Evolution of microstructure and phase composition of joints with ultrasonic-assisted resistance brazing

ABSTRACT

Zn–2Al alloy is used as a filler metal for brazing of aluminium 6063 alloys by ultrasonic-assisted resistance brazing (RB) technology. The evolution of the microstructure and phase composition under the effects of the electric field and ultrasonic vibration were studied. The results show that ultrasonic cavitation can remove the existing oxide films effectively and inhibit element segregation, which help obtain joints with good metallurgical bonding. In comparison to RB joints, which exhibit a coarse dendritic structure, a uniform dendritic structure was obtained in the ultrasonic-assisted resistance joints. Furthermore, the combined electric and ultrasonic field has a significant effect on grain size and on dissolution during brazing. With the application of ultrasonic vibration, the diffusion between the base metal and the Zn–2Al filler metal intensifies.     

Initial bond formation in thermosonic gold ball bonding on aluminium metallization pads

The morphological features of lift-off footprints on the aluminium metallization pads were investigated to gain an understanding of the effects of bonding parameters on formation of initial bonds during thermosonic gold ball bonding. The obtained results showed that metallurgical bonding initiated at the peripheral areas of the contact area situated along the direction of ultrasonic vibration. Those areas extended inwards with an increase in ultrasonic power. Both the external bonded area and central non-bonded area increased with increasing bonding force. Based on the evolution of footprints, the bonding models were proposed, and the effects of the bonding parameters on the formation of initial bonds were discussed.     

Temperature gradient transient liquid phase diffusion bonding: a new method for joining advanced materials

Abstract

A novel method for transient liquid phase (TLP) diffusion bonding of aluminium based materials has been developed which is capable of reliably providing excellent bonds with parent material shear strengths. This new method relies on imposing a temperature gradient across the bond line during TLP diffusion bonding and this leads to the formation of sinusoidal or cellular interfaces. Consequently, bond strengths are increased, possibly as a result of the higher metal–metal contact along the non-planar interfaces compared with the planar interfaces associated with either conventional TLP or solid state diffusion bonding processes. This paper describes the results achieved, implementing the new method and using copper interlayers, when joining Al–Mg–Si, Al–Li alloys, and an aluminium metal matrix composite with SiC particles. Patent protection has been filed in the UK under UK Patent No. 9709167.2.     

Capillary filling process during ultrasonically brazing of aluminium matrix composites

Abstract

Alumina borate whisker reinforced aluminium metal matrix composites were ultrasonically brazed in air. The liquid filler metal (FM) of a Zn–Al eutectic alloy could get into the clearance between composite couples by the action of ultrasonic capillarity. The velocity of the FM getting into the clearance decreases with increasing clearance width. The time exhausted to remove completely the oxide film on the surface increases with increasing clearance width. The oxide film could be removed completely and the metallurgical bonding formed under the action of ultrasonic vibration for sufficient time. The shear strength of the bond could reach up to 145 MPa (the nominal strength of the Zn–Al filler alloy).     

Perspectives offered by combining a laser beam with arc welding procedures

Summary

With regard to the fact that, during Cu wire stitch bonding of copper lead frames, the wire deformation strongly affects the bondability and strength of the bonds obtained, this paper describes an investigation of the wire deformation being varied through variation in the initial capillary tip load and capillary tip load during application of ultrasonic vibration as well as the bondability and bond strength then obtained. When stitch bonding with soft copper wire is performed by the two‐step load control method, which has a higher initial capillary tip load and a lower capillary tip load during the second stage, the range of action of the material being worked under ultrasonic vibration can be widened, the deformation during the working time of ultrasonic vibration can be suppressed, the ultrasonic energy can make a greater contribution to bonding, and a higher bond strength can be obtained. The results obtained in this study suggest that, during wire bonding by the two‐step load control method, the optimum capillary tip load is obtained. Within the presently adopted experimental range, the optimum values are a capillary tip load of 1.4 N before the application of ultrasonic vibration and a capillary tip load of 0.4 N during ultrasonic vibration.     

Pressureless diffusion bonding of ferritic stainless steel foil controlled by metal evaporation

Abstract

Bond formation of thin ferritic stainless steel foils containing 20Cr–5Al (wt-%) was observed under near vacuum pressure of ～10^-3 Pa. Debonded (peeled off) surfaces exhibited a white scaly morphology after bonding above 1400 K. No bond formation was observed below 1400 K, so the critical temperature for bond formation must exist around 1400 K. The critical temperature also depends on the surface roughness of the foil (the thickness of the gap between foils), which can be explained in terms of aluminium evaporation behaviour between foils. A theoretical model is proposed to estimate the effect of aluminium evaporation on the critical temperature. It is also suggested that aluminium evaporation between foils can protect the bonding surface from the residual oxygen gas.     

