选区激光熔化IN738LC合金成形温度场的数值模拟及实验研究

通过数值模拟根据熔池热行为变化规律对选区激光熔化工艺参数进行优化,是提高成形件质量的有效手段。为此,本论文采用ANSYS的APDL语言建立了全参数化的IN738LC合金选区激光熔化过程温度场有限元分析模型,并通过单熔道成形实验对热源模型进行校核。结果表明：随着激光功率的增加或者扫描速度的减小,粉末吸收的线性能量密度不断增加,熔池中心最高温度升高,熔融金属量增加,熔道形态由不规则断续状向规则连续长条状演化;随着扫描速度的增加或者激光功率的减小,粉末吸收的线性能量密度不断下降,熔体流动能力减弱,熔池宽度与熔化穿透深度也随之减小;有限元模拟与实验结果吻合较好,当激光功率为270 W,扫描速度为1150 mm/s时,单熔道具有连续少缺陷、规则良好的成形形貌。     

侧吹气流方向对大功率CO2激光焊缝成形的影响

激光深熔焊过程中会在熔池上方产生等离子体云，若该等离子体云过密，将降低激光入射到工件表面能量密度。在工业应用中，常采用侧吹辅助气体来减弱等离子体云的影响。本文通过试验，研究了在不同气体流量下，侧吹气体方向不同对焊接过程稳定性和焊缝成形的影响。结果发现：使用大功率CO2激光焊接AH32时。侧吹气体方向对焊缝成形有着明显的影响。从熔池和等离子体两个方面，解释了侧吹气体方向的影响机制。     

Problems in laser repair-welding a surface-treated tool steel

A research work is described on the development of defects in laser repair welding of a surface-treated tool steel. Repair welding tests were carried out on plasma-nitrided and on chrome-plated type 1.2738 steel plates. Welding defects in the chrome-plated samples were mainly due to chrome and oxygen overalloying of the weld metal, leading to extensive hot cracking. Cracks of the surface chrome deposit in the HAZ due to welding stresses were also detected. In the nitrided samples, welding resulted in the copious formation of gas pores due to nitrogen release during weld metal solidification. A procedure for combined laser pre-treating and laser welding of the nitrided samples was thus presented. The procedure is aimed at minimising the negative effect of the laser beam on the structural integrity of the base and welded material. The results of the microstructural analyses revealed that a significant reduction in defects could be achieved by the proposed method in the nitrided samples.     

Sensors for monitoring the quality of molten aluminum during casting

The quality of a cast product depends directly on the quality of the molten metal from which the product is cast. Techniques for determining melt quality often require the laboratory analysis of samples taken from the molten metal. In addition to reducing processing efficiency, delays associated with this laboratory analysis can lead to uncertainties because the melt conditions can change during holding at high temperatures. On-line monitoring of the melt quality provides more rapid feedback for improved process control. In this article, techniques for monitoring the important aspects of melt quality, specifically chemical composition, gas content, and inclusion content, are reviewed.     

Hydrodynamical phenomena in the process of laser welding and cutting

Abstract

The main objective of the paper is to outline the 'bridges' existing between the outcomes of fundamental researches and the results of investigations in the field of industrial laser materials processing (LMP). An analysis is presented on the models based on non-stationary hydrodynamic phenomena caused by deeply penetrating high power CW laser beam into materials. This is typical of laser welding (LW) and laser cutting (LC). A physical analysis pertaining to melt removal and melt layer instability mechanisms of gas jet assisted CW–CO₂ laser fusion cutting is presented. The models deliberated here are melt squeezing out by gas pressure gradient, melt dragging by the friction force between melt surface and gas flow, formation of moving shelves at the cutting front. In case of high laser intensity, radiative flux interacts with material causing dynamical thermal transport onto the surface and phase transition at solid–liquid–gas interfaces. The solution is based on the non-stationary variables. Under these conditions the Mach number varies significantly due to laser intensity associated with laser flux energy instabilities. The connection among material surface temperature, laser intensity, laser flux and pressure in the plasma cloud is brought out. In addition, novel mechanisms based on hydrodynamics are proposed.     

