Welding of Ti-6Al-4V alloy using dynamically controlled plasma arc welding process

A novel dynamically controlled plasma arc welding process was introduced,which is able tominimize heat input into the workpiece materials while maintaining desired full penetration,and it was used to weld Ti-6Al-4V alloy sheets.The microstructures,facture surfaces and microhardness of the welded joints were characterized by using optical microscope,scanning electron microscope （SEM） and Vickers microhardness tester.Comparing with welds such as gas tungsten arc and conventional plasma arc processes,the experimental results revealed the improvements when using the present process including：1） reducing prior-beta （β） grain size and prohibiting formation of hard martensite phases in the fusion zone due to the decreased heat input;and 2） better toughness and higher hardness.

大熔深K-PAW熔池及电弧视觉行为特征分析研究

穿孔等离子弧焊接(K-PAW)以其独特的优势,在中厚板的焊接制造中应用广泛,并逐渐向更大厚度的方向发展。然而,随工件厚度增加熔池穿孔出现不稳定等问题,制约了其在实际生产中的应用。分析了14 mm厚304不锈钢在“受控恒定穿孔”控制策略下的大熔深K-PAW熔池特征尺寸变化规律和焊接过程中的熔池行为。通过视觉检测与电信号同步采集系统,对大熔深K-PAW过程中熔池的形貌特征进行定量分析,探究在受控脉冲条件下的熔池特征参数变化情况,以及熔池特征与电信号的同步动态行为规律。以初始电流270 A进行焊接,在起弧的前14 s的盲孔阶段,随着电弧热量积累,熔池不稳定,尺寸出现震荡,最大震荡幅度超过10 mm,同时熔池长宽比也在1.0~1.6范围内变化;随着穿孔于第15 s发生,熔池达到平衡,且长宽比在19 s时达到峰值;维孔阶段,熔池保持稳定,形状未发生大变化。综上,可以通过熔池形状变化趋势来区分焊接阶段。比较了不同焊接电流(280 A、275 A、270 A)的盲孔阶段,发现采用较大的初始电流有利于缩短盲孔时间,由270 A时的22.9 s减小到了280 A时的19.8 s,并降低焊接气孔出现的可能性。最后,发现电弧中心与熔池前边缘的距离可以反映穿孔状态。结果验证了“受控恒定穿孔”控制策略的可行性,并验证了通过视觉特征判断焊接阶段及是否穿孔的可行性。

基于BP神经网络薄板P-PAW搭接的间隙自适应工艺参数优化

对大型LNG燃料薄膜舱中1.2 mm厚SUS304L不锈钢板薄板的搭接焊,存在的搭接间隙会带来焊缝成形不良等严重影响船舱安全性的问题。本文通过激光传感器检测搭接间隙变化,设计了不同的峰值电流和焊接速度在不同间隙下的工艺实验,研究了间隙对焊缝成形质量的影响机制;基于BP神经网络建立不同间隙下的脉冲等离子弧焊(P-PAW)的工艺参数和间隙及焊缝成形尺寸之间的拓扑关系模型,通过工艺实验获取模型的训练样本,实现了基于BP神经网络的间隙自适应工艺参数优化系统。结果表明,该系统实现了不同搭接间隙下进行实时工艺参数优化的功能,并对搭接间隙在0~0.6 mm变化时所带来的空洞等缺陷起到了有效的抑制作用,实现了良好的焊接成形一致性控制,提高了在LNG薄膜舱中不锈钢板焊接的自适应能力。

