Development of a new hybrid plasma torch for materials processing

The main objectives of this study were to construct a plasma generator device and to investigate its applications in welding, cutting, and surface hardening. The device was derived from the union of two plasma-generating technologies, non-transferred-arc plasma and magnetoplasmadynamic thruster (MPDT), and characterised by the simultaneous formation of two plasma arcs in one device, generating a plasma jet with high energy density. Initially, trials were conducted to analyse the influence of the physical variables (gas flow rate and electric current intensities – primary and secondary) on the plasma jet, for which the thruster and the length of the plasma jet expulsed from the chamber were determined. The relevant parameters for welding, cutting and surface hardening were determined by trial and error, in which the trials were conducted using various plate thicknesses and materials. The results have shown that this device can be used for welding, cutting and surface hardening.     

Influence of welding parameters on microstructure and wear behaviour of a typical NiCrBSi hardfacing alloy reinforced with tungsten carbide

Wear parts which are exposed to severe abrasive conditions must withstand high wear demands. Abrasive loading superposed with impact due to abrasive particles are a dominating wear mechanism restricting lifetime in many different industries, for example mining and farming. In practical application, different welding technologies such as plasma transfer arc (PTA), metal active gas (MAG) and laser are used to form wear resistant materials. The aim of this study is to evaluate the influence of welding parameters on the microstructure and wear behaviour of these wear resistant materials using MAG welding technology. To simulate real field conditions on a lab-scale, tests were performed with a standard ASTM G65 dry-sand rubber-wheel tester (3-body abrasion). In order to investigate impact abrasion, a special impeller-tumbler apparatus was designed and used for wear tests (combined impact and abrasion wear). Wear tests were performed on Ni-based alloys containing large amounts of hard phase.Within this work it was shown that welding parameters such as current intensity and number of layers strongly influence dilution with the base material and furthermore the formation of transition zones between welding layers and overlap zones. Concerning wear behaviour it was found that high content of uniformly distributed tungsten carbides in a metallic matrix show the best behaviour under a condition of pure abrasion, whereas under cyclic impact loading (high energy level) massive breaking of the tungsten carbides results in a high wear regime, compared to martensitic materials which perform best.     

辅助增强匙孔气流对激光焊接不锈钢组织和显微硬度的影响

以HR-2抗氢钢为试验材料,利用板条型CO2连续激光器,采用喷嘴在前、入射角为60°的匙孔增强气流,进行了不同参数下的激光焊接试验.结果表明,在气流辅助增强匙孔激光焊中,匙孔气喷嘴在前的布局对熔池扰动小,能获得较好的焊缝成形;与传统激光焊接相比,熔深增加、熔宽明显减小,焊缝形状呈葫芦型,这是匙孔气流压制等离子体、使匙孔向底部凹陷的结果;从焊缝组织来看,增强激光焊焊缝的柱状晶较短小,焊缝中心呈现等轴晶形态,属于FA的不锈钢凝固模式.与传统激光焊不同,气流辅助增强匙孔激光焊的焊缝、热影响区显微硬度与母材相当,这可能是引入的增强匙孔气流改善了焊缝金属的冷却条件,更少铁素体析出的结果.     

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.     

Comparison of formability of friction stir welded and laser welded dual phase 590 steel sheets

Abstract

The formability of welded dual phase 590 (DP 590) steel sheets was investigated, using both friction stir welding and laser welding. Similar and dissimilar gauge sheets were joined using both processes. The laser welded sheets were produced under process conditions typical of industrial production of tailor welded blanks. The friction stir welded specimens were produced in a lab, where different tool rotational speeds and translation speeds were investigated in order to obtain good weld properties. The formability of the welded sheets was evaluated using a series of mechanical tests, including transverse tension and plane strain formability testing. Friction stir welded specimens performed about the same as laser welded specimens in transverse tension testing; however, hardness profiles showed that the laser welds had greater peak hardness than the friction stir welds. Therefore plane strain formability tests were performed with the welds oriented along the major strain direction. When this type of weld stretching was performed the friction stir welded sheets were shown to be ～20% more formable than the laser welded sheets.     

