Fibre laser joining of highly dissimilar materials: Commercially pure Ti and PET hybrid joint for medical device applications

Laser transmission joining (LTJ) is growing in importance, and has the potential to become a niche technique for the fabrication of hybrid plastic-metal joints for medical device applications. The possibility of directly joining plastics to metals by LTJ has been demonstrated by a number of recent studies. However, a reliable and quantitative method for defining the contact area between the plastic and metal, facilitating calculation of the mechanical shear stress of the hybrid joints, is still lacking. A new method, based on image analysis using ImageJ, is proposed here to quantify the contact area at the joint interface. The effect of discolouration on the mechanical performance of the hybrid joints is also reported for the first time. Biocompatible polyethylene terephthalate (PET) and commercially pure titanium (Ti) were selected as materials for laser joining using a 200 W CW fibre laser system. The effect of laser power, scanning speed and stand-off distance between the nozzle tip and top surface of the plastic were studied and analysed by Taguchi L9 orthogonal array and ANOVA respectively. The surface morphology, structure and elemental composition on the PET and Ti surfaces after shearing/peeling apart were characterized by SEM, EDX, XRD and XPS.

Effects of plastic coating on K-alpha yield from ultra-short pulse

A study of the K-alpha radiation emitted from Ti foils irradiated with intense, similar to0.2 J, 67 fs, 800 nm laser pulses, scanning a range of intensities (similar to10(15)-10(18) W cm(-2)), is reported. The brightness of single-shot K-alpha line emission from the front of the targets is recorded. The yield from bare titanium (Ti) is compared to that from plastic (parylene-E) coated Ti. It is demonstrated that, for a defocused beam, a thin layer of plastic increases the yield.

3D FEM simulation of helical milling hole process for titanium alloy Ti-6Al-4V

As an emerging hole-machining methodology, helical milling process has become increasingly popular in aeromaterials manufacturing research, especially in areas of aircraft structural parts, dies, and molds manufacturing. Helical milling process is highly demanding due to its complex tool geometry and the progressive material failure on the workpiece. This paper outlines the development of a 3D finite element model for helical milling hole of titanium alloy Ti-6Al-4V using commercial FE code ABAQUS/Explicit. The proposed model simulates the helical milling hole process by taking into account the damage initiation and evolution in the workpiece material. A contact model at the interface between end-mill bit and workpiece has been established and the process parameters specified. Furthermore, a simulation procedure is proposed to simulate different cutting processes with the same failure parameters. With this finite element model, a series of FEAs for machined titanium alloy have been carried out and results compared with laboratory experimental data. The effects of machining parameters on helical milling have been elucidated, and the capability and advantage of FE simulation on helical milling process have been well presented.

Deformation of titanium alloy Ti-6Al-4V under dynamic compression

Deformation localisation is the main reason for material failure in cold forging of titanium alloys and is thus closely related to the production yield of cold forging. In the study of the influence of process parameters on dynamic compression, considering material constitutive behaviour, physical parameters and process parameters, a numerical dynamic compression model for titanium alloys has been constructed. By adjusting the process parameters, the severity of strain localisation and stress state in the localised zone can be controlled thus enhancing the compression performance of titanium alloys.

Effect of microstructure on the fatigue properties of Ti-6Al-4V titanium alloys

Through an analysis on microstructure and high cycle fatigue (HCF) properties of Ti-6Al-4V alloys which were selected from literature, the effects of microstructure types and microstructure parameters on HCF properties were investigated systematically. The results show that the HCF properties are strongly determined by microstructure types for Ti-6Al-4V. Generally the HCF strengths of different microstructures decrease in the order of bimodal, lamellar and equiaxed microstructure. Additionally, microstructure parameters such as the primary a (a) content and the a grain size in bimodal microstructures, the a lamellar width in lamellar microstructure and the a grain size in equiaxed microstructures, can influence the HCF properties. © 2012 Elsevier Ltd.

Performance and wear of coated carbide drill in machining of carbon fibre reinforced composite/titanium stack

In this research we investigate the performance of drilling process in carbon fibre reinforced composite (CFC) material, titanium alloy and the hybrid stack of these two materials, using coated carbide drill bit. We study the effect of the process parameters such as the feed rate and speed on the induced forces and torques, also on the wear of drill and surface roughness of the holes. In the composite material the percentage of surface damage in both drilling CFC on its own and drilling in stack form is estimated. Also, the effect of worn drill on the surface damage is identified. In the titanium, the burr formation in stack and non-stack form is investigated. The wear of the drill results in increased forces and torques required for drilling. This increases the surface delaminations substantially at the entrance in drilling of CFC. However, the surface roughness of the holes reduces with the wear of the drill in CFC drilling. Also, the surface delamination and surface roughness of the holes in the CFC whilst drilled in hybrid form reduces significantly. This is despite the increase of the forces and torques required in drilling CFC in stack form. Copyright © 2012 Inderscience Enterprises Ltd.

