Review of the Machining Difficulties of Nickel-Titanium Based Shape Memory Alloys

Review of the Machining Difficulties of Nickel-Titanium Based Shape

Memory Alloys

M.R. Hassan

, M. Mehrpouya

and S. Dawood

Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Malaysia ¹morhas@eng.upm.edu.my, ²mehrshad.mehrpouya@gmail.com, ³sarmd.dawood@gmail.com

Key words: SMA, NiTi, Nitinol, EDM, Residual stress

Abstract.The purpose of this review article is to identify machining difficulties of nickel-titanium based shape memory alloys. Nickel-titanium (Nitinol) is one the famous shape memory material which applied in many products like the equipments of aerospace, medical, and biomedical. NiTi alloy cannot be machined easily because of high tool wear, high cutting force, huge hardness and surface defects are made many problems into their machining. Investigation in micron precision

shows plenty surface defects in machining process, something like debris of microchips, feed marks, tearing surface, deformed grains, material cracking and chip layer formation which consists the main problem in the machining of shape memory alloys. Moreover, these defects can be reduced or eliminated by changing in the cutting parameters like feed rate, cutting speed and cutting depth so that, machining of nickel-titanium alloys would be improved.

Introduction

Many scientists in different years discover particular materials that denominate them Shape Memory Alloys (SMA). The unusual property of these materials being able to recover and sustain large strains without inducing invariable plastic deformation and to “remember” last configuration and come back to their principal shape with the changing of temperature [1-3]. Nakanishi in 1983 believed that SMA has a special thermo mechanical behavior in temperature and stress induced martensite transformation. Schroeder and Way in 1977, Saburi and Way in 1979 were the other scientists that have the same opinions about the shape memory materials [3, 4]. Nickel-Titanium alloy is one of the most famous shape memory alloys which applied in wide range of products especially in aerospace, medical, and biomedical [5, 6]. But, this considerable alloy has a problem in the industries that is related to its machining. Because conventional methods in machining such as; turning, milling or drilling face difficulties in considerable tool wear in these alloys. Also, this matter needs high experienced operators that it increases the costs. Moreover, the conventional techniques have high thermal condition clearly and this phenomenon cause to change in mechanical effects in shape memory materials [7, 8].

Nickel-titanium alloys are famous for their tensile deforming in a ductile manner (50% strain) before fracturing but some difficulties such as tool wear, high toughness, high ductility, work hardening and viscosity in the cutting processes have caused machining characteristics of NiTi to be high complicated

[9,10]. So, these problems can incur low workpiece quality with inferior chip breaking and burrs

formation that Fig. 1 presents some of these influences that the behavior of unconventional stress-strain creates them during cutting processes of shape memory alloys [11].

Figure 1: Some Problems in the Machining of NiTi: a) High Tool Wear, b) Adverse Chip Form, c) Burrs Formation after Turning, d) Grinding [11]

Literature and discussion

Machining has a main characteristic in a wide complex of manufacturing processes how it is designed for removing material from workpiece. The basic machining operation can be categorized to milling, drilling, turning, sawing, shaping, broaching and abrasive machining. Milling process is to remove surface by predetermined value of material from a workpiece how movement activity is between rotational cutting tool and workpiece. Drilling is another process that wants to create round holes in the material of workpiece. Turning is a cutting process that removes material by generating external and cylindrical forms and if the cutting process focuses on the internal turning, it is called Boring for internal shapes. Sawing cutting process works with the power of saw to make differential geometries. Shaping is a cutting process by removing material from surfaces how it uses a single point tool that support with a ram. The tool reciprocates in a linear motion despite the workpiece. Broaching process applies a cutting tool with particular cutting edge. In this operation, multiple transverse cutting with push-pull motion that remove material with axial cutting. Finally, Abrasive machining process can be defined in grinding how the small chips of material are removed from the workpiece [12,13].

There are many surface defects in Ni-Ti (Nitinol) machining process and the main forms such as feed marks, surface drag, debris of microchips, surface plucking, tearing surface, material cracking, surface cavities, adhered material particles, chip layer formation, deformed grains, slip zones, laps (material folded onto the surface), and lay patterns that some of them has shown in Fig. 2 [8,14].

Figure 2: Surface Damages in Machining of Nickel-Titanium Alloys: (a) Metallographic Microstructure after Turning Process (b) Lay Pattern after Dry Milling Process (c) Metal Debris after Turning Process,

and (d) Smeared Material and Feed Marks after Turning Process [8]

Chip redeposition to the surface and deformed grain are the other surface defect in this verification, because the plucking of bits and its related redeposition on the surfaces cause to different defect. First, these particles can cause tearing and dragging on the surface and second it is difficult to adjust the cutting parameters according to these surface defects. Fig. 3 has shown the tearing mechanism in machining process [14].

Figure 3: Surface Tearing Mechanism in Machining Process [14]

Residual stresses create after machining processes of nickel-titanium when the workpiece material abandoned thermo-mechanical load and this energy cannot be retrieved completely. In the other hand, residual stress is the remained stresses in material when the loading is removed. Cutting parameters and tool conditions cause to create tensile plastic deformation and this matter effect on morkpiece material,

because residual stress is a potential to create in term of propagation, crack initiation and fatigue failure. Therefore they should remove or prevent to happening during machining processes [15]. Plastic deformation is the principle threat of surface integrity that comes from the plastic deformation of the workpiece in the machining process. This surface defect affects on the workpiece deformation directly and a lot of parameters can create or prevent them, something like; workpiece parameters (material, grain size), tool parameters (edge radius, rake angle, wear shape, coating) and cutting parameters (feed, cutting speed and depth of cut) [13, 14, 16].

These defects can be affected by the cutting parameters. For example feed marks are really considerable parameter in machining but its severity can be changed by modifying and optimizing the

feed rate [17,18]. In the other case, cutting speed have affect on the amount of surface’s microchip

debris. Also, depth of cut can be affect on some surface defects such as tearing, smearing, dragging or material plucking. They are common problems in the machining of nickel-titanium alloys and the cutting parameters should be optimized to eliminate these obstacles [8,14]. Also, Some particular techniques have been used to machine nickel-titanium to overcome their difficulties such as Electric Discharge Machine (EDM), Laser cutting, wire cut machine, electrochemical machining and chemical milling but there are limited. Therefore it is better to solve these problems by some approaches how understanding of conventional machining be considerable [19, 20].

Conclusion

Nickel-titanium (Nitinol) is one of the famous shape memory materials which cannot be machined easily. Because some particular parameters such as high tool wear, high cutting force and huge hardness are made surface defects due to machining temperature. There are many

surface defects in Nickel-titanium machining process such as feed marks, surface drag, debris of

microchips, surface plucking, tearing surface, material cracking, surface cavities, adhered material particles, chip layer formation, deformed grains, slip zones, laps (material folded onto the surface), and lay patterns. By changing in the cutting parameters (like feed rate, cutting speed and cutting depth), these defects can be reduce or eliminate. Consequently, machining of nickel-titanium alloys would be improved.

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