Pore formation mechanism and its mitigation in laser welding of AZ31B magnesium alloy in lap joint configuration

Magnesium is one of the lightest structural metals that has been used in different industries such as automobile, aerospace and electronics. However, in fusion joining of magnesium alloys, porosity is one of the main drawbacks to achieve a weld with desirable properties. The oxide layer existing on the surface of magnesium alloy is one of the causes of pore formation in the weld bead. In the current study, a fiber laser with a power of up to 4 kW is used to weld samples in a zero-gap lap joint configuration. Two groups of samples are studied: as-received (AR) surfaces (where an oxide layer remains on the surface) and treated surfaces. The surface treatment includes two techniques: mechanically removed (MR) and the use of a plasma arc (PA) as a preheating source. Also, a separate set of experiments are designed for preheating samples in a furnace for comparison with the PA-treated results. To reveal the chemical compositions of the welds and metal sheet surfaces, an energy dispersive spectroscopy (EDS) is performed. Surface chemical compositions are tested by X-ray photoelectron spectroscopy-reflected electron energy loss spectroscopy (XPS-REELS) to characterize the surface composition on AR and PA-treated samples. The dynamic behavior of the weld pool and laser-induced plasma plume is monitored in real-time using a high speed CCD camera to investigate the stability of the laser welding process. The presence of the oxide layer at the faying surface of two overlapped sheets results in an unstable process. The obtained results reveal that the preheating procedure can effectively mitigate pore formation at the interface of the two overlapped sheets.     

Weld appearance prediction with BP neural network improved by genetic algorithm during disk laser welding

The appearance of welds is the external manifestation of welding quality. The morphology of molten pools is significantly associated with the weld appearance, but the approach to measure the morphology of molten pools during laser welding remains an outstanding challenge up to now. In this study, the shadows of molten pools were formed to describe the morphology of molten pools. Principal components analysis (PCA) is applied to analyze the characteristics of the molten pools’ shadow in order to reduce their redundancy. Then BP neural network improved by genetic algorithm (GABP) is established to model the relation between welding appearance and the characteristics of the molten-pool-shadows. The effectiveness of the established model is analyzed through two different welding speed experiments, and the results verify its prediction performance. The work provides an effective way to predict the weld appearance and assess the welding quality in real-time.     

Improvement of surface characteristics for long life of metal molds by large-area EB irradiation

This paper describes the applicability of large-area EB irradiation method to a new surface modification for metal molds. The previous papers clarified that the surface roughness on wide area uniformly decreased to less than 1 μm Rz in a few minutes by the large-area EB irradiation. Therefore, the large-area EB irradiation method has high practicability as an efficient surface finishing process for metal molds, and then the EB irradiation equipment has already been introduced into the market for practical use. In this method called EB polishing, the surface material melts and a very thin resolidified layer is formed on the surface. In this paper, the surface structure of EB polished metal mold steel SKD11 was first observed by TEM, and also component analysis was carried out by EDX. Then it was found out that the crystal grain size became small and chromium carbide is uniformly rearranged in the layer. Next, using heat conduction analysis model considering the electron penetration on the surface, the temperature distribution was calculated. The results showed that the surface temperature rapidly raised over its melting point until the end of EB irradiation, and cooled down rapidly. As the important practical surface characteristics for long life of metal mold, resistance to corrosion, water repellency, and the releasability of molded resin from metal mold surface were evaluated. The corrosion resistance and the water repellency could be improved by EB polishing, since the surface structure changes and Cr content distributed uniformly on the surface. Also, the releasability of molded resin from the surface became better.     

The bio-compatible dental implant designed by using non-stochastic porosity produced by Electron Beam Melting® (EBM)

The application of non-stochastic lattice structures for building a bio-compatible dental abutment is investigated. The bio-compatible dental implant should mimic the micro-motion of the natural tooth. Three different lattice structures including cross, honeycomb, and octahedral structures with different unit cell sizes were employed to produce lattice abutment made of Ti–6Al–4V by Electron Beam Melting^® (EBM). According to the results, by increasing the unit cell size, abutments showed more deformation; however, the maximum tolerated normal force decreased. The octahedral lattice structure with 2 mm unit cell size showed the best mechanical behavior under 400 N normal biting force, and was selected to investigate the effect of biting force angle on the stress distribution developed in lattice abutment. Numerical analysis showed that α = 30° is a critical biting force angle, for which the maximum equivalent stress increased noticeably.     

