Machinability of Additive Manufacturing Titanium Alloys: An Overview (1) _ Parts _ Tools _ Materials

Machinability of Additive Manufacturing Titanium Alloys: An Overview (1) _ Parts _ Tools _ Materials

Original Title: Machinability of Additive Manufacturing Titanium Alloys: An Overview (1) Jiangsu Laser Alliance Chen Changjun Guide: Based on the study of cutting force, surface finish and tool wear, this paper gives a comprehensive review of the cutting performance of titanium alloys prepared by various AM techniques. This paper is the first part. Summary Titanium (Ti) alloys are widely used in many industries due to their excellent physical and mechanical property. However, these characteristics can result in higher cutting forces and temperatures during machining, thereby reducing the machinability of titanium alloys. In recent years, additive manufacturing (AM) techniques have been used to fabricate titanium parts with complex contours. These AMed parts, although close to net shape, require finishing operations due to poor surface integrity. In this paper, the cutting performance of titanium alloys prepared by various AM techniques is comprehensively reviewed according to the research on cutting force, surface finish and tool wear. In addition, the effects of cooling/lubrication methods and material properties on AMed parts are analyzed. From this review, it was found that the improved mechanical property of AMed titanium resulted in greater cutting forces and higher temperatures, which significantly affected tool wear and surface quality after finishing operations. Nonetheless, the literature on significantly improving the machinability of AMed titanium components is very limited, which needs to be emphasized in future studies. 1 Introduction Titanium (Ti) alloy has been widely used in industry because of its excellent physical and chemical properties. They have relatively low density and excellent mechanical property, including high yield strength and modulus of elasticity, and these properties can be maintained in high temperature service environments. These characteristics apply to most aerospace components, such as landing gear assemblies and engine turbine blades. In addition, it has been reported that titanium alloys with dominant β phases are ideal for biomedical applications such as orthopedic bone implants, because the β-stabilizing elements should improve the biomechanical compatibility of titanium alloys. While titanium alloys have the most popular properties, they are also classified as difficult-to-machine materials. The high strength and low thermal conductivity of titanium alloys result in relatively large cutting force and high cutting temperature during machining. Some profiles of titanium components such as turbine blades and bone implants are very complex, which makes it time-consuming to manufacture parts with a good surface finish. In addition, a typical titanium part manufacturing process involves extensive machining of the original workpiece material, which results in significant material waste and high energy and time consumption, making titanium part production an expensive and unsustainable process. Microstructures at different cooling rates (a) 150 ° C/s; (B) 50 ° C/s; (C) 15 ° C/sec; (d) 5 ° C per second; (e) 1.5 ° C per second; (f) 0.5 ° C per seconds; (G) (H) 0.15 ° C. There is no doubt that metallographic examination is one of the more accurate and effective methods to study the microstructure evolution of titanium alloys. Upper panels a – G show a series of optical micrographs of the cooling rate from 150 ° C/s to 0.15 ° C/s. These micrographs show how the microstructure changes with decreasing cooling rate. Differences between these microstructures include variations in the magnitude of αp, the width of αs, and the thickness of the boundary α layers, as well as whether the microstructure has a colonial structure (a cluster of parallel α plates belonging to a single variant). Expand the full text Over the past two decades, additive manufacturing (AM) processes have been increasingly applied to the production of metal parts with complex geometric profiles. In this process, components are manufactured by melting and reconsolidating the raw material layer by layer, usually according to the digitized design of the part. Compared with traditional manufacturing processes such as casting, machining, forging, and powder metallurgy, AM can produce near-net-shape parts without roughing, thereby avoiding the waste of workpiece materials and saving the consumption of machining resources such as coolant, tools, and machining fixtures. However, most parts produced by the AM process cannot be used directly. This is because the surface integrity of additive manufacturing (AMed) parts is unpredictable. Specifically, the roughness,Titanium 6Al4V wire, hardness, and residual stress of the printed part surface are much higher than those of the forged material.