Nano-second pulsed DPSS Nd:YAG laser striation-free cutting of alumina sheets

Lasers are extensively employed in cutting alumina sheets in the electronic industries. An important quality factor of laser cutting is striation (periodical lines) formation on the cut surfaces, which affects the surface roughness and geometry precision of laser cut products. The elimination of striation is important for laser cutting of alumina since it is necessary for denser circuit patterns and device miniaturisation. The present study demonstrates, for the first time, striation-free cutting of alumina sheets with 1 mm thickness using a 400 W high repetition rate nano-second pulsed DPSS Nd:YAG laser. The short pulse duration minimises heat damages. The effects of gas type, gas pressure, nozzle standoff distance, average laser power, cutting speed, and pulse repetition rate on striation characteristics were studied. The specific operating conditions for nano-second pulsed laser striation-free cutting were presented. The present study supports one of the existing theories for continuous wave (CW) laser striation-free cutting of metallic materials and further extends it to the pulsed laser striation-free cutting of ceramic materials. A mechanism of pulsed laser striation-free cutting was proposed, by which a model to predict the operating window for the nano-second pulsed laser striation-free cutting of alumina sheets was developed. The operating window predicted by the model was in agreement with the experiment.     

Quantitative characterization of porosity in Fe–Al dissimilar materials lap joint made by gas metal arc welding with different current modes

5052 Al alloy sheets and galvanized mild steel sheets were joined by gas metal arc welding with three different current modes, including direct-current pulse gas metal arc welding (DPG), alternate-current pulse gas metal arc welding (APG) and alternate-current double pulse gas metal arc welding (ADG). The effect of current mode on size, distribution and volume fraction of pores generated in Fe–Al dissimilar materials lap joint was quantitatively studied. EDS result showed that pores in Fe–Al joint were mainly caused by trapped zinc metal vapor from galvanized steel. Volume fraction of pores in joints made by APG and ADG processes was larger than that in joint resulted from DPG process. Moreover, pores in joints made by APG and ADG processes had smaller diameter, and tended to distribute in the middle of the weld seam. On the contrary, pores with large diameter were inclined to distribute close to upper weld surface of the joint resulted from DPG process. These results are attributed to the difference of arc stirring force and linear heat input in these three processes caused by different current modes.     

Experimental study on turning of TC9 titanium alloy with cold water mist jet cooling

Titanium alloys, as difficult-to-cut materials, have poor machinability due to their superior mechanical properties, heat resistance and corrosion resistance. High cutting temperature that will greatly accelerate tool wear often occurs in titanium alloy cutting process. In this paper, cold water mist jet (CWMJ) cooling method, an eco-friendly cooling method, was used to obtain a lower cutting temperature during TC9 titanium alloy turning process. The effects of CWMJ were mainly discussed as compared with cold air jet and flood cooling methods. A comprehensive evaluation on the cooling effects of CWMJ was carried out by hydrodynamic tests, heat transfer tests and turning tests, respectively. Experimental results indicated that CWMJ had better cooling effects as compared with other two cooling methods. Cutting temperature was greatly reduced, and tool life was improved with CWMJ during TC9 turning process. Machined surface quality and chip morphology were also acceptable.     

Ti-6Al-4V合金的动态控制等离子焊接(英文)

介绍了一种新的动态控制等离子焊接工艺。该工艺能够在保持完全焊透的情况下减少对被焊材料的热输入。利用该工艺对Ti-6Al-4V合金实施平板对焊。利用光学显微镜、扫描电镜(SEM)和显微硬度计对焊接接头的显微组织、断口形貌和显微硬度进行表征。与钨极电弧焊接和常规的等离子焊接相比较,使用本工艺提高了焊接质量,其原因在于:1)由于热输入的减少,焊缝中先析出的β相晶粒大幅度减少,从而使马氏体的形成得到抑制;2)焊接接头具有更好的韧性和更高的硬度。     

Fiber Bragg grating sensors for on-line welding diagnostics

On-line monitoring of an arc-welding process is performed in this paper by means of fiber Bragg grating transducers (FBGs) in two different configurations. FBGs were glued to the plate surfaces next to the weld joint, and a FBG transducer was designed to sense the perturbations in the air surrounding the welding plasma column. Sensitivity to both temperature and vibrations during the process was analyzed during several tungsten inert gas (TIG) welding tests, showing the feasibility of the proposed design to perform an on-line detection of different welding perturbations. To enhance the analysis, a setup based on plasma optical spectroscopy was also employed to allow a comparison among the response of both techniques to the appearance of these perturbations.     