铝合金VPPAW穿孔焊接匙孔闭合处的微观组织与力学性能

铝合金因其密度低,比强度高等特性,被广泛应用到航空航天、军事工业等领域.与其他焊接工艺相比,变极性等离子弧焊接在大型铝合金结构件加工方面具有突出优势.针对轻金属合金等离子弧焊接过程中匙孔难以自由闭合的问题,提出了气电分离时序脉冲等离子弧环焊缝收弧方法,对比研究了环焊缝收弧方法与手工钨极惰性气体保护焊(tungsten inert gas welding, TIG焊)填补法焊缝匙孔闭合处的微观组织与力学性能.结果表明,通过调节焊接起弧、收弧策略,实现了等离子弧匙孔自由闭合,焊缝收弧区成形良好,组织为均匀的等轴晶;抗拉强度为329.9 MPa,断后伸长率为16.3%,比TIG填补法的抗拉强度提高了22.91%,断后伸长率提高了55.24%.平均硬度值为78.9 HV0.2,热影响区硬度呈下降趋势,熔合线处硬度呈升高趋势,焊缝区硬度波动范围为75~81 HV0.2.

Relationship between sound signal and weld pool status in plasma arc welding

The sound features of the weld pool status in plasma arc welding were systematically investigated after the sound signal was collected with a microphone. The results show that it is difficult to extract information about the weld pool status directly in time domain although the sound signal varies with the weld pool behaviors to some extent. The frequency spectra of the sound signal contain plenty of information about the weld pool behaviors. It is shown from the analysis of the sound generation mechanism that the sound signal of plasma arc welding is mainly caused by the weld pool oscillation, the power source fluctuation and so on. R S algorithm is designed to determine the weld pool status, and it is able to offer the feedback information for the closed loop control of the penetration quality of plasma arc welding. [

Research of precise pulse plasma arc powder welding technology of thin-walled inner hole parts

The inner hole parts played an oriented or supporting role in engineering machinery and equipment,which are prone to appear surface damages such as wear,strain and corrosion. The precise pulse plasma arc powder welding method is used for surface damage repairing of inner hole parts in this paper. The working principle and process of the technology are illustrated,and the microstructure and property of repairing layer by precise pulse plasma powder welding and CO2 gas shielded welding are tested and observed by microscope,micro hardness tester and X-ray residual stress tester etc. Results showed that the substrate deformation of thin-walled inner hole parts samples by precise pulse plasma powder welding is relatively small. The repair layer and substrate is metallurgical bonding,the transition zones( including fusion zone and heat affected zone) are relatively narrow and the welding quality is good. It showed that the thin-walled inner hole parts can be repaired by this technology and equipment.

Finite element analysis of keyhole plasma arc welding based on an adaptive heat source mode

An adaptive heat source mode is proposed to account for the keyhole effect and the characteristics of volumetric distribution along the direction of the workpiece thickness. Finite element analysis of the temperature field in keyhole plasma arc welding is conducted and the weld geometry is obtained. The predicted results are in agreement with the measured ones.

Modeling the keyhole shape and dimension in plasma arc welding

It is of great significance to model the keyhole shape and dimensions to optimize the plasma arc welding process parameters. In this study, through employing a combined volumetric heat source mode, the weld pool in keyhole plasma arc welding is determined firstly, and then the dynamic force-balance condition on the interface between the plasma jet and the molten metal is dealt with in describing the keyhole formation inside the weld pool. The effects of welding current on the shape and size of keyhole are numerically analyzed. The sharp transformation from a partial keyhole to a full-penetration keyhole is quantitatively demonstrated.

Determination of arc pressure pool surface in and current density on the molten plasma arc welding

A unified numerical model is developed to couple the plasma arc, weld pool and keyhole in a self consistent way. The plasma arc/anode interface and the melt/solid interface are treated specially, the VOF method is used to trace the moving keyhole wall, and the fluid flow and heat transfer in both the plasma arc and weld pool are numerically simulated. The distributions of current density and arc pressure on the weld pool surface during the keyhole formation process are analyzed using the coupled model. The predicted arc pressure and current density are compared with the experimentally measured results, and both are in good agreement.

