Difficult Machining Research Articles

Numerical, Mechanical, and Metallurgical Investigation of an Innovative Near Net Shape Titanium Selective Laser Melting Engine Component and Comparison with the Conventional Forged One

Selective laser melting (SLM) can be used to manufacture near net shape (NNS); the main benefits are a remarkable weight reduction, a lower environmental impact, the opportunity to integrate some functions, and an improvement of the performances. The current article covers the development of a Ti6Al4V NNS engine component produced by SLM. Finite element analyses of the main relevant operating conditions are performed to reach a topological optimization of the part. The main target is weight reduction keeping the same safety performances. The weight reduction achieved is 45% and 15% with respect to steel and titanium forging by replacing the original “H” section with an SLM multibranch structure. Other benefits are the manufacturing of the connecting rod (conrod) into two separate parts, avoiding the difficult machining to separate the cap from the main body and the integration of conformed cooling channels into the conrod. Then, the SLM components are produced and mechanical and metallurgical properties are investigated and compared with the titanium hot forging ones. Both the macrostructures present equiassic and isotropic behavior due to the heat treatment transformation. No defects are observed for both the technologies. The mechanical properties are verified to be aligned with the design targets.

Effects of laser polishing on surface morphology and mechanical properties of additive manufactured TiAl components

Abstract The laser deposition manufacturing (LDM) is a free-form metal deposition process, which allows generating a prototype or small series of near net-shape structures. Despite numerous advantages, one of the most critical issues of the technique is that produced pieces have a deleterious surface quality which requires post machining. Traditional machining methods such as milling and grinding have been used to improve the surface quality of the LDM components. However, these methods have some drawbacks such as tool wearing and narrow-area difficult machining. In this paper, the CW laser and the pulsed laser was applied to polish the surface of LDM TiAl alloy samples. The difference between these two polishing processes on the surface morphology, microstructures, corrosion resistance, micro-hardness and wear resistance were investigated. Results revealed that the surface roughness of CW laser polished TiAl can be reduced to about 1.76 μm, which is comparable to a precise machining surface. It was also found that a hardened layer about 600 μm was produced on the TiAl alloy surface after laser polishing, which can improve the mechanical properties of the component. The micro-hardness of the laser polished sample was improved about 8% compared with the initial TiAl alloy, the wear resistance and corrosion resistance were also improved compared with the as-received TiAl surface.

Ultrasonic assisted turning of AISI 52100 steel using nanoparticle-MQL method

High cutting force and high cutting tool-chip interface temperature occur during machining of high strength materials like AISI 52100 by using conventional turning method and this makes to the difficult machining operation. Besides, in conventional turning (CT) of these materials, which has high abrasion resistance and hardness properties, an additional process such as grinding is required to achieve the desired surface quality. Ultrasonic assisted turning (UAT) method has been widely used in the machining industry in recent years, as it exhibits better surface quality, lower cutting forces, and cutting temperature than the CT method. This paper focused on experimentally and statistically determining the effects of different cutting parameters and cooling conditions (dry and minimum quantity lubrication-MQL) on surface roughness, workpiece surface texture, tangential direction cutting force, and cutting tool-workpiece interface temperature in the machining of AISI 52100 material by using UAT and CT methods. The experimental results revealed that significant improvements determined in the cutting forces and the surface roughness occurred in UAT compared to CT in all the cooling conditions. However, the increasing cutting speeds caused to observe higher interface temperature values in dry turning by the UAT method. Variance Analysis (ANOVA) was carried out to determine the significance grade of independent variables on dependent variables. According to ANOVA results, while the cutting speed is the most important factor on the surface roughness and the cutting zone temperature, and the cooling method is the most important factor on the cutting force. The effects of turning methods on corresponding surface topography, including surface defects, were evaluated. It is determined that in the surface topography analysis, feed marks are the main texture of the workpiece in the CT method while micro-level indentations and protrusions occurred on the workpiece surface in the UAT method.

