用加速量热仪研究参比燃料的低温氧化特性（Ⅰ）

分析了两种参比燃料正庚烷和异辛烷氧化过程的化学动力学机理.在初始压力约1.5MPa和当量比为0.52左右的条件下,采用加速量热仪对两种基本参比燃料及其均匀燃/空混合物在450～650K之间的氧化特点进行了考察.实验结果表明两种参比燃料在450K时就有放热发生,并且随着自放热的发生正庚烷的压力逐渐升高,而异辛烷的压力在整个放热过程中基本保持不变,但当放热结束时却突然升高;正庚烷和异辛烷分别在598.5K和594.8K结束自放热,进入负温度系数区.     

Influence of pre- and post-weld heating on weldability of modified 9Cr–1Mo(V–Nb) steel pipe under shielded metal arc and tungsten inert gas welding processes

Abstract

Welding of modified 9Cr–1Mo(V–Nb) steel pipes has been carried out via shielded metal arc (SMA) and tungsten inert gas (TIG) welding processes. The weld joints have been produced using different preheating temperatures, followed by post-weld heat treatment (PWHT) at various temperatures. The microstructures of the weld and of the heat affected zone (HAZ) of the weld joints have been studied under the optical microscope and correlated with the preheating and PWHT. The average hardness of the weld and different regions of the HAZ, and tensile properties of the weld joints have also been studied and correlated with the preheating and PWHT. The tensile properties of the SMA and TIG weld joints produced using preheating and PWHT at various temperatures are compared and correlated with their microstructures. It is noted that a comparatively high preheating temperature of the order of 573 K is beneficial, and PWHT is necessary to reduce the susceptibility to cold cracking of weld joints of the present steel. The PWHT at 1123 K enhances ductility to fracture, but decreases the tensile strength of the base material, causing fracture of both the SMA and TIG weld joints from this region close to the HAZ. The tensile properties of SMA welds are found to be superior to those of the TIG welds, especially for PWHT at temperatures up to 1023 K.     

Finite element analysis of resistance spot welding in aluminium

Abstract

In the present work, an integrated simulation model has been developed for analysis of the resistance spot welding process in aluminium alloys via the finite element method. A coupled electrothermal analysis for an axisymmetric sheet-electrode geometry has been carried out that can predict the expected nugget diameter, penetration, and electrode face heating at any instant throughout the welding time. Several calculations have been carried out for different welding currents, welding times, and electrode forces and for different surface conditions of the aluminium sheets. Non-linear, temperature dependent, thermophysical material properties have been considered. An observation of interest is that in most instances the maximum nugget diameter is reached well within 0·02-0·04 s and further flow of welding current simply increases the electrode face heating. Also, the initial surface condition influences the growth of the fusion zone to a great extent. Various other conclusions have also been drawn.     

Parametric approach to positional fault detection in short arc welding

Abstract

This work investigates a parametric technique for short arc welding fault detection. The independent component analysis algorithm is applied to split welding voltage and current signals into arcing and short circuiting regions. A series of parameters including means, medians and normalised inner products are calculated by processing the different sections of welding electrical data. Out of position welds in overlap joints are studied with investigation of the connection between parameters and short arc welding features. The effect of changing shielding gas is also illustrated. Figures of merit to quantify the change in various parameters between faulty and good welds are compared and discussed. It is shown that the data separation into two regions with independent component analysis is useful in providing various parameters that can assist in short arc welding fault detection under different conditions.     

Time–frequency diagram applied to stability analysis in gas metal arc welding based on acoustic emission

Abstract

Time–frequency diagram is applied to estimating the stability of gas metal arc welding process. A methodology has been developed to obtain indexes based on image processing of the spectrogram in order to evaluate the relationship between the spectrogram and the stability of process. The acoustic emission (AE) signals have been processed and characterised to obtain the spectrogram of the acoustic signals generated by arc. Statistical analysis of image generated by spectrogram and temporal parameters of AE signals has been used as a tool to obtain a method for stability evaluation. As a main result of the research, it demonstrates the effectiveness of the application of image processing of the time–frequency diagram for evaluating the stability in the welding processes. The results demonstrate the validity of this method to characterise the stability using the image characterisation.     

