Special Technologies

Machining of Shape Memory Alloys (SMAs)

Mehrshad Mehrpouyamehrshad.mehrpouya@uniroma1.it

Sapienza University of RomeDepartment of Mechanical and Aerospace Engineering

Department of Mechanical and Aerospace Engineering

Special Technology

Department of Mechanical and Aerospace Engineering

Special Technology

https://it.linkedin.com/in/mehrpouya

Department of Mechanical and Aerospace Engineering

Special Technology

Machining of Nickel-Titanium AlloyMachining has a main characteristic in a wide complex of manufacturingprocesses how it is designed for removing material from workpiece. Thebasic machining operation can be categorized to milling, drilling, turning,sawing, shaping, broaching and abrasive machining.

Department of Mechanical and Aerospace Engineering

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Machining of Nickel-Titanium Alloy

Drilling, as with turning, requires careful control of feed and speed, and the use of chlorinated lubricant is recommended.

Cylindrical centerless grinding is a useful process for developing a good surface on tubing and wire.

Department of Mechanical and Aerospace Engineering

Special Technology

Machining of Nickel-Titanium Alloy

Abrasive methods such as abrasive wheel cut off and abrasive water jet cutting are also used in processing Nitinol.

Electro-discharge (EDM) machining is quite useful, although not really suitable for volume production.

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Laser cutting and machining

has become a preferred method forcreating stents from Nitinol tube.Very complicated geometries areproduced using CNC controlled partmotion and finely focused pulsedNd:YAG laser beams. Laser cuttingis fast and very flexible, and cutgeometry is readily changedthrough reprogramming of the CNCcontrol.

Department of Mechanical and Aerospace Engineering

Special Technology

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Department of Mechanical and Aerospace Engineering

Special Technology

The mechanism of a machining process

Nomenclature

𝑽𝑪 Cutting Speed

FC Cutting Force

𝒉𝒄 Chip Thickness

h Depth of cutting

𝜸𝟎 Rank Angle

𝜶𝟎 Clearance Angle

𝒌𝒓 Tool Cutting Edge Angle

Φ Shear Angle

Department of Mechanical and Aerospace Engineering

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Problem Statement

Machining of Nitinol is very difficult by reason of the very rapid work hardening of this alloy. Although with proper carbide tooling and control of tool geometry, speed and feed, excellent tolerance and finish can be achieved in turning operations.

NiTi alloy cannot be machined easily because of hightool wear, high cutting force, huge hardness and surface defects are made many problems into their machining.

Department of Mechanical and Aerospace Engineering

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Surface defects

Investigation in micron precision shows plentysurface defects in machining process, somethinglike;

High Tool Wear Chip layer formation Burrs Formation Lay Pattern Debris of microchips Feed marks Tearing surface

Deformed grains Material cracking Smeared Material Feed Marks after Turning Build-up edge (BUE)

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Some Problems in the Machining of NiTi: a) High Tool Wear, b)

Adverse Chip Form, c) Burrs Formation After Turning, d) Grinding(Weinert and Petzoldt 2004).

Department of Mechanical and Aerospace Engineering

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Surface Damages in

Machining of Nickel-TitaniumAlloys: (a) MetallographicMicrostructure after Turning(b) Lay Pattern after DryMilling (c) Metal Debris afterTurning, and (d) SmearedMaterial and Feed Marks afterTurning (Ulutan and Ozel 2011).

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The schematic of the build-up edge (BUE) in the machining process

Department of Mechanical and Aerospace Engineering

Special Technology

There are a lot of parameters that have influence on the workpiece’ssurface quality. workpiece parameters (material, grain size), toolparameters (edge radius, rake angle, wear shape, coating) and cuttingparameters (feed, cutting speed and depth of cut) (Falvo 2007, Ulutan and Ozel

2011, Mackerle 2003; Sun and Feng 2006).

Feed Rate Cutting Speed Tool Wear

Tool Geometry and Properties

Cutting DepthWorkpiece

Materials and Properties

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Case Study

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Department of Mechanical and Aerospace Engineering

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Nickel-Titanium shape memory alloy

Ni50.9 Ti49.1.

Cutting tool, TiCN/TiAlN multilayer

coating (is chosen as the appropriate

with capability of utilizing in high cutting

speed processes)

At room temperature

The experimental machining (Turning Process)

Department of Mechanical and Aerospace Engineering

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The optimum cutting speed is investigated as aconsiderable parameter and a principle factor inapplied stress to obtain a better machiningquality of NiTi. The interaction between variouscutting speeds and the temperature rise of theworkpiece has attracted much attention. Highstress can increase the hardness of NiTi due toits specific thermo-mechanical properties.

Department of Mechanical and Aerospace Engineering

Special Technology

Generally, the lower cutting force andconsequently lower stresses in themachining process improve themechanical properties, as well asreduction in hardness, distortion andresidual stress.

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Experimental results

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FEM provide suite powerful offers andcomplete solutions for both routine andsophisticated engineering problems coveringa vast spectrum of industrial applications. Inthe automotive industry engineering workgroups are able to consider full vehicleloads, dynamic vibration, multibody systems,impact/crash, nonlinear static, thermalcoupling, and acoustic-structural couplingusing a common model data structure andintegrated solver technology.

Finite Element Method (FEM)

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Generally, a complete simulation processbased on finite element method (FEM)enables to predict a comprehensive modelfor machining optimization and effectivelyreduces the cost of experimentationeffectively. Particularly, numerical modelingof the cutting operation reveals that thestress-strain rate, chip formations and toolstatement are costly and time consumable todetermine experimentally

Finite Element Method (FEM)

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JC constitutive material model

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Finite Element Method (FEM)

Mesh ModelSchematic of the machining modeling

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Von Mises stress contour plots in cutting speed, (a) 20, (b) 80, (c) 100, (d) 110, and (e) 130 m/min

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Shear stress contour plots in cutting speed, (a) 20, (b) 80, (c) 100, (d) 110, and (e) 130 m/min

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Von mises stress-Cutting speed

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Shear stress-Cutting speed

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Resultant stress-Cutting speed

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As it clear, the value of micro-hardness has reduced remarkably, when the cutting speed has risen. Additionally, this diagram depicts 100 m/min as the acceptable amount of cutting speed where the lowest value of hardness is 240 HV ± 7.5 (Kaynak et. al.).

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Department of Mechanical and Aerospace Engineering

Special Technology

The obtained cutting speed, as shownin the resultant graph of FEM, wouldbe acceptable since it has only 9%variation in comparison with theexperimental cutting force (100m/min).

Final result

Department of Mechanical and Aerospace Engineering

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