Strength and microstructure of laser fusion-welded Ti–SS dissimilar material pair

The ability to efficiently create robust and reliable dissimilar metal joints has the potential to enable new functionalities and reduce the manufacturing costs of medical devices. The need for dissimilar material welds in the medical device industry is driven by the unique properties exhibited by biocompatible materials such as stainless steel and titanium, as well as shape memory materials such as NiTi. Many material pairs, however, suffer from significant intermetallic phase formation during welding which greatly reduces their strength. This study investigates the microstructures and strength of the laser fusion-welded titanium–stainless steel dissimilar material pair as a simplified model of the NiTi–stainless steel pair. Compositional and structural analysis of the weld pool is performed and fracture morphologies are analyzed in different regions of the weld joint. The role of weld pool geometry, heat flow, and quench rate on the resultant phases, microstructures, and strength of the welds is discussed.     

TIG焊熔池表面流动行为的研究

针对钨极惰性气体保护(Tungsten inert gas,TIG)焊熔池表面流动行为,在确定TIG焊熔池表面可采用粒子示踪的方法来进行其表面流动行为示踪的基础上,在以激光为试验背光光源,通过激光在TIG焊熔池表面镜面反射后,使得熔池及示踪粒子清晰成像于成像屏上。在此基础上,开展对304不锈钢和Q235普通碳钢的熔池表面流动行为的试验研究,对所获得这两种材料的TIG焊熔池试验数据进行处理与对比分析,探究熔池表面流动规律。研究结果表明:在TIG焊过程中,其焊接熔池存在两种运动模式,在304不锈钢焊接过程中熔池表面的液态金属由边缘向熔池中心流动;在Q235碳钢焊接过程中熔池表面的液态金属不定向、不规则地由熔池中心向熔池边缘流动,并测量304不锈钢TIG焊过程中熔池表面的液态金属流动速率为12 mm/s左右,Q235碳钢的熔池表面的流动速率为15 mm/s左右。     

Investigation on the resistance spot-welded austenitic/ferritic stainless steel

In this work, the effect of weld current on joining capability of austenitic stainless steel (AISI 304) and ferritic stainless steel (AISI 430) sheets with application of resistance spot welding process was investigated. Macrostructure, microstructure, microhardness, tensile shear strength, and failure mode of welded materials were evaluated for different weld currents. The values of weld current were 2.5, 3.75, and 5 kA. It was found that when the weld current increased, the nugget size and the weld strength were increased. Two distinct failure modes including interfacial and pullout were observed during tensile shear test. Finally, an adequate weld current was obtained.     

TIG焊熔池表面张力的测定及表面活性剂的影响

利用熔池振荡及熔池谐振信号的检测,以熔池自身固有振荡频率与熔池尺寸的内在关系测定熔池金属表面 张力,研究了薄板ＳＵＳ３０４不锈钢ＴＩＧ焊不同熔地尺寸的表面张力变化以及活性剂对熔池表面张力的影响。试验研 究方法为焊接熔池表面张力测定中的最初应用。     

304不锈钢大功率光纤激光焊成形研究

为了研究大功率光纤激光焊在304不锈钢上的焊缝成形,使用5~7kW的激光功率,10~100mm/s的焊接速度在16mm厚的304奥氏体不锈钢上进行全覆盖参数试验。随后观察了焊缝的熔深、熔宽、焊缝形状等成形参数。结果表明,焊接速度低于20mm/s时,焊缝表面会形成隆起,熔深随速度减慢,迅速增加;焊接速度在30~40mm/s时,焊缝表面变得凹凸不平且两边存在咬边,熔深随速度减慢且小幅增加;焊接速度介于50mm/s和90mm/s之间时,焊缝的熔深和熔宽几乎不变;而当速度达到100mm/s时,熔深急剧减小,且钉头形焊缝的形状发生了很大的改变。通过以上试验结果结合小孔效应和熔池特性分析了激光焊缝的成形机理,对大功率光纤激光焊接形成了更全面的认识。     

奥氏体不锈钢铅液池裂纹修复

纯奥氏体不锈钢焊接难度较大。奥氏体不锈钢铅液池进行国产化制作,要求焊接后进行固溶处理,固溶处理后经探伤发现焊缝上有大量的裂纹。通过对材料、固溶处理工艺、焊接工艺研究分析,找出了产生焊接裂纹缺陷的原因,采用适于奥氏体不锈钢焊接修复的焊接材料,并采用适当的焊接工艺,成功地进行了焊接修复。     