Therefore, finishing of AMed parts is one of the critical post-processing steps that needs to be performed to obtain a smooth surface finish for part assembly. A lot of work has been done on the workability of different wrought titanium alloys. Most studies have focused on cutting forces, tool design, coolant, tool wear, and other key factors. The cutting force is one of the key factors, which is greater due to the high yield strength of titanium alloys. This leads to higher cutting temperatures and severe wear at the tool/chip interface, which limits the efficiency of machining titanium alloys. Excessive tool wear can affect the surface integrity of the workpiece, including roughness and residual stress after machining. Therefore, different cooling and lubrication methods, such as high pressure coolant, low temperature coolant and minimum quantity lubrication (MQL), are used to reduce cutting forces and reduce tool/chip and tool/workpiece wear. In order to improve the machining efficiency of titanium alloys, laser assisted machining can also be used to soften the surface of the workpiece material and significantly reduce the machining stress in the material removal process. Meanwhile,titanium sheet grade 5, if the laser power is not properly selected, tool wear is still inevitable and the surface quality is affected. The reason for the decrease in cutting temperature with increasing feed rate in the LAM process may be that heat diffusion is more efficient in thicker chips (where more material is present and acts like a heat sink) produced at high feed rates than in smaller chips produced at low feed rates. In this case, the temperature decrease due to the effective heat conduction between the chips generated at the high feed rate overcomes the temperature increase due to the increased material removal rate. At low feed rates, the unprocessed material is reheated in the next cutting operation because the laser spot size (2 mm) is at least 10 – 15 times larger than the feed, thus building on the previous heat content, showing a higher cutting temperature as shown in panel (a) compared to the thinner heating zone at high feed shown in panel (B) below. (A) IR thermograph of feed 0.054 mm/rev showing thicker heating zone than (B) IR thermograph of feed 0.28 mm/rev. Although much research has been done on the processing of wrought titanium alloys, little is known about the machinability of AMed titanium alloys. It is well known that the mechanical property of AMed titanium alloys are different from those of wrought titanium alloys; the hardness, yield strength and ultimate tensile strength of AMed Ti6Al4V are significantly higher than those of wrought Ti6Al4V. These enhanced mechanical property may influence the development of cutting forces, temperatures, and tool wear when machining AMed titanium alloys. Secondly, the microstructure of AMed titanium alloys is significantly different from that of wrought titanium alloys due to the rapid melting and solidification phenomena that occur in most AM processes, which may largely affect the surface integrity of the machined workpiece. Third, different AM processes require different levels of machining operations. In addition, the use of preheating techniques such as plasma or laser-assisted machining is highly undesirable when machining AMed titanium alloys because external heat sources can have a detrimental effect on part performance. Therefore, it is necessary to study the machinability of AMed titanium alloy to ensure the quality of the machined surface. At present, there are many review papers on the mechanical property and microstructural properties of titanium alloys prepared by different methods. However, there is little information on the machinability of AMed titanium parts. It is important to understand how the different inherent properties of different AM methods affect the machinability of AMed titanium alloys. In this paper, the AM process and its workability for titanium alloys are reviewed. In the next section, the principles of mainstream AM technologies are introduced, including electron beam melting (EBM), selective laser melting (SLM), wire arc additive manufacturing (WAAM), laser additive deposition (LAD) and cold spray additive manufacturing (CSAM). The mechanical property and surface properties of AMed titanium alloys are also presented in this section. In the subsequent sections, the latest knowledge on the machinability of AMed titanium alloys is presented in detail. Finally, the main conclusions drawn from this review are summarized through technical discussions and future research avenues.
2. Additive manufacturing of titanium alloy The additive manufacturing (AM) process is based on the use of CAD design files that can be easily converted into adaptable print files, allowing step-by-step, slice-by-slice, ti6al4v ,titanium round bar, and layer-by-layer controlled printing of parts to net or near-net shapes. This freely designable mode of manufacturing components on demand allows components to be produced without the use of multiple manufacturing machines such as welding, grinding, slatting, and milling. Other cost cutting factors include expensive skilled machine technicians, a variety of cutting and machining tools, and resource-intensive fixtures that require precise alignment. Some commonly used metal additive manufacturing (MAM) processes include powder bed fusion (PBF), powder feed fusion (PFF), wire arc additive manufacturing (WAAM), and cold spray additive manufacturing (CSAM). The PBF process