Surface integrity in high speed end milling of titanium alloy Ti–6Al–4V

Abstract

Machined titanium components, such as medical prosthesis, require the greatest reliability, which is determined by process induced surface integrity. However, surface integrity of milled titanium components easily deteriorates due to the poor machinability of titanium alloys and cyclic chip loading during milling. Milling induced surface integrity, including anisotropic surface roughness, residual stress, surface microstructure alterations and microhardness, has received little attention. In the present study, a series of end milling experiments were conducted to comprehensively characterise surface integrity at various milling conditions of titanium alloy Ti–6Al–4V with TiAlN coated carbide cutting tools. The experiments were carried out under dry cutting conditions. For a range of cutting speeds, feeds and depths of cut, analyses of machined surface roughness, residual stress, microhardness and the microstructural observations were carried out. The present work aims to evaluate the influence of different milling conditions on workpiece surface integrity.     

Experimental investigation on machinability of DMLS Ti6Al4V under dry drilling process

Additive Manufacturing (AM) technology is being extensively utilized in many industries concerning its superior advantage of easily fabricating the 3D parts possessing relatively high geometrical complicacy. Nevertheless, the AM parts generally need conventional machining, such as drilling, to meet the practical requirements, indicating disparate machinability compared with those manufactured by conventional wrought processes. Hence, to investigate the effects of machining parameters on the drilling characteristics of AM parts is of great significance. This work experimentally studied the machinability of the additive manufactured Ti6Al4V by covering a variety of topics including the cutting forces, temperatures, machined surface roughness, tool wear modes and chip morphology with selected cutting speeds and feed rates under dry drilling process. The material utilized in experiments is the Ti6Al4V manufactured by Direct Metal Laser Sintering (DMLS) technology. The experimental results highlight the parametric influence on the machinability of DMLS Ti6Al4V and present the possibility to validly redesign the machining parameters accordingly.     

Application of ultrasonic vibration assisted MQL in grinding of Ti–6Al–4V

This work presents experimental investigations performed to evaluate the improvement in grinding performance of Ti–6Al–4V alloy using ultrasonic vibration assisted minimum quantity lubrication (UMQL) technique. The grinding experiments have been performed using an indigenously designed and fabricated UMQL setup. In UMQL, the ultrasonic vibration of the horn has been used to atomise the cutting fluid into ultra-fine droplets of uniform size. Sunflower oil in 1, 5 and 10% of the volume have been added in water to prepare the biodegradable emulsions and used as grinding medium. The grinding performance during UMQL has been evaluated by comparing the grinding forces, surface roughness and the ground surface topography obtained during dry and conventional MQL (CMQL) techniques. Surface quality and chip morphology have been studied using microscopic imaging techniques. The UMQL grinding results in smaller grinding forces, and improved surface quality as compared to CMQL grinding. The experimental findings demonstrate that the UMQL has a strong potential to enhance the grindability of Ti–6Al–4V. The present work is also a step forward in finding a sustainable grinding technique for high strength materials using vegetable oil as a coolant.     

Ultrasonic Assisted Electrical Discharge Machining of Ti–6Al–4V Alloy

In this research, an attempt was made to investigate the influence of copper tool vibration with ultrasonic frequency on output parameters in the electrical discharge machining of Ti–6Al–4V. The selected input parameters for the experiment comprise of ultrasonic vibrations of tool, current and pulse duration and the outputs are tool wear ratio (TWR), material removal rate (MRR), and stability of machining process and surface integrity of a workpiece, including surface roughness, thickness of recast layer, and formation of micro cracks. Scanning electron microscope and X-Ray diffraction were employed to examine the surface integrity of the workpiece. The results revealed that tool vibration with ultrasonic frequency enhances MRR via increasing normal discharges and decreasing arc discharges and open circuit pulses. Also, by using ultrasonic vibrations in finishing regimes, the density of cracks and TWR decrease while in roughing regimes, the thickness of recast layer, density of cracks, and TWR increase.     