表面活性元素对Marangoni流和熔池形状影响的数值分析

根据合金熔化与凝固过程液相区、固相区和糊状区共存的特点，建立了适合于ＧＴＡ焊接熔化与凝固过程的统一模型控制方程和辅助方程，并用控制容积积分的有限差分法对其进行了数值求解。能量方程中考虑了固／液相变潜热、表面合金元素的气化、表面对流和辐射热损失， 动量方程中考虑了浮力、电磁力和Ｍａｒａｎｇｏｎｉ力驱动流。重点讨论了Ｆｅ－１８Ｃｒ－９Ｎｉ合金中表面活性元素—硫 （Ｓ） 对Ｍａｒａｎｇｏｎｉ力诱导的熔池内涡流环路和熔池形状的影响。结果表明， Ｓ质量分数较小时， 每半个熔池内存在两个相反的涡流环路， 随着Ｓ质量分数的增加向外的流体环路逐渐减弱并消失， 面向内的流体环路逐渐增强与扩大， 造成熔池深度的增加。     

Analyzing mechanical properties and nondestructive characteristics of brazed joints of NiTi shape memory alloys to carbon steel rods

Research is being conducted on the use of shape memory alloys, in particular NiTi, in civil engineering, due to the superelastic behavior of NiTi which can be used for damping. In this particular application, NiTi has to be joined to steel, which constitutes a major difficulty due to the considerably different properties of materials involved and the poor weldability of NiTi. Brazing was investigated for this application, since it is an economic and efficient process to joint dissimilar materials. This paper presents a study on the mechanical behavior in the superelastic regime of dissimilar NiTi/steel joints and the feasibility of a nondestructive method based on eddy currents to characterize the weld metal. Brazed joints in lap configuration were produced with a 20 % Ag braze alloy and tested under fatigue conditions in the superelastic regime. Lap joints of steel rods to NiTi ribbons have undergone up to 60 cycles of load/unload without rupture at a maximum load of the superelastic plateau close to the pull-out load of the joint. Measurements of the electrical impedance, with a helicoidal cylindrical coil along the lap joint, allowed the identification of the location of the joint, with a good spatial resolution characterizing the morphology of the brazed joints.     

CO2 laser welding of zinc-coated steel sheets

Welding of zinc-coated steel sheets for the automotive industry has been investigated experimentally and theoretically, using a continuous wave 2 kW CO₂ laser. The specimens of 0.8, 1.0 and 1.2 mm thickness were welded as butt joint and lap joint. Argon gas was shielded co-axially to reduce the plasma and to protect the molten, pool from atmosphere. The mechanical tests of specimens were carried out to investigate the ductility of welds in butt joint and lap joint, using the Erichsen test, ball punch test and tensile shear test. The value of transverse weld pattern is higher than others. The fatigue life of longitudinal weld is superior, but that of circular weld pattern is inferior due to the high tensile residual stresses in the weld. The maximum Erichsen value was obtained as 96% and the deformability of zinc coated steel butt-welded was found to be 80% in the ball punch test. The high pressure formed by vaporization of zinc with the low boiling temperature during laser lap-joint welding splattered the molten pool and created porosities in the weld. The optimum gap was calculated to be 0.1 mm in the lap joint welding of zinc-coated steel sheet which was a good agreement with the experimental result.     

Theoretical and experimental study of microstructures and weld pool geometry during GTAW of 304 stainless steel

In this work, temperature field and weld pool geometry during gas tungsten arc welding of 304 stainless steel are predicted by solving the governing equations of heat transfer and fluid flow under quasi-steady state conditions. The model is based on numerical solution of the equations of conservation of mass, momentum, and energy in the weld pool. Weld pool geometry, weld thermal cycles, and various solidification parameters are then calculated by means of the model predictions. The model considers the effects of various process parameters including welding speed and heat input. It is found that the weld pool geometry, predicted by the proposed model, is in reasonable agreement with the corresponding experimentally measured ones. In addition, the solidification behavior of the weld pool can be predicted properly by the model predictions.     

钨极氩弧焊熔透熔池塌陷倾向的预测

对钨极氩弧焊(GTAW)全熔透熔池进行受力分析,建立熔透熔池的力学模型,提出熔池塌陷的力学判据.利用所建立模型计算并讨论影响熔池塌陷的各个作用力的动态变化、大小和百分比组成,并预测钨极氩弧焊焊接不锈钢和低碳钢薄板时在不同焊接速度下的合适焊接电流范围.试验结果与理论预测相一致,验证了所建立模型的实用性.     