includes direct metal laser sintering (DMLS), selective laser melting (SLM1), and electron beam melting (EBM) AM techniques, while the PFF process includes laser additive deposition (LAD2) and thermal spraying. In addition, the MAM process can be broadly divided into three application sectors, namely: near-net-shape fabrication, metal repair and maintenance, and coating, as shown in Figure 1. Near-net-shape manufacturing can be defined as batch 3D printing of parts that are relatively close to the desired size. Metal repair involves replacing or refurbishing areas of metal components that have experienced wear and deterioration in performance during use, where several millimeters of worn areas are repaired. Coatings are commonly used to enhance the performance of a part through surface strengthening, where material from microns to a few millimeters in thickness is deposited on the substrate/part surface. All MAM processes produce different part characteristics due to process differences. This will directly affect the post-processing required for different applications, in particular machining. Therefore, it is important to understand not only the MAM process but also its application areas to develop appropriate processing strategies. Figure 1 classifies MAM processes according to application areas. 2.1 Selective Laser Melting (SLM) This AM-technology uses a layer-by-layer approach to fabricate dense 3D components with enhanced mechanical property. It is considered to be one of the most widely used MAM technologies, offering high resolution and high dimensional accuracy. The key process parameters that affect the characteristics of SLM printed parts are laser power, scan speed, pattern fill spacing, layer thickness, part orientation, scan strategy, and post-manufacturing heat treatment. Failure to optimize these parameters can result in blowholes and other harmful defects in the print. The properties of titanium alloys depend on a set of correct process parameters. Unintentional changes in process parameters can result in microstructural changes that directly affect the mechanical property of the part. For commercially pure (CP) titanium, Attar et al. reported a large variation in mechanical property due to microstructural inhomogeneities and porosity inside the printed part caused by unoptimized process parameters. Wysocki et al. reported that the UTS of SLM-printed titanium parts was three times higher than that of cast parts; however, the ductility was significantly reduced. Likewise, Attar et al. also reported a significant improvement in the intensity of oxygen-doped SLM-printed CP Ti. Three scanning strategies are studied, namely, scan "O", scan "X", and scan "H". For scan "O", the laser beam scans the alternating layers at angles of 45 ° and -45 ° to the y-axis, respectively. The surface properties of SLM printed parts depend on the process parameters as well as surface defects such as pores. This can be attributed to rapid solidification and segregation phenomena. These surface characteristics greatly affect the workability after molding. 2.2 Electron Beam Melting (EBM) The EBM process uses a similar technique to the SLM, but uses an electron beam as the heat source. It also operates in a vacuum environment, generating complex shapes with few machining steps. In contrast to SLMs operating in an inert gas environment, the vacuum environment prevents electron beam deflection due to the presence of gas molecules. Cross section of SLM Ti – 6Al – 4V sample (120 W and 360 mm/s). Due to the high preheating temperature, the components produced by EBM are composed of ultrafine lamellar eutectic structure. The quality of EBM Ti6Al4V components is reported to be almost equivalent to that of cast components after forging and machining.
EBM components do require close control of build parameters to reduce porosity and undesirable microstructural changes. Unoptimized process parameters may result in a change in the mechanical property of the titanium alloy. The beam and scanning speed are considered to be the most critical control parameters, directly affecting defects at the microstructural level and thus the mechanical property of the printed part. The study of the porous matrix Ti6Al4V structure of the EBM assembly shows that the increase of density will improve the strength and Young's modulus. 2.3 Wire Arc Additive Manufacturing (WAAM) WAAM is known for its fast build speed and fast time to market. The WAAM system is mounted on a base plate with a programmable robotic arm to produce a fully functional part with a near-net shape. It has been widely used to produce prototypes and large components with acceptable mechanical property and structural strength. EBSD analysis of heat-treated Ti – 6Al – 4V (A – D) and Ti – 6Al – 4V – 0.13B (E – G) samples: (a) Macroscopic inverse pole figure of Ti – 6A1 – 4V sample (step size = 0.7 μm). (B) High magnification inverse pole figure of the highlighted region in (A) (step size = 0.2 μm). (C) The extreme graphs of { 10 – 12 } and 〈 10-1-1 〉. (D) Line trace of the area indicated by the arrow in (B), showing the misorientation angle of the twin. (E) Macroscopic inverse pole figure of Ti – 6Al – 4V – 0.13B sample (step = 0.5 μm). (F) High magnification inverse pole figure of the highlighted area in (E) (step size = 0.1 μm). (G) The extreme graphs of { 10 – 12 } and 〈 10-1-1 〉. (H) Line trace of the area indicated by the arrow in (F), showing the misorientation angle of the twin. (I) Orientation of the EBSD sample with respect to the vertical