Experimental study of magnetic arc blow for plasma arc cutting

Oxygen plasma arc cutting (PAC) is widely used in various industrial fields. In the case of cutting magnetized plates, the magnetic field is concentrated around the cutting front according to the progress of the cutting, and the electromagnetic force induced by leakage magnetic field deflects the plasma jet. The deflected plasma jet causes poor cutting quality and sometimes causes fatal damage on the electrode and the nozzle by a double arc as abnormal discharge. This phenomenon is called magnetic arc blow, and it is a critical issue of applying plasma cutting on magnetized plates. The purpose of this study is to investigate the arc blow behaviour and to devise a method to prevent it. We examined the relationship between operating conditions and double arc with external magnetic fields on the plasma jet. We found criteria of operation conditions that induce the arc blow. In addition, we have succeeded in suppressing the double arc generation attributed to the external magnetic field with a mild steel shield cap attachment around the nozzle tip.     

外加横向磁场对高速GMAW焊缝成形的影响

对于高速熔化极气体保护焊接（GMAW）过程,当焊接速度超过临界值后,会出现驼峰焊道,焊缝成形变差.研究证明,熔池中动量很大的后向液体流是产生驼峰焊道的主要原因.研发了外加横向磁场发生装置,通过产生的电磁力来抑制后向液体流的动量,从而抑制驼峰焊道的形成.应用特斯拉计测试和考察了工件上磁感应强度大小及分布的影响因素.通过开展焊接工艺试验分析了不同强度的外加磁场作用下的焊缝成形规律.结果表明,外加横向磁场能明显调控熔池流态,有效抑制驼峰焊道和咬边等缺陷,显著改善焊缝成形,提高临界焊接速度.     

Structural and chemical evolution of super duplex stainless steel on activated tungsten inert gas welding process

Abstract

In the present work, the welding parameters of tungsten inert gas (TIG) and activated tungsten inert gas (ATIG) welding processes were compared on duplex stainless steel with two protective gases (Ar and ArHeN₂). The addition of an activating flux can improve the TIG welding process. The ATIG process involves an increase in the penetration as well as a reduction in the number of passes for a thickness higher than 2 mm for stainless steel or other metallic materials. Metallographic observations, chemical analyses and mechanical tests were performed. The results show the importance of the welding parameters, the protective gas and the use of a flux on the characteristics of the weld beads, its microstructure, its hardness and its behaviour in corrosion after welding.     

Analysis and influence of acetylene and propane gas during oxyfuel gas cutting of 1045 carbon steel

Many metal-manufacturing industries include oxyfuel cutting among their manufacturing processes because cutting and welding are often required in metal-cutting processes, specifically in the fabrication of pressure vessels and storage tanks. The oxyfuel cutting process uses controlled chemical reactions to remove preheated metal by rapid oxidation in a stream of pure oxygen. Previous research has demonstrated that metal cutting surfaces varied depending on the gas used for the combustion as well as the cutting speed (Vc) used during the process. In this research, AISI 1045 carbon steel was cut using an oxyacetylene and an oxypropane cutting process. Different tests, such as surface roughness, cut drag displacement, groove width, microhardness, and microstructure, were used to analyze the influence of the Vc and the combustion flame (oxyacetylene and oxypropane). The results showed, in general, better cut surfaces when using propane gas. Also, it was demonstrated that oxyacetylene cutting is almost 85% more expensive than oxypropane cutting.     

Hump formation mechanism in gas-jet-assisted keyhole laser welding of HR-2 stainless steel

Gas-jet-assisted keyhole laser welding offers the possibility of a breakthrough in the limitations of penetration depth in laser welding,which currently suffers from equipment restrictions.A gas jet of sufficient intensity to assist the keyhole should be used to obtain suppressed plasma,a deepened keyhole,and increased penetration depth.However,an excessively strong gas jet gives rise to humps.The incident angle of the keyhole-assisted gas jet is 60°,with a nozzle ahead of the laser beam.A series of experiments were carried out with different welding velocities and gas parameters by using HR-2 hydrogen-resistant stainless steel and a slab CO_2 continuous-wave laser welding machine.The weld profiles can be categorized into four types,welds of traditional laser welding,welds of enhanced laser welding,undercut welds,and humping welds with increased gas pressure.A high-speed camera was employed in the experiments to monitor the formation of humps under an excessively strong gas jet.The results of analysis show that hump formation can be divided into six stages.Its main driving force is the intense turbulence of gas jet.There are two main reasons for hump formation:premature solidification of the molten pool caused by the large temperature gradient between the front and rear parts of the molten pool,and the emergence of a thin layer liquid bridge with one-directional flow under the enhanced cooling effect of excessively strong gas.     

