Cooling (IFC) applied during the HP can increase productivity by decreasing the required hold time,can restrict austenite grain growth,and can also improve the subsequent strain penetration in thick plate with further metallurgical benefits.Multi-pass plane strain compression (PSC) tests have been performed on the thermomechanical compression (TMC) machine at Sheffield University including different severities of IFC.Clearly it is impossible to simulate all aspects of the temperature and strain gradients present in thick plates in laboratory specimens,and most of the tests were conducted at temperatures and strains calculated by Finite Element modelling as relevant to specific positions through the plate thickness.However,some aspects of the gradients were addressed with tests using cold platens.The results have indeed shown that IFC can shorten the HP and reduce austenite grain growth and its variation across thick plate.