Influence of wire-EDM textured conventional tungsten carbide inserts in machining of aerospace materials (Ti–6Al–4V alloy)

Importance of this present investigation is to identify the influence of modified tool (tool with texturing) on the process of orthogonal turning of Ti–6Al–4V work material. To achieve the enhanced turning conditions, four different types of textures (plain conventional, cross, perpendicularly textured and parallel textured tool to the chip flow direction) were fabricated on the rake face of the tool insert and the lubricant used during the machining process is molybdenum disulfide (MoS₂). Machining forces (the force of cutting and feed), angle of shear, chip morphology, temperature distribution between tool and chip were measured. Shear strain and strain rate were also computed and compared with all type of cutting tools. Experimental results revealed that the cross-textured cutting tool exhibit an effective reduction in cutting force, friction, shear strain and strain rate. The favorable metal removal condition of curling chip with low diameter was achieved through cross-textured tool.     

Simplified MQL system for drilling AISI 304 SS using cryogenically treated drills

In machining operations, cutting fluids have been comprehensively used to improve the cutting tool life, but the issues related to manufacturing cost, environment and health call for reducing their use by possible methods. Minimum quantity lubrication (MQL) is a technique that overcomes these problems by spraying a small amount of cutting fluid (<100?ml/hr) as mist using compressed air. In this work, the basic MQL technique is used to achieve flow rates slightly higher (～880?ml/hr) than MQL using simple techniques like paint sprayer and compressor, which is more generally called reduced quantity lubrication (RQL). Another method to increase the tool life is by cryogenic treatment, which increases the hardness of the tool. Tungsten carbide drill bits were subjected to cryogenic treatment (?185 °C). Drilling studies were carried out on AISI 304 stainless steel (SS) using untreated and cryo-treated WC drill bits under RQL and conventional wet lubrication conditions. The tool wear on the treated WC drill bits with RQL was comparatively less than on the untreated ones with RQL and wet lubrication. These improvements were established through microhardness, SEM images, XRD, wear studies and surface roughness measurements comparisons.     

Experimental investigations on cryogenic cooling by liquid nitrogen in the end milling of hardened steel

Milling of hardened steel generates excessive heat during the chip formation process, which increases the temperature of cutting tool and accelerates tool wear. Application of conventional cutting fluid in milling process may not effectively control the heat generation also it has inherent health and environmental problems. To minimize health hazard and environmental problems caused by using conventional cutting fluid, a cryogenic cooling set up is developed to cool tool–chip interface using liquid nitrogen (LN₂). This paper presents results on the effect of LN₂ as a coolant on machinability of hardened AISI H13 tool steel for varying cutting speed in the range of 75–125 m/min during end milling with PVD TiAlN coated carbide inserts at a constant feed rate. The results show that machining with LN₂ lowers cutting temperature, tool flank wear, surface roughness and cutting forces as compared with dry and wet machining. With LN₂ cooling, it has been found that the cutting temperature was reduced by 57–60% and 37–42%; the tool flank wear was reduced by 29–34% and 10–12%; the surface roughness was decreased by 33–40% and 25–29% compared to dry and wet machining. The cutting forces also decreased moderately compared to dry and wet machining. This can be attributed to the fact that LN₂ machining provides better cooling and lubrication through substantial reduction in the cutting zone temperature.     

不同冷却润滑方式对切削SiCP/Al复合材料刀具磨损的影响

为探究不同冷却润滑方式对切削SiC_P/Al复合材料刀具磨损的影响,进行了干切削（Dry）、微量润滑（MQL）、液氮（LN₂）、切削油（Oil）和乳化液（Emulsion）共五种冷却润滑条件下的车削实验,分析了冷却润滑方式对刀具边界磨损、刀具破损和后刀面磨损的影响。结果表明:MQL和LN₂有更佳的流体冲刷效果,可以将脱落的SiC颗粒及时带离切削区,减少边界磨损; Oil和Emulsion冲刷效果较差,会加剧边界磨损。LN₂的使用会增加刀具受到的热应力和机械冲击,积屑瘤发生完全脱落,造成切削过程不平稳,当切削距离达到1 100 m时,刀具发生破损; Oil切削时,严重的边界磨损导致刀尖部位尺寸减小,强度降低,当切削距离达到825 m时发生了刀具破损。MQL良好的润滑渗透性和LN₂有效的冷却效果可以减少后刀面磨损。因此,MQL兼具冷却、润滑和流体冲刷效果,更加适合作为切削SiC_P/Al复合材料的冷却润滑方式。      