TIG焊接熔透熔池形状和表面变形的数值模拟

利用建立的三维TIG焊接熔池瞬时行为数值分析模型,对移动热源作用下不锈钢薄板全熔透时熔池动态行为进行了数值分析。结果表明,同时采用电弧热流和电弧压力的双椭圆分布模式,相对于高斯分布模式,计算出的熔池形状和表面变形与试验结果吻合得更好。给出运动电弧作用下熔池表面变形的动态演变,分析了与固定电弧焊接时的异同,为运动电弧作用下TIG焊接的数值模拟与智能控制提供了理论依据和试验数据。     

Vibration cavitation behaviour of selected Ni-Mn-Ga alloys

Vibration cavitation erosion tests were carried out on Ni-Mn-Ga alloys of three different crystal structures: (1) the cubic austenite, (2) the non-modulated tetragonal martensite (T) and (3) the five-layered martensite (5M). All Ni-Mn-Ga alloys exhibited cavitation behaviour characterized by a step-wise curve of mass loss versus test time. This behaviour is correlated to the microstructural nature of the alloys as well as the surface conditions of the pre-test samples. The type and concentration of the defects at the surfaces were critical to the cavitation resistance of the alloys. The best cavitation resistant alloy was of a cubic austenitic structure, followed with the alloy of a tetragonal T-martensite. The largest material loss was found in the alloy with a 5M martensite. All the studied Ni-Mn-Ga alloys had an excellent cavitation resistance compared to that of the reference stainless steel, and they even excelled some NiTi alloys found in literature. This may be due to the superelasticity of the cubic austenite and the twinning of the martensitic phases.     

Analysis of characteristics of molten pool using cast shadow during high-power disk laser welding

On-line monitoring and control of laser welding process play an important role in welding quality assessment. The morphology of molten pool during welding process influences welding quality. Volume, tilt, and height were characteristics of molten pool, but they cannot be measured directly during the welding process. An experiment of surfacing weld of Type 304 austenitic stainless steel with high-power disk laser welding was implemented. An active vision system with an auxiliary laser light source was designed to acquire the images of molten pool and its shadow during the welding process. The images were pre-processed to analyze the characteristics of the casting shadow of molten pool instead of analyzing the characteristics of the molten pool itself. The area, maximal distance between the shadow and keyhole, maximal width, and the tilt of the shadow were selected as the characteristics to analyze the relationship between the morphology of molten pool and welding quality by linear and 10th non-linear fitting. The results indicated that the welding quality could be dynamically detected by observing these characteristics. A different welding process was also conducted to confirm the proposed method. The research in this paper provides a method for on-line monitoring and control during high-power disk laser welding.     

Use of a multivariate optimization algorithm to develop a self-consistent numerical heat transfer model for laser spot welding

Laser spot welding as a joining method offers many outstanding advantages, such as localized heating and melting, high weld-strength-to-weld-size ratio, and minimal heat affected zone. These provide the benefits of low heat distortion, repeatability, ability to automate and high throughout that are always in demand in industry. An accurate knowledge of the temperature-time history of the weld pool is a prerequisite for reliable prediction of the weld dimensions, final microstructure and mechanical properties of the weld joint. Measurement of the weld thermal cycle in the laser weld pool is nearly impossible due to high peak temperature, rapid melting and solidification, and the complex flow of liquid metal within a small weld pool. Mathematical modeling of the laser spot welding process has emerged as a useful tool for the prediction of the temperature-time history and weld pool dimensions. However, the reliability of the predicted values of temperature history and weld dimensions significantly depends on the accuracy of the input parameters provided in such models. For example, the value of the absorption coefficient is a significant input parameter for modeling the laser spot welding process. However, the same is rarely available with adequate reliability and is also difficult to assign from scientific principles alone. This work presents a novel mathematical framework where the values of a set of uncertain input parameters for mathematical modeling are identified inherently by integrating a finite element based heat transfer simulation using adaptive volumetric heat source and a multivariate optimization algorithm.     

Laser-arc hybrid welding of wrought to selective laser molten stainless steel

Selective laser melting (SLM) is a successful tool-free powder additive technology. The success of this manufacturing process results from the possibility to create complex shape parts, with intrinsic engineered features and good mechanical properties. Joining SLM steel to similar or dissimilar steel can overcome some limitations of the product design like small dimension, undercut profile, and residual stress concentration. In this way, the range of applications of the SLM process can be broadened. In this paper, the hybrid laser welding of selective laser molten stainless steel was investigated. A high-power fiber laser was coupled to an electric arc and austenitic stainless steel wrought and SLM parts were welded together. The power and speed parameters were investigated. The joints were analyzed in terms of weld bead profile, microstructure, microhardness, and tensile test. The efficiency of the welding process was evaluated through the line energy input versus the weld molten area.     