引线键合中多向超声波能量传输特性研究

引线键合主要是采用超声波热键合技术，而超声波振动传播和能量传递是超声波键合设备中一个非常关键和复杂的问题。利用波在弹性体中传播的机理，给出超声波聚能杆和劈刀的振动传递特性，利用多普勒振动计PSV-300-F(1．5MHz)测量了振动位移和速度，证实了理论分析结果，并对双向垂直传输超声波能焊具的性能进行了研究，阐明垂直双向加载是一种高效的传能模型。     

Accelerated method for testing soldering tendency of core pins

Abstract

An accelerated method for testing die soldering has been proposed and tested. High intensity ultrasonic vibration has been applied through a core pin to molten aluminium in order to simulate service conditions under die casting. Such conditions include high pressure and high impingement speed of molten metal on the pin. Soldering tendency of H-13 steel pins with or without commercial coatings was tested using this accelerated method. The experimental results indicate that soldering occurs within a few minutes of testing using this new method, much faster than that using the conventional methods. The coating failure mechanism identified in this new method is identical to that observed in the conventional methods, suggesting that the new method is suitable for testing soldering tendency of core pins under die casting conditions.     

键合时间对粗铝丝超声引线键合强度的影响

试验研究了不同超声功率条件下,键合时间对粗铝丝引线键合强度的影响规律.试验中记录了每种键合试验的键合时间,采集了每一个键合点的剪切测试力作为键合点抗剪强度的表征,记录了每个键合点的状态.结果表明:(1)在小超声功率条件下,键合强度对键合时间敏感;在大超声功率条件下,敏感性下降;(2)短键合时间条件下主要键合失败形式为剥离和无粘接,表明键合界面的原子扩散不够;(3)大超声功率长键合时间条件下的键合失败形式多为根切,表明键合界面的原子扩散虽然足够,但长时间的超声振动也会使粗铝丝产生疲劳断裂,形成过键合.     

Fabrication of high‐strength steel fibre reinforced metal matrix composites by explosive bonding and their tensile properties

Summary

Steel fibre reinforced metal matrix composites (FRM) consisting of high‐strength ductile steel wire and aluminium or titanium foil were fabricated by explosive bonding. The strength properties were evaluated by tensile tests. The optimum explosive loading (explosive mass/driver plate mass) increases with the volume fraction of fibre. For the aluminium matrix composites, the micrographs obtained show sound bonding between the adjoining foils and non‐bonding between the foil and steel wire. The tensile strength of the aluminium matrix composites is 80% of the value predicted by the rule of mixtures. For the titanium matrix composites, the micrographs obtained show both bonded and unbonded regions between the adjoining foils or between the foil and steel wire. The tensile strength of the titanium matrix composites agrees with the value predicted by the rule of mixtures.     

超声键合装置中劈刀稳态振动的试验建模

利用PSV-400-M2(1.5 MHz)高频型扫描式激光多普勒测振仪, 对超声键合装置中受正弦激励、超声加载作用下的键合工具(劈刀),在不同松紧度下的稳态振动进行了连续扫描测试.结果表明,当劈刀连接的松紧度发生改变时,其稳态振动表现出不确定性;同一松紧度下振动形态中出现单节点,但不同松紧度下节点出现的位置不同;劈刀末端的速度振幅并不是人们期望的那样是最大.此外,基于弹性梁振动的基本理论对劈刀的稳态振动进行了探讨,得到其振动的近似模型.     

热超声键合压电换能器的动力学特性

基于有限元方法和试验测试,研究了芯片封装用压电超声换能器的动力学特忡。借助ANSYS压电耦合和非线性接触分析功能,对换能器自由和约束状态下的振动特性进行了分析。探讨了超声能量在空间域、时域和频域的传递规律。由模态分析得到换能器的振动形式,通过谐响应分析提取其在正弦电压激励下的振动信息,经瞬态分析获得换能器的瞬态响应。结果表明,螺栓径向尺寸和预紧力影响换能器的模态分布和动态特性,压电晶堆加载电压的频率影响超声能量传递特性。通过键合试验考察了焊点质量与螺栓径向尺寸的关系。分析和试验结果为换能器设计和键合工艺优化提供了指导。     

超声键合换能系统的非稳态特性试验

测得超声键合振动系统的输入电流、电压,用激光多普勒测速仪测得换能杆系统的响应速度,研究了超声引线键合工艺中超声振动系统的非稳态动力学特性.据此对超声加卸载过程分段,进行频谱分析和非稳态特性建模,并研究了相关的试验方法.结果表明,振动系统拟共振态腹点的速度均方值可作为系统机械能的等效量,供系统动力学特性试验研究;快速键合时系统惯性使功率源的开关不等同于键合机超声振动系统的启停.这些均有助于键合机换能系统的工艺建模和优化.     

Ultrasonic evaluation of TiAl and 40Cr diffusion bonding quality based on time-scale characteristics extraction

To solve the problem of ultrasonic pulse-echo method in the evaluation of kissing bond and unbond in TiAl and ⁴⁰Cr diffusion bonding, a characteristics extraction algorithm was proposed. The algorithm was based on continuous wavelet transform to convert ultrasonic TiAl and ⁴⁰Cr diffusion bonding interface signals into time-scale domain. The ultrasonic tests were performed by an ultrasonic C-scan imaging system using a 10 MHz focused transducer. The time-scale amplitude and phase of the interface signals were calculated and analyzed to distinguish the kissing bond and the unbond from the perfectly bonded interface. The kissing bond can be detected by the scale-dependent amplitude combined with phase variation and the unbond can be measured by the opposite phase. The amplitude and phase characteristics were extracted to reconstruct the amplitude and phase characteristics images for TiAl and ⁴⁰Cr diffusion bonding specimens evaluation. The amplitude and phase characteristics images are effective in the evaluation of bonding quality.     