The effect of cutting speed and heat treatment on the fatigue life of Grade 5 and Grade 23 Ti–6Al–4V alloys

High speed machining is a necessary manufacturing method for ensuring productivity and profitability. However, research has demonstrated that the high speed machining process impairs the surface characteristics of materials such as Ti–6Al–4V including surface roughness and subsurface microstructural damage. Therefore, there is concern that high speed machining detrimentally influences the fatigue properties of Ti–6Al–4V components. This paper investigates the effect of cutting speed on the surface integrity and fatigue properties of Ti–6Al–4V (ASTM Grade 5) and Ti–6Al–4V ELI (ASTM Grade 23) alloys in the beta annealed and mill annealed heat treated conditions. It was found that the surface roughness and fatigue properties are not significantly influenced by cutting speed, however, the microstructure substantially influences the properties.     

Effect of ceramic conversion surface treatment on fatigue properties of Ti6Al4V alloy

Samples of Ti6Al4V alloy were ceramic conversion (CC) treated. The fatigue properties of untreated and CC treated samples were evaluated with a conventional Wöhler cantilever rotating-bending fatigue machine under both plain fatigue and fretting fatigue conditions. Experiment results showed that CC treatment had a detrimental effect on plain fatigue properties and reduced the plain fatigue limit of the investigated Ti6Al4V alloy by about 24%. However, the fatigue property loss could be fully recovered by an additional shot peening after CC treatment. More importantly, shot peening after CC treatment resulted in considerably longer fatigue life over untreated and CC treated materials in the finite-life region. In contrast to its detrimental effect on plain fatigue, CC treatment appeared to be beneficial to fretting fatigue property. It increased fretting fatigue strength (FFS at 10⁷ cycles) of the Ti6Al4V alloy by approximately 10%. Shot peening after CC treatment further enhanced the fretting fatigue properties, with an improvement in FFS (10⁷ cycles) of about 20% over the untreated material under the present treatment and testing conditions.     

Residual stress formation after re‐contouring of micro‐plasma welded Ti−6Al−4 V parts by means of ball end milling

During the regeneration of damaged components such as compressor blades, the weld repair is followed by machining processes. Excess weld material is removed in a cutting process in order to reproduce the final contour. Therefore, both processe have to be considered regarding the resulting surface and functional properties of the repaired component. In this study, bead on plate welds are produced on 10 mm Ti‐6Al‐4 V sheets using micro‐plasma welding with different levels of welding current. They are subsequently re‐contoured via ball end milling using tools of different cutting edge radii. The residual stress depth profile is measured using X‐ray diffractometry. It is shown that the final residual stress depth profile is mainly influenced by the milling process when machining with rounded cutting edges.     

Experimental Study of Cutting Force,Microhardness, Surface Roughness,and Burr Size on Micromilling of Ti6Al4V in Minimum Quantity Lubrication

In the present study, the effects of various cutting conditions on the surface integrity of titanium parts (Ti6Al4V) have been investigated during the micromilling process. In addition, to have a better understanding of the results, the cutting force was measured. The experiments were performed in the Minimum Quantity Lubrication condition using the tungsten carbide microtool with 0.5 mm in diameter. Micromilling parameters including feed rate, spindle speed and axial depth of cut were considered as process inputs, each in three levels, and their effects on the surface roughness, burr width, surface and in-depth microhardness as well as mean cutting force were evaluated. In the range of experimental parameters and according to the results, cutting speed and feed per tooth had the highest impact on the surface integrity characteristics of this alloy, respectively. While most research works concentrated on the feed per tooth as the main parameter in the micromilling process, the result of the study showed that the variation of cutting speed as one of the influential factors could also be used in order to decrease cutting forces and to improve surface quality.     