The influence of the pressure formation at the tip of the melt delivery tube on tin powder size and gas/melt ratio in gas atomization method

The production of metal powder using gas atomization technique is a wide spread process for manufacturing a wide range of spherical metal powder alloys. Metal powder properties generally improve with smaller powder particle size. Close-coupled atomizers are of great interest and controlling their performance parameters is critical for metal powder producing industries. In this study a new designed close-coupled nozzle system was used to produce tin powder to investigate the effect of the protrusion length of the melt delivery tube on the pressure formation at the melt tip. Observed improvement in particle refinement cannot be directly attributed to an increase in atomizing pressures and gas kinetics. Results from this study indicated that the observed metal flow rate was not behaving as what was earlier assumed, namely that, deeper aspiration enhanced metal flow rate. The melt flow rate was reduced with increasing the atomizing gas pressure. So that gas to melt mass flow ratio was increased for the same protrusion length and this ratio increase caused the finer powder particle size.     

Laser welding of 3 mm thick laser-cut AISI 304 stainless steel sheet

The objective of the present work was to study the laser weldability of laser-cut 3 mm thick AISI 304 austenitic stainless steel sheet (using oxygen as an assist gas). For minimizing heat input during laser cutting, which is an important factor influencing the thickness of the oxide layer on the cut surface, laser cutting was performed in pulsed mode. The results of the study demonstrated that although the laser welding of laser-cut specimens did not result in the formation of weld defects, the resultant laser weldments exhibited reduced ductility with respect to base metal and bead-on-plate laser weldments. Laser-cut and laser-welded specimens also displayed higher notch sensitivity than the base metal. However, laser-cut and laser-welded specimens still possessed enough ductility to pass guided bend tests.     

Welding with a defocused laser beam

The results of experiments carried out to investigate the welding of austenitic stainless steels using a defocused laser beam are presented. It was assumed that the application of the defocused laser beam would increase the volume of the weld pool, which in turn would reduce the requirements on the preparation of edges and the gap between the workpieces. Microhardness was measured, 2D-computer tomography carried out, specimens were examined by metallographic techniques and the phase components of different zones of the weld pool were analysed by quantitative methods. Special features of the formation of the weld pool dependent upon the welding conditions were investigated. The results indicate that the application of the defocused beam produces high-quality welded joints and the method has also a positive effect on the stability of the vapour–gas channel and the phase composition of the weld metal.     

中频电磁场作用下金属液面稳定性的研究

为了控制电磁铸造过程中金属液面的稳定性 ,借助于激光测量传感器研究了低熔点伍德合金在 2 5 0 0Hz交变电磁场作用下液面波动情况及液面形状。试验发现 ,在电磁场作用下沿金属液面半径方向各点的波动规律相似 ,当金属液面与感应线圈上平面平齐 ,并且施加适当功率的电磁场时 ,能获得更好的液面形状 ,并使液面波动更平缓。     

Using copper nanomarkers for revealing microcracks and regions of microdamage on the surface of pipelines and parts of oil and gas machinery

It has been shown that copper nanomarkers can be used for revealing microcracks and regions of microdamage on the surface of pipelines and parts of oil and gas machinery. The synthesis of copper nanoparticles by hydrazine-induced reduction from aqueous salt solutions in the presence of surface active agents (SAAs) and their silicon-dioxide stabilization, which is needed to prevent surface oxidation processes, are described. Testing of the deposition of optical nanomarkers on a metal surface has been performed. It has been discovered that copper nanoparticles become trapped in microcracks on the surface of metal samples and stay there after the surplus substance is removed from the surface. It has been shown that synthesized copper nanoparticles can be detected in microcracks and regions of microdamage by irradiating metal surfaces of constructional materials with a femtosecond laser.     

基于熔池监测的激光沉积制造成形工艺参数的分析及预测

激光沉积制造(LDM)过程中,熔池宽度是成形精度的关键,主要受到工艺参数的影响。本文建立了基于CCD高速摄像机的熔池在线监测系统。为了提高熔池宽度检测精度,应用卡尔曼滤波技术对熔池宽度测量值进行了去噪处理。采用正交实验设计方法和多元回归分析,建立了熔池宽度与3个主要工艺参数(激光功率、扫描速度和送粉速率)间的关系模型。并设计单一变量实验对模型进行了验证。最后,利用粒子群算法(PSO)对薄壁结构的成形过程参数进行了预测。结果表明,对LDM成形过程进行工艺参数的分析和预测为实现沉积层成形精度的控制提供了依据。     

Prevention of porosity by oxygen addition in fibre laser and fibre laser–GMA hybrid welding

Prevention of porosity in partial penetration fibre laser and fibre laser–gas metal arc hybrid welding is investigated. It is found that modulation of laser power prevents porosity formation in fibre laser welding, but that this method is not effective in hybrid welding. However, the addition of a small amount of oxygen to the molten pool can prevent porosity formation in both fibre laser and hybrid welding. This is attributed to stabilisation of the keyhole. During welding, oxygen reacts with dissolved carbon to form carbon monoxide (CO) in the keyhole. The CO partial pressure in the keyhole prevents the intense interaction between laser beam and molten metal, thus stabilising the keyhole. The role of CO formation is confirmed by the enhancement of porosity suppression with increased carbon content of the base metal.     