An investigation report on sustainable strength of amalgamation materials

Modern manufacturing mainly focused on the development of various amalgamation (composites) materials having superior qualities and good properties in comparison with metals and especially the aluminium matrix composites have a wide range of acceptance in all engineering industries and bio medical applications. In these materials having low density with high sustainable strength of the components and also with good machinability and surface roughness. There is a need to increase the mass production while reducing the cost of the components or product. The aluminium composite alloys have high strength, excellent corrosion, and high wear resistance. But presently, some of the amalgamation material complexity in the fabrication process of new material and its arrives difficult machining. Some of these manufacturing difficulties are high tool wear and dynamic forces when machining which consequently limiting its applications. Also from literature it has been understood that by increasing the wt % of fly ash with AL6061 particles in the aluminium matrix, improves the mechanical properties brefily reviewed.

Difficult Cutting Property of NiTi Alloy and Its Mechanism

This paper describes the difficult machinability of nickel titanium alloy (NiTi alloy) and its mechanism. As a result of examining the difficult cutting machinability via a turning experiment, NiTi alloy cutting showed larger cutting force, higher cutting temperature, and severe tool wear with plastic deformation of the tool compared to Ti-6Al-4V. In addition, the discharged chips were tangled with the jaw chuck and the cutting tool. As a result of investigating the cause of these difficult machining properties by orthogonal cutting, it was found that the progression of severe flank wear is affected by the elastic recovery due to the super elasticity of the material. The verification of the results according to the shear plane theory suggest that the large deformation resistance of the material is the cause of the increase in cutting temperature. Furthermore, because the cutting temperature exceeds the shape memory transformation temperature, the generated chips are shape memory processed. It was also found that because the generated chips are super elastic, chips are not easily broken and they are lengthened, and are easily entangled with a cutting tool and a jaw chuck.

An investigation on material removal mechanism in ultra-high-speed grinding of nickel-based superalloy: three-dimensional simulation and experimental verification

Ultra-high-speed grinding is a significant method for the advanced manufacturing of the difficult-to-machine material components, e.g., nickel-based superalloy. However, unclear mechanism of material removal behavior is still the main problem. The present work intends to establish the three-dimensional (3D) finite element simulation model of ultra-high-speed grinding with single abrasive grain. The effects of undeformed chip thickness and grinding speed on the morphology of grinding traces and chips are investigated. The parameters, such as pile-up ratio and shear slip frequency, are characterized experimentally, and the mechanism of material removal behavior in the process of ultra-high-speed grinding has been accordingly researched deeply. The results indicate that the pile-up ratio decreases firstly and then increases with an increase of undeformed chip thickness; the lowest pile-up ratio would be obtained when the undeformed chip thickness is 0.8~1 μm. When the grinding speed is below 240 m/s, with the increase of grinding speed, the distance between the sawtooth of the grinding chips is reduced, while the sawtooth degree is increased. When the grinding speed exceeds 400 m/s, the chip shape changes from serrated fracture to banded continuity, and the frequency of adiabatic shear slip goes up with the increase of grinding speed, which helps to reduce the grinding force, making the grinding process more stable and decreasing the difficult machinability of Inconel 718 nickel-based superalloy.

Near-net shaped Al/SiCP composites via vacuum-pressure infiltration combined with gelcasting

Abstract To solve the problem of difficult machining, the near-net shaped Al/SiCP composites with high volume fraction of SiC particles were fabricated by vacuum-pressure infiltration. The SiCP preform with a complex shape was prepared by gelcasting. Pure Al, Al4Mg, and Al4Mg2Si were used as the matrices, respectively. The results indicate that the optimal parameters of SiCP suspension in gelcasting process are pH value of 10, TMAH content of 0.5 wt.%, and solid loading of 52 vol.%. The Al matrix alloyed with Mg contributes to improving the interfacial wettability of the matrix and SiC particles, which increases the relative density of the composite. The Al matrix alloyed with Si is beneficial to inhibiting the formation of the detrimental Al4C3 phases. The Al4Mg2Si/SiCP composite exhibits high relative density of 99.2%, good thermal conductivity of 150 W·m−1·K−1, low coefficient of thermal expansion of 10.1×10−6 K−1, and excellent bending strength of 489 MPa.