Cryogenic Drilling of Ti–6Al–4V Alloy Under Liquid Nitrogen Cooling

This article is focused on experimental study of the effects of cryogenic liquid nitrogen (LN₂) coolant during drilling of Ti–6Al–4V alloy material with three different levels of cutting speed (V_c) and feed rate (f) at a constant depth. Cutting temperature (T), thrust force (F_z), torque (M_z), surface roughness (R_a), and hole quality are the output responses investigated by using cryogenic LN₂ coolant compared with a wet coolant. Tool wear and chip morphology were examined with the changes in cryogenic LN₂ coolant. It is found that cryogenic LN₂ coolant results in lowering cutting zone temperature which helps more removal of heat from the cutting zone. Lower thrust forces and surface roughness were observed due to less friction and better chip breaking in cryogenic LN₂ condition. Also better chipping results in improvement in hole quality, viz., circularity and cylindricity in cryogenic LN₂ condition. Less serration and uniform segmentation results in better chip morphology and no damage to the cutting inserts resulted in improved tool life in cryogenic LN₂ condition. The main application of cryogenic LN₂ coolant in the cutting zone provides better lubrication and is more effective than wet coolant. The effects of this investigation show that cryogenic LN₂ coolant is an alternative approach for a wet coolant in the drilling process.     

Tool wear in cryogenic turning of Ti-6Al-4V alloy

Though titanium alloys are being increasingly sought in a wide variety of engineering and biomedical applications, their manufacturability, especially machining and grinding imposes lot of constraints. Rapid tool wear encountered in machining of titanium alloys is a challenge that needs to be overcome. Cryogenic machining with liquid nitrogen as coolant is being investigated by researchers to reduce the cutting zone temperatures and enhance the tool life. The effects of cryogenic cooling have been studied on growth and nature tool wear in the present investigation while turning Ti-6Al-4V alloy bars with microcrystalline uncoated carbide inserts under dry, wet and cryogenic cooling environments in the cutting velocity range of 70-100 m/min. Cryogenic cooling by liquid nitrogen jets enabled substantial improvement in tool life through reduction in adhesion-dissolution-diffusion tool wear through control of machining temperature desirably at the cutting zone.     

Experimental study on mechanical drilling of carbon/epoxy composite-Ti6Al4V stacks

This paper concerns the experimental studies on the drilling process of multilayer carbon/epoxy composite-Ti6Al4V stacks and their individual material layers using tungsten carbide drills. The significance of the current work aims to reveal the impact of tool wear on the drilling process for CFRP/Ti6Al4V, CFRP and Ti6Al4V. Four groups of machining tests including drilling individual CFRP, individual Ti6Al4V, multilayer CFRP/Ti6Al4V stacks with and without a cooling treatment were conducted. Drilling forces, cutting temperatures and hole quality attributes were experimentally investigated in terms of the drill wear extents. The mechanism controlling the tool wear effect on the drilling machinability of CFRP/Ti6Al4V was revealed, providing several implications for the industrial manufacturers. The results highlight the significant impact of the titanium chip ejection on the composite surface quality and the necessity of a rigorous tool wear control to guarantee the damage-free drilling of CFRP/Ti6Al4V stacks.     

Development of a novel cooling system-assisted minimum quantity lubrication method for improvement of milling performance

This paper presents a novel lubrication method for milling processes that employs cooling system-assisted minimum quantity lubrication (CSMQL) using a thermoelectric cooling system. The CSMQL method improves the cooling effect in the cutting area and enhances processing quality, in addition to reducing energy consumption. Four different coolant strategies including CSMQL, dry, minimum quantity lubrication (MQL), and wet methods were compared in processing mill die steel (SKD11), which is widely used in industry. Different aspects of the milling performance (e.g. surface roughness, morphology, milling temperature, and milling forces) were investigated using these coolant strategies. The experimental results show that not only is the surface roughness of steel milled using CSMQL better than that of steel milled using dry and MQL methods, but CSMQL also produces fewer tool marks on the workpiece surface. In addition, it was found from observations of chip color that using the CSMQL method reduced the cutting temperature by 27% and the cutting force by 22%, compared with dry machining. In summary, the use of CSMQL can not only improve the surface roughness and reduce the cutting force and cutting temperature, but also promote processing quality. This study will help researchers develop more efficient cooling strategies in the future.     