build direction (VD) and the substrate (BP). Note that the compression axis lies in VD. The inverse pole plots (upper panels A and B) reveal a strong (0 0 0 1) substrate texture of the α phase, which facilitates large-scale twinning transport across multiple α phase laths. The material deposition rate is typically around 50 – 130 G/min or 4 kg/H. WAAM uses a plasma arc to melt the wire into a molten pool, which is also a challenge for this AM method because large molten pools can cause surface tension effects that can lead to undesirable defects in printed parts. Other limitations of WAAM include the difficulty of fabricating radii, sharp corners, and curvatures along the geometry, which makes the dimensional accuracy very poor. Heat input, if not properly controlled, can produce large stresses on manufactured components. This AM process is disfavored because of its high accuracy, significantly higher induced residual stresses in the printed parts, poor surface finish, variations along the deposition direction, and voids formed along the inter-rail gap. Schematic drawing of tensile and fatigue specimens extracted from (a) Build 1 and (B) Build 2. In labeling the specimen, the first class indicates the direction: V indicates that the specimen is machined parallel to the build direction, and H indicates perpendicular. The second category indicates the type of test: T for tension and F for fatigue. M indicates that both the specimen used for the microstructure study and the blank specimen were not used in this study It is observed that the titanium alloy prepared using WAAM exhibits an anisotropic microstructure, typically presented by large columnar β grains. The ductility and strength of the WAAM Ti6Al4V tensile specimens are less variable than those of the extruded Ti6Al4V, and the ductility observed by Wang et al. indicates that the ductility in the horizontal direction decreases with increasing strength. The tensile strength and fatigue life of WAAM components are reduced when defects are present. It has been reported that the surface roughness of parts manufactured using WAAM is much higher compared to SLM. Again, the WAAM sample shows less elongation and tensile strength compared to the SLM sample, as shown in Figure 2. Therefore, it is essential to understand the influence of WAAM process parameters on the surface characteristics, which will directly affect the machinability of printed parts. Figure 2 Tensile strength, yield strength and elongation compared to WAAM and SLM samples. 2.4 Laser additive deposition (LAD) The LAD process utilizes laser beam assisted fusion of the metal powder material.
The process is similar to welding, so it is valuable to apply resurfacing or deposition to the repair of existing parts. Dinda et al. reported that the tensile strength and yield strength of LAD printed Ti6Al4V (1163 MPa and 1105 MPa, respectively) are reasonably higher than the ASTM standard for Ti6Al4V implants, however, the ductility is very low. 2.5 Cold Spray Additive Manufacturing (CSAM) Also known as cold gas dynamic spraying, CSAM is a solid-state process that does not involve the melting of metal powders. The process involves a gas pre-chamber followed by converging and diverging accelerating nozzles. The powder is introduced through a carrier gas and a propellant gas is introduced into the antechamber. The carrier gas is maintained at a higher pressure than the propellant gas, thereby propelling the powder at cryogenic temperatures to impact the target substrate at extremely high velocities. Nitrogen (N2) and helium (He) inert gases are used to achieve these critical supersonic velocities. Some of the advantages of using CSAM to fabricate titanium components are significantly greater deposition and better adhesion to the substrate surface. The CSAM process is used to deposit temperature sensitive and oxygen sensitive materials because little or no heat is required during processing. Compared to other MAM processes, CSAM imposes fewer constraints on build size and geometry. In the CSAM process, the bonding strength between the Ti6Al4V layer and the Ti6Al4V substrate can reach 60 MPa, but it is still considered to be porous compared with Cu and Al, which have a denser microstructure. The mechanical property of CSAM titanium alloys are considered to be poor due to the difficulty in achieving high tensile strength. The CSAM Ti6Al4V deposited on the Ti6Al4V substrate shows high residual stress near the free surface of the print. The stresses found were mainly tensile, but compressive stresses were also recorded in the vicinity of the interface region. Compared to SLM and EBM processes, CSAM parts have a higher surface roughness and therefore require more machining of the finished part, and if further laser melting is introduced, the hardness is found to be higher than the originally produced part, which results in a change in its mechanical property.
Source: Machinability of additively manufactured titanium alloys: Acomprehensive review, Journal of Manufacturing Processes,  doi.org/10.1016/j.jmapro.2022.01.007 References: X. Gao, et al.,  A study of epitaxial growth behaviors of equiaxed alpha phase at different cooling rates in near alpha titanium alloy,  Acta Mater, 122 (2017), pp. 298-309 Original works by Chen Changjun of Jiangsu Laser Alliance! Return to Sohu to see more Responsible Editor: (function() { function getBrandHtml() { var brands = [],ti6al4v eli, html = ''; for(var i = 0; i < brands.length; i++) { var brand = brands; if(brands.length i+1) { html+= '