Neutron Shielding Performance of 3D-Printed Boron Carbide PEEK Composites.

Polyethylene is used as a traditional shielding material in the nuclear industry, but still suffers from low softening point, poor mechanical properties, and difficult machining. In this study, novel boron carbide polyether-ether-ketone (PEEK) composites with different mass ratios were prepared and tested as fast neutron absorbers. Next, shielding test pieces with low porosity were rapidly manufactured through the fused deposition modeling (FDM)-3D printing optimization process. The respective heat resistances, mechanical properties, and neutron shielding characteristics of as-obtained PEEK and boron carbide PEEK composites with different thicknesses were then evaluated. At load of 0.45 MPa, the heat deformation temperature of boron carbide PEEK increased with the boron carbide content. The heat deformation temperature of 30% wt. boron carbide PEEK was recorded as 308.4 °C. After heat treatment, both tensile strength and flexural strength of PEEK and PEEK composites rose by 40%–50% and 65%–78%, respectively. Moreover, the as-prepared composites showed excellent fast neutron shielding performances. For shielding test pieces with thicknesses between 40 mm and 100 mm, the neutron shielding rates exhibited exponential variation as a function of boron carbide content. The addition of 5%–15% boron carbide significantly changed the curvature of the shielding rate curve, suggesting an optimal amount of boron carbide. Meanwhile, the integrated shielding/structure may effectively shield neutron radiation, thereby ensuring optimal shielding performances. In sum, further optimization of the proposed process could achieve lightweight materials with less consumables and small volume.

Research on different cooling methods in the machining of CGI and GCI

Gray cast iron (GCI) is the most common material used in diesel engine blocks. Compacted graphite iron (CGI) is a new material for high-power diesel engine blocks, which can allow an increase in the cylinder-pressures and achieve better fuel economy and a higher power output. However, the difficult machinability of CGI seriously hinders its large scale production. At high-speed machining, the tool life of machining GCI is 10–20 times that of GCI. To improve the tool life of machining CGI, this work focuses on the influence of uncooled, cutting fluid cooled and liquid nitrogen (LN2) cooled methods in the machining of CGI and GCI. The results indicate that LN2 cooled method can effectively reduce the highest temperature and temperature diffusion in the processing. Furthermore, for machining CGI, LN2 cooled method increases the tool life by 192.6% and 66.7% compared with uncooled and cutting fluid cooled methods, respectively. However, for machining GCI, LN2 cooled method plays a negative role. For CGI and GCI, the wear modes of the tools are similar, which are mainly material adhesion, coating wear and adhesion and built-up edge.

FEM analysis of the distortion of thin-walled sealing part affected by the machining-induced residual stress

Thin-walled sealing parts are typical difficult machining parts due to their lower stiffness and higher machining accuracy requirements. Among all the influential factors on the distortion of thin-walled parts, machining-induced residual stress (MIRS) plays a key role. In this paper, the effects of the MIRS changed with the number of cuts and the allocation of cutting depth on the distortion of the sealing part are investigated. Firstly, the MIRS is obtained by a finite element cutting simulation model of the AISI304L stainless steel material. Then, the obtained MIRS is applied to the workpiece, and the distortion of the workpiece is simulated after releasing the clamping constraints. The simulation results show that the multi-cutting processes and the reasonable allocation of cutting depth have a remarkable influence on the control of the workpiece distortion.