Tailoring surface quality through mass and momentum transfer modeling using a volume of fluid method in selective laser melting of TiC/AlSi10Mg powder

A selective laser melting (SLM) physical model of coupled radiation transfer and thermal diffusion is proposed, which provides a local temperature field. A strong difference in thermal conductivity between the powder bed and dense material is taken into account. Both thermo-capillary force and recoil pressure induced by the material evaporation, which are the major driving forces for the melt flow, are incorporated in the formulation. The effect of the laser energy input per unit length (LEPUL) on the temperature distribution, melt pool dynamics, surface tension and resultant surface morphology has been investigated. It shows that the surface tension plays a crucial role in the formation of the terminally solidified surface morphology of the SLM-processed part. The higher surface tension of the lower temperature metal near the edge of the melt pool and the thermal-capillary force induced by the surface temperature gradient tend to pull the molten metal away from the center of the melt pool. For a relatively high LEPUL of 750 J/m, the molten material in the center of the melt pool has a tendency to flow towards the rear part, resulting in the stack of molten material and the attendant formation of a poor surface quality. For an optimized processing condition, LEPUL=500 J/m, a complete spreading of the molten material driven by the surface tension is obtained, leading to the formation of a fine and flat melt pool surface. The surface quality and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.     

Preparation and Performance of Spherical Ni Powder for SLM ProcessingEI北大核心CSCD

3D printing has attracted increasing interests in the field of metallic materials as it can effectively shorten the production cycle and create parts with complex shapes, which can hardly be produced by traditional methods. However, the gas atomization, as the mainstream method of preparing metal and alloy powders to meet the requirements of the processing of selective laser melting (SLM) at present, still has some limitations, such as hollow and/or satellite balls in the powder. This influences directly the density and performance of the printing parts. Moreover, the laser absorption in the smooth surface of powder particle is generally less than 10% in the laser processing, which hinders rapid heating of the powder. It has been found that the material can obtain multiple absorption of laser energy by increasing the surface roughness of powder particles, which can effectively improve the laser absorption rate and is beneficial to get the dense printing parts. Based on this, a novel method combining low temperature spray-drying with heat treatment was developed to prepare Ni powder with high purity, good sphericity, high flowability and narrow particle size distribution. The microstructure and laser absorptivity of the prepared Ni powder were compared with those of the commercial Ni powder prepared by gas atomization, and their influences on the microstructure and properties of the 3D printed bulk materials were investigated. It is found that the laser absorptivity of the Ni powder prepared by spray-drying is more than 2 times as high as that of the commercial Ni powder. This leads to a wider melting channel, smaller surface ten-sion and liquid-bridging force between particles in the printing process. As a result, the spheroidization phenomenon occurred on the surface of the printed bulk material can be avoided by the use of the spraydried powder, and the relative density is achieved as 99.2% at the as-printed state. In the microstructure of the printed bulk material, in addition to the cellular crystals, there are a number of fine columnar crystals, grown across the interlaminar boundaries, which is favorable for a high bonding strength between the interlayers.     

气保护药芯焊丝熔滴过渡的形式及特点

根据气保护药芯焊丝的特点,设计了以激光为背光的高速摄影试验。基于试验结果,总结出了药芯焊丝熔滴过渡的各种形式及存在条件并分别做了描述,阐述了药芯焊丝熔滴过渡的特点。认为药时丝的熔滴过渡形式芯焊丝相比,既有共同点,又有不同点。与实时丝熔滴过渡过程相比,药芯焊丝熔滴过渡的特点是,焊丝中与熔池之间始终存在着半熔化状态的渣柱,其长度取决于药芯材料的熔点。由于熔滴过附着于柱周围以附渣的形式进行过渡及渣柱的导     

Effect of compressibility on flow field and fiber orientation during the filling stage of injection molding