Effects of welding parameters on melting energy of CO2 laser–GMA hybrid welding

Abstract

A series of CO₂ laser–gas metal arc (GMA) hybrid welding experiments were carried out on the mild steel workpiece to investigate the effects of the welding parameters, such as laser power, arc current and the distance between laser and arc D _LA, on the melting energy. A dimensionless parameter psi was introduced to indicate the change in the melting energy of hybrid welding. The results showed that with different welding parameters, the melting energy of hybrid welding was changed by the two heat sources (laser and arc) interaction. With an optimal combination welding parameters, psi can be increased up to 23%. Finally, the role of the two different mechanisms in the heat sources interaction was quantitatively discussed in terms of psi. It can be concluded that when D _LA<4 mm, the interaction between the laser induced plasma and the arc plasma dominates the heat sources interaction, therefore the changes of melting energy, whereas the heat sources interaction is only dominated by the preheating mechanism when D _LA≥4 mm.     

Study of grain size and microhardness of submerged arc welded joint of AISI 1060 steel

Abstract

Change in microstructure, grain growth, hardness and residual stress in a weldment are very much dependent on the temperature distribution, peak temperature and cooling rate. In the present work, three-dimensional transient finite element analysis has been used to predict the cooling rate and peak temperature at different points of the submerged arc welded joint. Grain size and microhardness of the submerged arc welded joint of AISI 1060 steel were experimentally measured and explained on the basis of estimated peak temperatures and cooling rates.     

FEM simulation of Cu97Si3 filler metal droplets spreading under arc brazing

Abstract

The spreading behaviour of Cu₉₇Si₃ filler metal droplets under arc brazing is studied by finite element method (FEM) simulation using Surface Evolver software. The mathematical model of arc pressure force acceleration added to the droplet microelement as the form of gravity acceleration is used in numerical simulation. The 3D filler metal droplet profile for different welding currents is then simulated. Finally, the simulation results were compared with experimental results, showing good correspondence. It was seen that the spreading height decreases and the diameter increases with increase of the welding current in an approximate linear relation.     

Brave New Now

Under Tomorrow's Sky is a fictional future city. For the project, speculative architect Liam Young assembled a think-tank of scientists, technologists, futurists, illustrators and science-fiction authors to collectively develop this imaginary place, the landscapes that surround it and the stories it contains. The city forms a stage set for a collection of fictions, emerging infrastructures and design experiments. It is an imaginary landscape extrapolated from the wonders and possibilities of emerging biological and technological research. Here, speculative fiction provides a critical tool, which is visionary in its trajectory while provoking us to examine some of the most pertinent questions facing us today.     

Effect of mixed LiBOB and LiPF6 salts on electrochemical and thermal properties in LiMn2O4 batteries

The effects of mixed LiBOB and LiPF₆ salts as electrolyte for lithium ion batteries are investigated by electrochemical testing and thermal stability analysis. Elevated temperature cycle, specific power, DCIR and EIS tests reveal that mixed salts electrolyte has a distinct effect on cycle life and effectively stabilize impedance increase, as optimum concentration of LiBOB is between 0.1 and 0.25 M. Fading mechanism analyses demonstrate that the mixed salts system could decrease Mn dissolution and LiBOB decomposition at elevated temperature application. Accelerating rate calorimetry (ARC) shows that the thermal stability of mixed salts electrolyte is also acceptable. In short, the mixed salts of LiBOB and LiPF₆ provide a possible solution to improve the instability of LiMn₂O₄ as cathode in lithium ion batteries at elevated temperature conditions and show a promising capability in high current discharge for high power application such as hybrid electrical vehicles.