Bonding mechanism of ultrasonic wedge bonding of copper wire on Au/Ni/Cu substrate

The ultrasonic wedge bonding with d 25 μm copper wire was achieved on Au/Ni plated Cu substrate at ambient temperature. Ultrasonic wedge bonding mechanism was investigated by using SEM/EDX, pull test, shear test and microhardness test. The results show that the thinning of the Au layer occurs directly below the center of the bonding tool with the bonding power increasing. The interdiffusion between copper wire and Au metallization during the wedge bonding is assumed negligible, and the wedge bonding is achieved by wear action induced by ultrasonic vibration. The ultrasonic power contributes to enhance the deformation of copper wire due to ultrasonic softening effect which is then followed by the strain hardening of the copper wedge bonding.     

Constitutive equation with residual hardening effect for modeling the ultrasonic vibration enhanced friction stir welding process

ABSTRACT

A modified acoustic-plastic constitutive model characterising both acoustic softening and residual hardening effects on aluminium alloys is developed and applied into a computational fluid dynamic model for the ultrasonic vibration enhanced friction stir welding (UVeFSW) process. The flow stress and material flow field in UVeFSW are quantitatively analysed. The effect of acoustic residual hardening on the streamlines density at rear advancing side is examined by comparing with that based on the model considering just acoustic softening. The numerical simulation results are experimentally validated, and it is found that the modified constitutive equation improves the prediction accuracy of the flow stress and material flow field in UVeFSW.     

Modelling thermal cycles and intermetallic growth during friction melt bonding of ULC steel to aluminium alloy 2024-T3

Abstract

Dissimilar materials, aluminium 2024-T3 and ultralow carbon steel, have been welded by a novel process called friction melt bonding. A finite element thermal model is developed to predict temperature cycles and to estimate the fusion pool geometry and the intermetallic bonding layer thickness. The total mechanical power input in pseudo-steady state is inferred from in situ measurements at the tool torque and rotational speed. Temperature dependent properties, including the latent heat of fusion, and proper contact conditions between the welded plates and the backing plate are included. Predicted temperatures are in agreement with the measurements at various distances from the weld centreline. Molten pool geometries and intermetallic thicknesses, whose control is crucial to insure good weld mechanical performances, are also in accordance with the experimental observations.     

Transient liquid phase diffusion bonding under a temperature gradient: modelling of the interface morphology

Imposing a temperature gradient when transient liquid phase (TLP) diffusion bonding results in reliable joints with shear strengths as high as those of the parent material (e.g. aluminium-based alloys and composites). The key feature of this new method relies on the formation of non-planar interfaces as a consequence of morphological instability at the solid/liquid interface during solidification of the liquid layer (normally with a eutectic composition) at the bonding temperature. A model is proposed in this paper which is capable of predicting interface morphology and the bonding conditions which result in the formation of non-planar bond lines when using this new bonding method. The results of model provide an insight to the nature of the solidification process. The proposed model was verified experimentally when bonding pure aluminium and also an aluminium alloy (6082) using copper interlayers.     

Cratering on thermosonic copper wire ball bonding

Copper wire bonding offers several mechanical and electrical advantages as well as cost saving compared to its gold wire predecessor. Despite these benefits, silicon cratering, which completes the fracture and removal of bond pad underlayers, has been a major hurdle to overcome in copper wire bonding. Copper wire is harder than gold, and thus needs greater ultrasonic power and bond force to bond it onto metal pads such as aluminum. This paper reports a study on the influence of wire materials, bond pad hardness, and bonding-machine parameters (i.e., ultrasonic power and bond force) on silicon cratering phenomenon. Ultrasonic power and z-axis bond force were identified as the most critical bonding machine parameters in silicon cratering defects. A combination of greater bond force and lower ultrasonic power avoids silicon cratering and gives the desired effects. Results also show that a harder bond pad provides relatively good protection from silicon cratering.     

Ultrasonic evaluation of the bonding strength of dissimilar metal bonds

Diffusion bonding of steel plates to titanium plates was carried out, and shear testing and ultrasonic measurement of bonds were performed to obtain the relationships between bonding strength, state of bonding interface and ultrasonic parameters. Fourier spectra of the ultrasonic wave reflected from the bonding interface were dependent on the state of the bonding interface; when an interlayer did not melt the spectrum showed a simple profile with one peak, and when the interlayer melted, the spectrum showed a profile with large irregularity. Transfer functions obtained from the spectrum were analysed with the subspace method (a kind of principal component analysis) and were represented by two major components. The bonding strength was quantitatively evaluated by two major components and a diameter estimated from the ultrasonic measurement.