Experimental determination of the weld penetration evolution in keyhole plasma arc welding

Keyhole plasma arc welding experiments are conducted to measure the weld geometry and penetration at different moments during the initial phase from igniting arc to quasi-steady state. Indirect information on keyhole formation and evolution in plasma arc welding can be extracted based on the weld macrophotograph at cross section. It has laid foundation to verify the mathematical models of keyhole plasma arc welding.

A computer-based control system for keyhole plasma arc welding

A new kind of control system for keyhole plasma arc welding （K-PAW） was developed based on the computer and the Graphics Language--LabVIEW. It can set and output the required current waveforms with desired decreasing slopes so that the corresponding ＂opening and closing＂ of keyhole can occur periodically. With this control strategy of welding current waveforms, the workpiece is fully penetrated while no burn-through Occurs. Keyhole plasma arc welding experiments were conducted to verify the stability and reliability of the developed system.

Analysis of keyhole geometry in plasma arc welding

The key problem for numerical simulation of plasma arc welding （PAW） process is to develop a suitable and adaptive volumetric heat source mode which reflects the physical characteristics of keyhole PAW. To this end, the keyhole geometry under different PAW process conditions must be predicted. In this paper, a mathematical model for determining the keyhole shape is developed with considering the mass and momentum conservation of the in-keyhole plasma jet as well as the pressure equilibrium at the plasma jet/liquid metal boundary. A suitable heat source model related to the keyhole shape is applied to the calculation of PAW weld dimensions. The predicted results are in good agreement with the experimental ones.

Sensing controlled pulse key-holing condition in plasma arc welding

According to the strategy of controlled pulse key-holing,a new sensing and control system was developed for monitoring and controlling the keyhole condition during plasma arc welding(PAW). Through sensing and processing the efflux plasma voltage signals,the quantitative relationship among the welding current,efflux plasma voltage and backside weld width of the weld was established. PAW experiments show that the efflux plasma voltage can reflect the state of keyhole and backside weld width accurately. The closed-loop control tests validate the stability and reliability of the developed keyhole PAW system.

Formative mechanism of the undercut defect in horizontal variable polarity plasma arc welding

Abstract Horizontal welding is important for heavy or huge welding structures. Keyhole mode variable polarity plasma arc welding of aluminum alloy plates with medium thickness was carried out in horizontal position. The characteristic of welding defects was introduced. Preliminary experiments indicated that the undercut defect could not be eliminated easily. The relationship between welding parameters and the undercut defect showed that this deject could be lessened by using higher heat input. The fluid flow of weld pool was observed by a high speed camera. The fluid flow in weld pool was not symmetric and much of molten metal gathered in the lower part. The fluid flow velocity in the lower part was bigger than that in the upper part. To this end, the formative mechanism of the undercut defect was proposed. The flowability of the molten metal was an influential factor for the undercut defect. A preheating method was designed to verify the formative mechanism.

Analysis of the 3-D fluid velocity and temperature profile in keyhole plasma arc welding

Modeling and simulation of fluid flow and heat transfer in keyhole plasma arc welding is of great significance for optimizing the process parameters. In this study, a three-dimensional transient model is established to analyze numerically the heat transfer and fluid flow phenomena in keyhole plasma arc welding. VOF （volume of fluid） method is used to track the boundary of the keyhole. The dynamic developments of keyhole geometry, the fluid velocity field and temperature profiles are numerically simulated. And the changing Of the fluid velocity and pressure distribution on the keyhole wall in the forming process of the keyhole are analyzed.

In-situ weld-alloying plasma arc welding of SiC_p/Al MMC

Plasma arc welding was used to join SiCp/Al composite with titanium as alloying filler material. Microstructure of the weld was characterized by an optical microscope. The results show that the harmful needle-like phase Al4C3 is completely eliminated in the weld of SiCp/Al metal matrix composite(MMC) by in-situ weld-alloying/plasma arc welding with titanium as the alloying element. The wetting property between reinforced phase and Al matrix is improved, a stable weld puddle is gotten and a novel composite-material welded joint reinforced by TiN, AlN and TiC is produced. And the tensile-strength and malleability of the welded joints are improved effectively because of the use of titanium.