Probabilistic seismic fragility and loss analysis of concrete bridge piers with superelastic shape memory alloy-steel coupled reinforcing bars

Concrete bridge piers with conventional steel reinforcing bars are vulnerable to strong earthquakes by inducing significant residual deformations, which substantially weakens the seismic resilience of bridges. Superelastic shape memory alloy (SMA) bars showing superior self-centering capacities are desirable substitutes to steel reinforcements to minimize the seismically-induced residual deformations of piers. Nevertheless, high cost, difficult machining, and lack of sufficient energy dissipation are the primary restraining factors to a wide implementation of SMA reinforcements. This study proposes a novel SMA-steel coupled reinforcement for concrete bridge piers, which is intended to achieve the balance between self-centering and energy dissipation capacities. Probabilistic seismic fragility analyses are conducted on the prototype bridge with either pure steel, SMA-steel coupled, or pure SMA reinforcements to evaluate their probability of damage at different limit states. Seismic loss analyses are further performed to compare the relative cost-effectiveness of different patterns of reinforcements. The results indicate that an optimal amount ratio between SMA and steel bars can be found for the coupled reinforcements, which shows lower vulnerabilities and higher resilience under earthquakes than the other reinforcement patterns. The direct repair loss and the indirect downtime loss after earthquakes are considerably reduced when the SMA reinforcing bars are introduced. The coupled reinforcement with the optimal SMA-steel amount ratio shows the most effectiveness in mitigating the long-term economic impacts induced by seismic hazards within the lifetime of the bridge.

System design and experimental research on ultrasonic assisted elliptical vibration grinding of Nano-ZrO2 ceramics

Aiming at the low toughness, high hardness and difficult machining of hard and brittle materials such as Nano-ZrO2 ceramics, a new type of ultrasonic assisted elliptical vibration grinding (UAEVG) system was designed to improve its machinability and grinding quality. The overall frequency equation of the ultrasonic grinding cup-shaped tool transformer (UGCTT) was derived by the integrated nonresonant design method and the length of the conical horn was optimized. The trajectory of abrasive particles was simulated by finite element software and verified by ultrasonic resonance test. Finally, through the experiment of UAEVG Nano-ZrO2 ceramics, the effects of UAEVG method on its surface morphology, roughness and residual stress were explored. The results showed that when the length of the horn was 65 mm after optimization, the transformer had good vibration mode. Simulation and ultrasonic resonance test results showed that the abrasive particles periodically moved in an elliptical shape in the longitudinal radial plane, and the radial and longitudinal amplitude were AR = 7.5 μm and AL = 3.1 μm. The grinding experiments of Nano-ZrO2 ceramics showed that UAEVG could change the grinding mechanism of conventional grinding (CG), reduce the generation of grinding debris and material fracture. When the grinding depth was increased, the plastic processing ratio of the material under UAEVG was significantly larger than that of CG, which could form a better surface topography. The surface roughness increased with the increase of the grinding depth and feed rate, and the surface compressive stress decreased with the increase of grinding depth. UAEVG could reduce the surface roughness by about 30% and increase the residual compressive stress by about 20% compared with CG, which could achieve better grinding surface quality. Therefore, it can be concluded that UAEVG could expand the plastic processing domain, improve the surface integrity and fatigue strength of Nano-ZrO2 ceramics to some extent.

Study on ASJ Cutting of TC18, Based upon Multivariate Nonlinear Regression and SA-BP-AGA

TC18 titanium alloy has been widely applied, but is considered as a difficult machining material. Taking the kerf angle as the quality criterion, this paper studied the cutting performance of TC18 by the use of an abrasive slurry jet (ASJ), based upon multivariate nonlinear regression and SA-BP-AGA. Cutting experiments were carried out according to the Taguchi orthogonal method. The experimental factors included traverse speed, standoff distance, pressure and slurry concentration, with five levels set, respectively. Meanwhile, a characterization method of the major influencing factors was proposed. A multiple nonlinear regression model and a back propagation artificial neural network (BP) prediction model, based on adaptive genetic algorithm (AGA), were established. The reliability was verified by statistics equations for the 22 groups of the fitting or training model and the three groups of experimental results. The BP-AGA and Simulated annealing algorithm (SA) were used to form a set of prediction optimization systems, called integrated SA-BP-AGA. Finally, the results showed that the main factor influencing the kerf angle is the slurry concentration. BP-AGA is easier to model, offers better robustness and is more accurate than a multivariate nonlinear regression model. The best kerf angle can be predicted by the integration system. The study results can improve the performance for the machining of TC18 by ASJ.