斜面激光熔覆粉末流场及光粉耦合机理研究

目的 扩展增材制造在斜面复杂零部件修复领域的应用,弥补倾斜熔覆过程中粉末流动行为研究的空缺。方法 采用RSM进行模拟方案设计与数据处理,通过拟合输入参数与输出之间的数学模型,探究送粉电压、气流量、基体倾斜角度对倾斜基体上粉末浓度和粉斑直径的影响规律。结合数值模拟和试验研究对光粉耦合机理进行分析,探究光粉平衡关系对涂层形貌的影响机理。结果 粉末颗粒速度随气流量的增加而增大;倾斜基体上最大粉末浓度随气流量的增大而降低,随送粉电压的增大而增高;倾斜基体上的粉斑直径随基体倾斜角度和气流量的增大而增大。以倾斜基体上的粉末浓度最大、倾斜基体上的粉斑直径最小为优化目标,0°、10°、20°、30°倾斜基体上最大粉末浓度模拟值与预测值的误差分别为4.34%、3.61%、5.82%、13.15%,基体上粉斑直径模拟值与预测值的误差分别为2.95%、3.22%、3.57%、4.10%,说明该模型对倾斜基体上最大粉末浓度及粉斑直径的预测精度较高。结论 气流量对粉末颗粒速度影响显著,送粉电压、气流量对基体上最大粉末浓度的影响显著,倾斜角度、倾斜角度与气流量的交互项对倾斜基体上的粉斑直径影响显著。研究结果为激光...     

Effect of combined plasma-carburizing and deep-rolling on notch fatigue property of Ti-6Al-4V alloy

This paper discusses the effects of a combination of plasma-carburizing and deep-rolling on notch fatigue properties of a Ti-6Al-4V alloy. Circumferentially V-notched cylindrical Ti-6Al-4V alloy specimens were plasma-carburized at a relatively low temperature for the improvement of wear resistance, and then, deep-rolled at the notch root for inducing compressive residual stress. Scanning electron microscopy, optical microscopy, laser scanning microscopy, surface roughness tester, and micro-hardness tester were used to characterize the modified surface layer at the notch root. Axial loading fatigue tests (R = 0.1) were performed using a servo-hydraulic testing machine in a laboratory atmosphere at an ambient temperature. The notch fatigue life of the specimen was reduced by plasma-carburizing due to the brittleness caused by the higher hardness in addition to the disappearance of compressive residual stress on the notched surface, but remarkably improved by the subsequent deep-rolling. The surface layer containing the compressive residual stress and the work hardening induced by deep-rolling effectively prevented and delayed the fatigue crack initiation and propagation of deep-rolled carburized specimen.     

Porosity formation mechanism and its prevention in laser lap welding for T-joints

A high-speed camera and X-ray transmission observation system were used to observe the keyhole and molten pool dynamic behavior in laser lap welding T-joints. The oscillation frequency of the molten pool and the keyhole increases with increasing gap. The lower keyhole becomes slant with the large gap and large quantities of bubbles are formed at the bottom tip of the keyhole. The molten pool is divided into three different zones by the large gap and a small eddy is formed at the lower molten pool. The bubbles are difficult to escape from the lower molten pool and the gap when the gap is large, resulting in the formation of porosity at the gap and root of weld seam. The distribution characteristics of porosity in different gap have an excellent agreement with the keyhole and the molten pool dynamic behavior. Porosity can be suppressed by maintaining a small gap or adopting high welding speed. The paper provides fundamental insights into the mechanism of porosity formation during laser lap welding T-joints and guidance to aid in its elimination.     

On the evolution of surface roughness during deformation of polycrystalline aluminum alloys

Surface roughening of polycrystalline Al–Mg alloys during tensile deformation is investigated using white light confocal microscopy. Materials are tested that differ only in grain size. A height–height correlation technique is used to analyze the data. The surface obeys self-affine scaling on length scales up to a correlation length which approximately equals the grain size and above which no height correlation is present. The self-affine scaling exponent increases initially with strain and saturates at a value around 0.9. A linear relation is observed between root-mean-square roughness and both strain and grain size. The observed roughness is explained as the result of the combined effect of a self-affine roughening on a subgrain scale and a grain scale roughening caused by orientation differences between neighboring grains.