Study on Weld Quality Characteristics of Pulsed Current Micro Plasma Arc Welding of Inconel625 Sheets

Nickel alloys had gathered wide acceptance in the fabrication of components which require high temperature resistance and corrosion resistance, such as metallic bellows used in expansion joints used in aircraft, aerospace and petroleum industry. In the present paper an attempt is made to study various weld quality characteristics like weld bead geometry dimensions, micro hardness, microstructure, grain size and tensile properties of Pulsed Current Micro Plasma Welding of Inconel625sheets. Weld joint was prepared by fusing the two parent metals of Inconel625 sheets. Square butt joint is used and welding was carried out using Pulsed DCEN, without filler wire. Peak current, back current, pulse and pulse width are considered as the main influential input variables during the welding.

Study on Effect of Welding Speed on Micro Structure and Mechanical Properties of Pulsed Current Micro Plasma Arc Welded Inconel 625 Sheets

Nickel alloys had gathered wide acceptance in the fabrication of components which require high temperature resistance and corrosion resistance, such as metallic bellows used in expansion joints used in aircraft, aerospace and petroleum industry. Micro Plasma Arc Welding (MPAW) is one of the important arc welding processes commonly using in fabric- cation of Nickel alloys. In the present paper welding of Inconel 625 sheets using pulsed current micro plasma arc weld- ing was discussed. The paper mainly focuses on studying the weld quality characteristics like weld pool geometry pa- rameters, microstructure, grain size, hardness and tensile properties of Pulsed Current Micro Plasma Arc Welded In- conel 625 sheets at different welding speeds. Results reveals that at a welding speed of 260 mm/minute better weld quality characteristics can be obtained.

Closed-loop control of weld penetration in keyhole plasma arc welding

To improve the penetrating ability and the welding quality of keyhole plasma arc welding, a novel penetration closed loop control system was established. In the system, welding current and plasma gas flow rate were selected as adjusting variables. The wavelet method was used to detect penetration status from welding arc voltage in real time. The control strategy of one keyhole per pulse was adapted to fulfill stable and high quality welding process. Experimental results show that the developed system can apparently increase the penetrating force of plasma arc and keyhole plasma arc welding is realized successfully in stainless steel with 10 mm in thickness. Moreover, the disturbances of gradual change and break change from 3 mm to 6 mm in thickness are come over due to the good response property of the developed system.

System Identification of Controlled Pulse Keyhole Plasma Arc Welding Process

The development of closed-loop control systems is one of the most effective ways to improve the stability of the keyhole status during keyhole plasma arc welding （K-PAW）. Due to the disadvantages of the ＂one-pulse-one-keyhole＂ technology based on the conventional square current waveform, the controlled pulse welding current waveform is newly applied to control the keyhole open and close periodically. In order to realize the real-time control on the keyhole behavior with this advanced current waveform, welding experiments and system identification are conducted based on the classical control theory. One complete welding cycle can be divided into 3 periods. The keyhole establishing time is the most important time variable, which determines the keyhole behavior and welding process stability. At the same time, the averaged effiux plasma arc voltage during one pulse cycle can reflect the real keyhole dimension and status in a real-time manner. Therefore, two single-input-single-output （SISO） systems are proposed, in which keyhole establishing time and keyhole average dimension are taken as the system controlled variables respectively. Welding experiments are designed with the peak current varying randomly. Experiments show that the keyhole establishing time changes in an opposite direction to the varied peak current, and the averaged efflux plasma arc voltage varies with the same trend as the peak current. Based on the least squares technique and F test of classical system identification, second order difference equation for keyhole establishing time/peak current system and first order difference equation for keyhole average dimension/peak current system are obtained. It is proved that the calculated data by the two mathematical expressions are well matched with the measured data. The proposed research provides mathematical expressions and theoretical analysis to develop closed-loop systems for the controlled pulse K-PAW.