Research on High Efficiency and Low Destructive Manufacture Process of Complex Structure Honeycomb Core

With the development of modern aircraft, complex structure honeycomb core parts are widely used. As a kind of difficult machining material, it’s easy to destroy the surface quality of the parts during machining the honeycomb core parts because of the special structure. The article studies the technology to increase efficiency and reduce damage during the machining process of complex structure honeycomb core. The problem can be effectively solved by using proper clampings, proper cutting parameters and proper tool path interval.

Laser-multi-pass-narrow-gap-welding of nickel superalloy—Alloy 617OCC

Stringent requirements on creep strength and corrosion/oxidation resistance mean that nickel-based superalloys will be the material of choice for high-temperature and high-pressure (HT/HP) applications in the future. Components for these HT/HP processes in turbines, housings, or piping are often characterized by their large dimensions and wall thicknesses of ≥20 mm. Because of the high material costs and difficult machinability of Ni-based components in comparison to steel components, an efficient and reliable welding technology that produces crack-free weld seams is a necessary precondition for economic manufacturing. Conventional arc welding technologies require low welding speeds, large quantities of filler material, and large input energy per unit length, which can result in damages to microstructure and consequently diminished HT properties. Laser-multi-pass-narrow-gap-welding (Laser-MPNG) is the solution to this problem. This paper presents the technological approach of Laser-MPNG, which uses a brilliant fiber laser with a maximum laser power of 4 kW and remoweld®MPNG, a welding head that was specifically developed to join the nickel superalloy Alloy 617OCC as a promising candidate for future industrial welding applications in pipe elements of up to 72.5 mm wall thickness in modern power stations.

Wear mechanisms of cutting tools in high-speed turning of Ti6Al4V alloy

Titanium and its alloys are associated with low density and excellent strength/weight ratio, biocompatibility in surgical implants, high mechanical strength at high temperatures and excellent corrosion resistance. The Ti6Al4V alloy is applied in the aerospace, biomedical, chemical, naval and petrochemical industries. However, these alloys present problems during machining and are therefore considered materials of difficult machinability at high cutting speeds. This paper aims to analyse the wear mechanisms in cutting tools using cemented carbide (H13A) and synthetic polycrystalline diamond (PCD) in the high-speed turning of Ti6Al4V alloy, under the following cooling conditions: dry, jet and minimum quantity lubricant (MQL). To measure wear, a microscope was used to measure flank wear in the tools. Next, the images of these tools were obtained using scanning electron microscopy for the identification of the wear mechanisms. According to the results, the variation in the cutting parameters (cutting speed and feed rate), type of tool material, as well as the cooling conditions significantly affects the behaviour of the wear mechanisms. The wear mechanisms presented in the cutting tools are not the result of one single mechanism, but a combination of many mechanisms. Overall, the primary type of wear is flank wear and the mechanisms are abrasion, adhesion and attrition (adherence and drag).

Investigation of the Ni-Co-Cr alloy microstructure for the manufacturing of combustion chamber GTE by selective laser melting

Selective laser melting implementation by gas turbine engine combustor chambers manufacturing difficult machining material Ni-Co-Cr alloy is an actual task to directly form complicated shapes. The paper shows the study of Ni-Co-Cr powder such as chemical, morphology, grain size, flow rate, and pour density analysis. The study results demonstrate the structure of Ni-Co-Cr alloy manufactured by selective laser melting in vertical and horizontal directions in original condition and after hot isostatic pressing. It is established that hot isostatic pressing offers the possibility to reduce the porosity and improve the mechanical properties. The influence of the hot isostatic pressing process on the density and microstructure of Ni-Co-Cr alloy is investigated by means of X-ray phase analysis and scanning electron microscopy. The results of the experiments show that the majority of the pore can be densified by means of hot isostatic pressing. The study presents the SLM technology implementation by producing the small size gas turbine engine combustor chamber.

