Micro Machining

As the demands for the manufacture of parts with complex micro-scale features increase for micro-components and systems, it is critical to develop technologies for processing engineering materials of high strength, good corrosion and wear resistance with good precisions.  Micromachining, micro-milling in particular, due to its great process flexibility, has been implemented as an alternative technology to generate accurate three dimensional (3D) geometries.  The size effect such that the material specific cutting force at the micro-scale is much higher than at the macro-scale makes micromachining of difficult-to-machine materials even more difficult.  The high specific cutting force cannot be sustained by micro-sized tools, which usually results in a catastrophic failure of the tool and poor finished surface.

 

Laser-Assisted Micro Machining (LAMM) is a promising technology offering desired capability of producing complex 3D and high aspect ratio micro features in various difficult-to-machine materials such as hardened steels, titanium alloys, and nickel-based superalloys.  Softening the workpiece material using a controlled laser ahead of the cutting position reduces the cutting forces and has the potential for extending the practical applications of the process.  

 

Micro Milling of Tool Steel (journal paper)

LAMM Side Cutting (journal paper)

LAMM Slotting (journal paper)


 

Experimental evaluation of micro milling of hardened tool steel

 

This study is focused on experimental evaluation of micro-milling of hardened H13 tool steels.  Micro-milling experiments were carried out on:

  • Three-axis CNC controlled micro-milling system that includes a Precise SC-40 spindle with a maximum rotation speed of 90k RPM and provides movement of the workpiece relative to the tool with a 1 µm resolution.  A flexible nozzle was attached to the spindle mounting fixture allowing for an adjustable flow of assist gas during machining.  
  • A differential acoustic emission (AE) sensor with an operating frequency range of 100-1000 kHz was securely mounted to the workpiece vise, and was connected to a matching preamplifier and data acquisition card with Physical Acoustics software being used for all signal processing.  The AE sensor was used as an indicator of tool contact and to collect qualitative data during the cutting process. 
  • Post-inspections after micro-milling experiments were carried out on surface integrity, machined part size and tool wear.  A JEOL JSM-T330 scanning electron microscope (SEM) and a Zeiss optical microscope were used to examine machined workpieces and tools in this study.  3D surface maps and surface roughness measurements were obtained using a non-contact interferometric surface profiler (ADE Phase Shift Micro-XAM). 

 

 

Size Effect

In micromachining, the cutting edge radius (re) of the micro tools is comparable to the undeformed chip thickness (h).  Ploughing eventually becomes dominant as re increases to be much greater than h and no chip forms beyond this condition.  

 

Micro Endmill

Two-flute endmills of ultra-fine tungsten carbide in a cobalt matrix were used.  The composition of the tool was 92% tungsten carbide with an average grain size of 0.4 microns and 8% cobalt as a binder to hold carbide together.  

 

Side cutting

Tool path in the side cutting configuration

Dimension Control

Left figure shows the workpiece geometry machined after 15 side cutting passes or 3 minutes of cutting time. Clean step geometry with burrs largely remaining on the top surface was observed along the whole machined section.  Machining marks can be observed on the end surface and the small steps on the machined end surface were caused by the change of surface contacts after loading and unloading the workpiece. 

 

Surface Roughness 

The surface roughness on the machined side surface was found to be around 0.5 µm. 

Surface defects on the machined side surface can be observed.   

Real-time monitoring

Acoustic emissions were recorded and analyzed for side cutting experiments. 

The recorded AE root mean square (RMS) voltage indicates the acoustic emission signal strength.  

 

Tool Wear

The maximum tool flank wear, maximum tool edge radius and average tool edge radius gradually reached about 25 µm, 10 µm and 4 µm, respectively, and the ratio λ decreased from about 2 to 0.2 before the tool catastrophic failure.  

 

 

LAMM Side Cutting of Difficult-to-Machine Alloys

 

This study is focused on numerical modeling analysis of laser-assisted micro-milling (LAMM) of difficult-to-machine alloys, such as Ti6Al4V, Inconel 718, and stainless steel AISI 422.  Multiple LAMM tests are performed on these materials in side cutting of bulk and fin workpiece configurations with 100-300 µm diameter micro endmills. 

  • A 3D transient finite volume prismatic thermal model is used to quantitatively analyze the material temperature increase in the machined chamfer due to laser-assist during the LAMM process. 
  • Novel 2D finite element (FE) models are developed in ABAQUS to simulate the continuous chip formation with varying chip thickness with the strain gradient constitutive material models developed for the size effect in micro-milling. 
  • The steady-state workpiece and tool cutting temperatures after multiple milling cycles are analyzed with a heat transfer model based on the chip formation analysis and the prismatic thermal model predictions. 
  • An empirical tool wear model is implemented in the finite element analysis to predict tool wear in the LAMM side cutting process. 
  • The FE model results are discussed in chip formation, flow stresses, temperatures and velocity fields to great details, which relate to the surface integrity analysis and built-up edge (BUE) formation in micro-milling.

 

LAMM Side Cutting

Two LAMM configurations: Laser beam on the side surface for bulk side cutting, and on the top surface in fin side cutting. The narrow width of the fins allowed for a more uniform temperature profile.  

Thermal Modeling

Thermal modeling of temperature fields of Ti6Al4V undergoing LAMM side cutting

FE Modeling

Chip formation and cutting temperatures of LAMM bulk side cutting of 422SS at 60 µs cutting time under the condition of 422bulk-3 (V=18.85m/min)

Cutting Temperature

Workpiece nodal temperature histories in conventional micro-milling and LAMM of 422SS.

Tool Wear

Tool wear comparison of conventional and LAMM fin side cutting of 422SS and Inconel 718

   

 


 

LAMM Slotting of Difficult-to-Machine Alloys

 

This study is focused on a numerical modeling analysis of LAMM slotting for difficult-to-machine biomedical implant alloys, such as Ti6Al4V and stainless steels. 

LAMM Slotting

An elliptical laser beam is positioned on the workpiece top surface ahead of the tool.

Thermal Modeling

Left figure shows the predicted workpiece temperature fields of Ti6Al4V undergoing laser heating.

Temperature Profile

Temperature profile along the round slot. Tmr changes from 400°C to 580°C as the tool rotates from 0° to 90° for Ti6Al4V.

Surface Integrity

The study showed that LAMM reduced the edge burr formation significantly and greatly improved surface integrity by thermally softening the material.