Experimental investigation of tool wear and chip formation in cryogenic machining of titanium alloys

Titanium alloys are one of the most important design materials for the aircraft industry.The high strength-to-density-ratio and the compatibility with carbon fibre reinforced plastic are the reasons for a raising application in this field.The outstanding properties lead to challenging machining processes.High strength and low heat conductivity affect high mechanical and thermal loads for the cutting edge.Thus,the machining process is characterized by a rapid development of tool wear even at low cutting parameter.To reach a sufficient productivity it is necessary to dissipate the resulting heat from the cutting edge by a coolant.Therefore the cryogenic machining of two different titanium alloys is investigated in this work.The results point out the different behavior of the machining processes under cryogenic conditions because of the reduced thermal load for the cutting tool.According to this investigation,the cryogenic cooling with CO_2enables an increase of the tool life in comparison to emulsion based cooling principles when machining theα+β-titanium alloy Ti-6Al-4V.The machining process of the high strength titanium alloy Ti-6Al-2Sn-4Zr-6Mo requires an additional lubrication realized by a minimum quantity lubrication(MQL) with oil.This combined cooling leads to a smoother chip underside and to slender shear bands between the different chip segments.     

Effect of surface roughness on fatigue performance of additive manufactured Ti–6Al–4V

Additive manufacturing is increasingly considered for production of high quality, metallic, aerospace parts. Despite the high potential of this manufacturing process to reduce weight and lead time, the fundamental understanding of additive manufactured Ti–6Al–4V material is still at an early stage, especially in the area of fatigue and damage tolerance. This paper covers the effects of inherent surface roughness on the fatigue life. In the as built condition, metallic parts have a poor surface texture, which is generally removed in fatigue critical areas. It is shown that the fatigue properties of Ti–6Al–4V samples, produced by direct metal laser sintering and electron beam melting, are dominated by surface roughness effects. A simple model based on an equivalent initial flaw size is formulated.     

Effects of intense cooling on microstructure and properties of friction-stir-welded Ti–6Al–4V alloy

Friction stir welding (FSW) was used to join Ti–6Al–4V alloy in air and under intense cooling conditions. The results show that the application of liquid nitrogen is beneficial in decreasing the peak temperature and in reducing the extent of the high-temperature region during welding, leading to a smaller stir zone (SZ). Intense cooling can lead to refined and homogeneous grains in the SZ, resulting in increased microhardness. The FSW joint produced with intense cooling had a tensile strength of 1020?MPa, which is nearly equivalent to that of the base material and is up to 2.6% higher than for the air-cooled joint. The fractographs for both types of joint were characterised by dimples, indicating that the fractures were ductile.     

Plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy

Abstract

The titanium affinity for oxygen is one of the main factors that limit the application of its alloys as structural materials at high temperatures. The objective of this work was to estimate the influence of the plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy, focusing on the determination of the experimental parameters related to the creep stages. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was air plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on the Ti–6Al–4V alloy in air for coated and uncoated samples and in a nitrogen atmosphere for uncoated samples at 600°C to evaluate the oxidation protection on creep of the Ti–6Al–4V alloy. The steady-state creep rate of the coated alloy is smaller than that of the uncoated alloy in air and nitrogen atmosphere. Results about the activation energies and the stress exponent values indicate that the primary and stationary creep, for all test conditions, was probably controlled by dislocation climb. The plasma-sprayed coatings increased the time to rupture and the strain at rupture is smaller than for uncoated samples tested in air.