Dissimilar metal laser spot joining of steel to aluminium in conduction mode

Dissimilar joining of thin (～1 mm) 6111-T4 aluminium alloy and DC04 uncoated low carbon steel used in automobile structures was carried out using laser spot joining in conduction mode. Two sets of experiments were carried out, using copper and aluminium backing bars, respectively. The welds were produced in overlap configuration with steel on the top. The steel surface was irradiated by the laser, and the heat was conducted through the steel into the aluminium. Temperature at the interface was controlled using the fundamental laser energy parameters so that aluminium melts and wets the steel surface. Reaction between the two metallic alloys resulted in the formation of intermetallic compounds (IMC). The formation pattern of IMC was dependent on the temperature profile and the distribution across the interface and was thicker in the centre of the weld and thinner near the edges. The stoichiometry of the IMC formed was varied across the layer and was principally composed of two different layers of Fe₂Al₅ and FeAl₃. Micro hardness tests were carried out to characterise the IMC layer. Mechanical shear tensile tests showed a maximum joint shear strength of up to 68 % of the shear strength of the aluminium alloy.     

钢-铝异种合金脉冲激光焊焊缝表征分析

采用Nd:YAG激光焊对304不锈钢和5052铝合金进行异种金属焊接,分别以峰值功率、焊接速度、离焦量和脉冲频率等工艺参数设计24组工艺试验,并对比分析未熔合、熔合和焊穿3种焊缝表面表征。运用激光点位移传感器测量焊缝高度,探索焊缝高度随激光功率等工艺参数的变化趋势,得出钢-铝焊缝3种表面形貌的工艺参数区间。分析结果表明,焊缝的表面形貌是由激光单点能量、离焦量和脉冲频率等因素共同决定,焊缝高度与峰值功率、焊接速度、离焦量和脉冲频率等工艺参数有一定的变化规律。     

Effects of the presence of probiotic bacteria in the aging medium on the surface roughness and chemical composition of two dental alloys

The aims of this study were: (a) to determine if the presence of probiotic bacteria in an aging medium, that is, artificial saliva in this study, has relevant effects on the surface roughness and the chemical composition of two main alloys used in dentistry (NiTi and stainless steel [SS]) and (b) in the case of NiTi, if these effects are influenced by the coating of the alloy (rhodium and titanium nitride). Atomic force microscopy and Raman spectroscopy were used to study the surface morphology and identify metal oxides formed on the surface of the alloys. Experiments demonstrated that the probiotic bacteria Lactobacillus reuteri can induce processes that alter some features of the surface such as roughness and chemical composition. The effect is dependent on the type of alloy and coating. The bacteria increased roughness in the case of uncoated NiTi more than saliva alone (pH = 4.8). Probiotic bacteria tend to decrease the corrosive influence of saliva on NiTi when the alloy is coated with rhodium or titanium nitride and this effect was also evidenced on SS. Raman spectroscopy confirmed that only SS samples are prone to oxidation processes, predominantly associated with exposure to saliva rather than probiotic bacteria.     

Friction Properties of Medical Metallic Alloys on Soft Tissue–Mimicking Poly(Vinyl Alcohol) Hydrogel Biomodel

In order to investigate the tribological behavior of medical devices in contact with tissue, friction tests for four kinds of medical metallic alloys (316L stainless steel, CoCr, NiTi and TiMoSn) on soft tissue–mimicking poly(vinyl alcohol) hydrogel (PVA-H) biomodel were carried out at low normal load. XPS analysis and wettability tests for them were prepared to understand the difference in friction. According to the surface oxide compositions, these alloys can be divided into two groups: “Fe/Cr-oxide-surface alloys” for 316L and CoCr, and “Ti-oxide-surface alloys” for NiTi and TiMoSn. From the wettability test, Fe/Cr-oxide-surface alloys show lower polar components of surface free energy than Ti-oxide-surface alloys. Fe/Cr-oxide-surface alloys show higher friction coefficients in the elastic friction domain than those of Ti-oxide-surface alloys, while there was no significant difference in the hydrodynamic lubrication. Since elastic friction is governed by the adsorption of hydrogel polymer on counterbody, the surface characteristic of alloys plays an important role in friction. A tentative explanation for this tendency is expressed by linking two different theories describing the adsorption force of hydrogel and wettability of countermaterial.