Surface microstructure of titanium alloy thin-walled parts at ultrasonic vibration-assisted milling

Milling of the titanium alloy thin-walled workpiece is a critical challenging task and the machining vibration is a major effect on the accuracy of the final part due to difficult machining properties and low stiffness. The common milling (CM) is very hard for titanium alloy thin-walled parts to get high-quality machined surface microstructure, so finding an effective way of processing to obtain better surface microstructure is particularly important. In this article, the longitudinal-torsional composite ultrasonic vibration-assisted milling (LTCUVM) is proposed to overcome the problems of the common milling. The cutting method of the high-frequency vibration separation of a workpiece and the tool reduces an average cutting force. Thereby suppressing the tool relieving and further providing a good machined surface. In this study, firstly, the kinematic model of LTCUVM was established and the variations of the movement of the cutting edge flank with the spindle speed change were analyzed. Then the finite element model (FEM) of the two processing methods was developed and significant differences between the CM and LTCUVM in the micro-milling were revealed. Further, related experiments were designed and conducted. Finally, the experimental analysis showed that the LTCUVM effectively reduced an average cutting force, and further reduced surface roughness and height of chatter marks compared to the CM with the same processing parameters.

Wear analysis of PVD-coated twist drills under MQL

PurposeDeep drilling of hardened steels is a difficult machining operation because of the high wear level of tools. This paper aims to present the main wear mechanisms observed in physical vapor deposition (PVD)-coated twist drills during deep drilling of SAE4144M steel under minimum quantity lubrication, assessed in the production of injection holders.Design/methodology/approachTwo PVD coatings were tested: TiAlN and AlCrN, industrially processed, the last one being a multilayer coating. The workpiece was heat treated for a hardness of 39 HRC to be applied in a diesel engine component. The tests were performed in an industrial environment for a fixed number of holes. Two levels of cutting speed and feed rate were selected for the experiments. In addition, minimum quantity of lubrication (MQL) was compared with conventional lubrication. Scanning electron microscope was used to reveal the wear mechanisms.FindingsSpalling of PVD-coating was revealed for conventional lubrication, while adhesion was observed in MQL conditions. The use of multilayered AlCrN-based coating promoted a significant reduction of adhered material on the twist drill, which is the reason for this selection in industrial operation.Practical implicationsResults showed that the MQL regime can be applied for this industrial application.Originality/valueA detailed description of wear mechanisms, which allows a suitable selection of coating and machining variables was found for a very difficult operation, using a more economic process in terms of lubrication.

A review on laser beam welding of titanium alloys

In recent years, there is an increased in used of titanium alloys for some parts of mass-produced automobiles and aerospace. However, titanium alloys are characterized by difficult machinability, high melting temperature, high strength, low thermal conductivity, and high reactivity to oxygen, which overshadowed conventional manufacturing processes. To this end, there is a pressing need for more efficient technologies for the manufacture of low-cost titanium structures. Over the years, several joining techniques have been considered for fabrication of titanium alloys. Nevertheless, laser beam welding presents a viable option for welding of titanium due its versatility, high specific heat input, and flexibility. To date, under optimum processing conditions, the strength of the laser-welded titanium alloys can be close to the original material; however, there are still some processing problems such as lower elongation and corrosion resistance coupled with inferior fatigue properties. In this document, the laser beam welding of similar and dissimilar titanium alloys is reviewed, focusing on the influence of the processing parameters, microstructure-property relationship, metallurgical defects, and possible remedies.