The anisotropy caused by the fiber orientation that is inevitably generated by the flow during the injection molding of short fiber reinforced polymers, greatly influences the dimensional accuracy, the mechanical properties and other qualities of the final product. Since the filling stage of the injection molding process plays a vital role in determining the orientation of the fiber, an accurate analysis of the flow field for the filling stage becomes a necessity. Unbalanced filling occurs when a complex or a multi-cavity mold is used, leading to the development of regions where the fiber suspension is under compression. It is impossible to make an accurate calculation of the flow field during filling within analysis assuming an incompressible fluid. In this study, a FEM/FDM hybrid scheme with consideration of compressibility was developed to calculate the flow field. At the moment of complete filling, the three-dimensional fiber orientation field was estimated by solving the equation of orientation change for the second-order orientation tensor with the fourth-order Runge-Kutta method. A mold with four cavities with different filling times was produced to compare the results of numerical analysis with experimental data. There was good agreement between the experimental and theoretical results when the compressibility of the polymer melt was considered for the numerical simulation. Also, qualitative and quantitative comparisons of fiber-orientation states for compressible and incompressible fluids were made.     

Plasma channel formation for triggering arc welding

Abstract

A method of starting arc welding using a plasma channel formed between electrodes in a tungsten inert gas arc welding system was demonstrated. The plasma channel was generated by gas breakdown in the laser beam path. In a previous study by the present authors, the arc welding could be started using a laser produced plume. Results in the present study indicated that the laser energy required to start the process using the plasma channel was lower than that using the plume.     

Modelling of selective laser melting process with adaptive remeshing

ABSTRACT

Selective laser melting involves manufacturing of a part layer-by-layer by localised melting and solidification of metal powders. The geometry, composition, structure and property of the solidified deposit are significantly influenced by repeated exposure to the laser beam at a high scanning speed. A computationally efficient heat transfer model of selective laser melting is reported here using the finite element method with adaptive remeshing. The model has incorporated a very fine mesh, moving with the laser beam, to capture the local thermal gradient in and around the melt pool. A gradually varying coarser mesh is used away from the melt pool. The computed results are verified with that calculated using a uniform fine mesh and against reported experimental observations.     

工业纯钛光纤激光-MIG复合焊接工艺及性能

采用光纤激光与熔化极惰性气体保护电弧焊(MIG焊)复合焊接工业纯钛,分别对激光焊、MIG焊和复合焊接头的焊缝表面成形、横断面进行了观察,并进行了激光焊和复合焊接头的拉伸试验及杯突试验.结果表明,复合焊的电弧稳定性比MIG焊显著提高,焊接速度可提高7倍;复合焊与激光焊接头的抗拉强度高于母材;复合焊接头的杯突值优于激光焊接头的杯突值,这是因为复合焊焊缝的微观组织有利于接头的塑性.因此,采用光纤激光-MIG电弧复合焊接方法很好地实现了工业纯钛的高速焊接,焊缝成形良好,接头的塑性优于单一激光焊的塑性.

Abstract：

Fiber laser-metal inert gas (MIG) arc hybrid welding was used to weld the commercial pure titanium (CP-Ti). The weld appearance, cross section, tensile strength, Erichsen value and microstructure of the CP-Ti welded joints were studied. The results show that the arc stability is substantially improved and the welding speed can be increased to 7 times by fiber laser-MIG hybrid welding. The welded joints by laser welding and the hybrid welding exhibit the higher ultimate tensile strength than those of the base metal. In addition, the welded joint by the fiber laser-MIG hybrid welding has higher Erichsen values than that by laser joints. The difference in plasticity is attributed to the microstructure changes in the welded joint of hybrid welding. Thus, the fiber laser-MIG hybrid welding of CP-Ti can be carried out suecessfully at higher welding speed with a good combination of weld bead appearance and plasticity.     

铸造镁合金旋转喷吹除气的试验研究

研究了旋转喷吹技术在AZ91镁合金熔体除气净化处理上的应用。采用减压凝固法考察了旋转喷吹的除气净化工艺的净化效果，确定了最佳的除气时间。利用金相显微镜观察了除气前后合金的微观组织。对除气前后的合金，进行了金属型和低压砂型铸造试验浇注，并对其力学性能进行了测试和比较。研究结果表明，经过30min旋转喷吹除气处理后，能快速显著地降低镁合金中的含气量，AZ91合金金属型和低压砂型试样的力学性能都明显地得到提高，特别是合金的伸长率得到大幅度的提高。