Comparative study of tool life and hole quality in drilling of CFRP/titanium stack using coated carbide drill

The use of hybrid materials including carbon fiber reinforced plastics (CFRPs) and lightweight metals such as titanium are increasing particularly in aerospace applications. Multi-material stacks require a number of holes for the assembly purposes. In this research, drilling trials have been carried out in CFRP, Ti-6Al-4V and CFRP/Ti-6Al-4V stack workpieces using AlTiN coated tungsten carbide drill bit. The effects of process parameters have been investigated. The thrust force, torque, burr formation, delamination, surface roughness and tool wear have been analyzed at various processing condition. The experimental results have shown that the thrust force, torque, burr formation and the average surface roughness increase with the increased feed rate and decrease with the increased cutting speed in drilling of Ti-6Al-4V. In drilling CFRP, delamination and the average surface roughness has similar tendency with the cutting parameters however thrust force and torque rises with the increased cutting speed. The results showed that after making 15 holes in CFRP/Ti-6Al-4V stack, measured thrust forces were increased by 20% in CFRP and by 45% in Ti-6Al-4V. Delamination was found to be much smaller in drilling of CFRP in stack from compared to drilling single CFRP. Tool life was significantly shortened in drilling of stack due to the combination of the wear mechanisms.

Effect of fretting on fatigue performance of Ti-1023 titanium alloy

The plain fatigue and fretting fatigue tests of Ti-1023 titanium alloy were performed using a high-frequency push-pull fatigue testing machine. Both σmax versus number of cycles to failure curves were obtained for comparative analysis of the fretting effect on fatigue performance of the titanium alloy. Meanwhile, by analyzing the fracture of plain fatigue and fretting fatigue, the fretting scar and the fretting debris observed by scanning electron microscopy (SEM), the mechanism of fretting fatigue failure of Ti-1023 titanium alloy is discussed. The fretting fatigue strength of Ti-1023 titanium alloy is 175 MPa under 10 MPa contact pressure, which is 21% of plain fatigue strength (836 MPa). Under fretting condition, the Ti-1023 titanium alloy fatigue fracture failure occurs in a shorter fatigue life. When it comes to σmax versus number of cycles to failure curves, data points in the range of 106–107 cycles under plain fatigue condition moved to the range of 105–106 under fretting fatigue condition. The integrity of the fatigue specimen surface was seriously damaged under the effect of fretting. With the alternating stress loaded on specimen, the stress concentrated on the surface of fretting area, which brought earlier the initiation and propagation of crack.

Microstructure and high cycle fatigue fracture surface of a Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy

Because of the requirements for the damage tolerance and fatigue life of commercial aircraft components, the high cycle fatigue (HCF) properties of Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy forgings are important. The effects of microstructure types of the α+β titanium alloy on fatigue properties need to be understood. In this paper, by analysing the fracture surfaces of the titanium alloy having four types of microstructure, the effects of microstructure are investigated. The differences of initiation areas and crack propagation among different microstructures were studied. It was found that the area of the initiation region decreases in the order of coarse basketweave, fine basketweave, Widmanstätten, and bimodal microstructure.

Hole-making Process and Its impacts on the Fatigue Response of Ti -6Al-4V Aircraft Alloy

In this work, the impact of conventional drilling and helical milling processes on the fatigue response Ti-6Al-4V (grade 5 titanium alloy) has been presented. Results show that the work pieces produced by helical milling has a 119% longer fatigue life compared with the drilled pieces under dry machining condition, and a 96% longer fatigue life for helical milled piece under lubricated condition. The use of cutting fluid has led to longer fatigue lives – 15% longer for drilling and 3% longer for helical milling. Other results such as the machined surface roughness, alloy surface and sub-surface microstructures have also been studied in details.

FEM Analysis to Optimally Design End Mill cutters for Milling of Ti-6Al-4V

This paper presents an FEM analysis conducted for optimally designing end mill cutters through verifying the cutting tool forces and stresses for milling Titanium alloy Ti-6Al-4 V. Initially, the theoretical tool forces are calculated by considering the cutting edge on a cutting tool as the curve of an intersection over a spherical/flat surface based on the model developed by Lee & Altinas [1]. Considering the lowest tool forces the cutting tool parameters are taken and optimal design of end mill is decided for different sizes. Then the 3D CAD models of the end mills are developed and used for Finite Element Method to verify the cutting forces for milling Ti-6Al-4 V. The cutting tool forces, stress, strain concentration (s), tool wear, and temperature of the cutting tool with the different geometric shapes are simulated considering Ti-6Al-4 V as work piece material. Finally, the simulated and theoretical values are compared and the optimal design of cutting tool for different sizes are validated. The present approach considers to improve the quality of machining surface and tool life with effects of the various parameters concerning the oblique cutting process namely axial, radial and tangential forces. Various simulated test cases are presented to highlight the approach on optimally designing end mill cutters.

Differential Scanning Calorimetry Study and Computer Modeling of b a Phase Transformation in Ti-6Al-4V Alloy

The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically, using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations for different systems based on the results from DSC study. In the present work, the kinetics of the transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling–transformation (CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the Johnson–Mehl–Avrami (JMA) theory and by applying the "concept of additivity." The JMA kinetic parameters were derived. Good agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters, the transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results from the point of view of activation energy for nucleation is also presented.