变极性等离子电弧压力的影响因素分析

利用U形压力计测试了不同工艺参数条件下的变极性等离子焊接的电弧压力,利用正交试验测试了不同正极性电流、反极性电流、正反极性时间比和等离子气流量四种参数条件下的电弧压力.结果表明,电弧压力随着这些因素的增加而增加,而四种因素对电弧压力影响的大小依次为:等离子气流量>正极性电流>正反极性时间比>反极性电流.分析原因,等离子气流量是通过冷却压缩提高电弧压力,压缩了电弧尺寸,而焊接电流则通过电磁压缩,但也提高了电弧半径.由于正反极性阶段电弧压力的差异较为明显.因此正反极性时间比对电弧压力也有显著的作用.

Abstract：

VPPA pressure was measured at different welding parameters by U-tube barometer method. The effects of different welding conditions on arc force, including straight polarity current, reverse polarity current, time ratio, plasma gas flow rate were analyzed by the orthogonal experiment. The results indicates that VPPA pressure increased with these four parameters, and the influences of the welding parameters are in order of plasma gas flow rate, straight polarity current, time ratio, and reverse polarity current. Then the mechanisms were discussed. The plasma gas can cool and compress the arc column and compress the arc radius, which increases the VPPA pressure; while the welding current increases the arc radius by electromagnetic compression, which enhances the VPPA pressure. Moreover, due to the great difference of VPPA pressure between straight polarity phase and reverse polarity phase, time ratio is also one of key factors to VPPA pressure.     

热输入对超薄443铁素体不锈钢组织性能的影响

针对443铁索体不锈钢焊接接头存在粗晶组织与性能脆化等问题进行研究,采用光学显微镜(OM)、扫描电子显微镜(SEM)分析了不同焊接方法焊后接头组织和性能的变化.采用钨极氩弧焊(TIG)、脉冲钨极氩弧焊(P-TIG)和脉冲激光焊(P-LBW)三种焊接方法进行焊接,分析了不同热输入对超薄443铁素体不锈钢焊接接头微观组织、拉伸性能和杯突性能的影响规律.结果表明,焊接热输入是影响该铁素体不锈钢焊接接头组织和性能的重要因素,采用脉冲焊接方法来细化组织可以有效提高焊接接头综合性能.     

Effects of surface treatments of galvanized steels on projection welding procedure

A group of projection welding experiments and joints tension-shear tests are carried out for cold-rolled steel sheets, galvanized steel sheets (GSS) without treatment, GSS with phosphating and GSS with surface greasing, respectively. The experimental results are regressively analyzed on the computers, then the projection welded joint tension-shear strength curve and the perfect welding currents range of each material are obtained. The results show that surface treatments of galvanized steels have effects on their spot weldabilities. Among the four kinds of materials, GSS with surface greasing have the worst spot weldability, for they need higher welding current and have a narrow welding currem range.     

Joining of molybdenum and its application

In this work fusion and solid state welding techniques for joining PM refractory metals are adressed. The weldability is discussed and evaluated by electron beam and friction welding experiments. EB-welding experiments were performed on rolled sheets made of different Mo-based materials. While a significant decrease in the mechanical properties was found for the weldments of unalloyed molybedenum and the alloy TZM, good weldability is given for Mo41Re. A welding procedure was established and used for the manufacturing of thin walled tubes. Rotary friction welding experiments were carried out on rod material of the Mo-alloys TZM and MHC. Suitable process parameter were determined and successfully transferred for the joining of tubular TZM parts.     

低功率等离子体炬生产钛合金粉末的理论与实验研究

探讨使用低功率等离子体炬生产钛合金金属粉末的可能性.设计一种氩气直流非转移电弧等离子体炬,并对其等离子体射流特性和导线温度进行数值分析.喷嘴附件内的最高射流速度为838～1178 m/s,不同气体流速下顶点处的射流速度为494～645 m/s.等离子体气体流速对有效等离子体射流长度无显著影响.利用等离子体射流的温度和速...