Laser-Assisted Machining

Figure 1 Schematic diagram of LAM

The use of high strength materials, such as nickel-based superalloys and titanium alloys, is becoming increasingly common in aerospace, automotive, energy, medical, and mining applications. However, these materials are traditionally considered to be difficult-to-machine.  Conventional machining processes for these materials are notoriously affected by slow machining speeds and/or frequent tool changes due to short tool lives. 

Laser-Assisted Machining (LAM) has begun to emerge as a viable industrial option for machining of difficult-to-machine materials.  In LAM a laser provides intense localized heating to the workpiece ahead of the cutting region (see right).  By lowering the material strength in the cutting area at a certain elevated temperature, LAM can achieve lower cutting force, slower tool wear progression rate, higher material removal rate and better surface quality.

Goals of LAM research:

  • Higher material removal rates
  • Improved surface quality
  • Longer tool life
  • Less cost

Hardened Steels (journal paper)

Heat Resistant Alloys (nickel-base superalloys journal paper)

Wear Resistant Alloys (high chromium white cast irons journal paper)

 


 

 

LAM of Hardened Steel

 

Machining of hardened steel components such as gears, bearing rings, crankshafts, camshafts, etc., has traditionally relied on grinding-based technologies.  One-step LAM process is proposed to replace the hard turning and grinding operations.  It will also allow for a higher material removal rate without compromising the surface integrity.  

Schematic comparison between current and proposed processes with the material removal and surface finish after each step. 

 

 

Challenges

  • Precise temperature control
  • Maintain surface integrity 

Dimension control: To measure the effect of thermal expansion on size control, the same cutting tool was used throughout the LAM and conventional machining tests.  The diameters of multiple parts produced by LAM and conventional cutting were measured when the parts cooled down.  Although both the stiffness and thermal expansion are sources to the dimensional error, precise size control is achievable by improving the machine rigidity and finding a suitable LAM depth of cut to minimize the dimensional error.  

Left figure shows that the actual depth of cut during LAM was 0.035 mm more than that of conventional machining. 

But the final dimensions or the tolerance of the parts produced by LAM were as consistent as by conventional machining.

Hardness: The surface hardness was measured at 5 different locations for each cylindrical part before and after LAM.  The following figure compares the surface hardness histograms before and after LAM.   The surface hardness measurements indicate that the machined surface produced by LAM was work hardened uniformly.  

 

The hardness of the as-received parts varies between 44 to 50 HRC, and in comparison the hardness after LAM becomes more concentrated and ranges from 47 to 48.5 HRC. 

 

   

Subsurface microstructures: after LAM and no microstructural change was observed when comparing the machined subsurface after conventional machining with those after LAM.  Neither phase change nor white layer forms on the machined surface under the LAM condition.  

Optical microscopy of microstructures of the subsurface after LAM at speed of 180 m/min, feed of 0.075 mm/rev and various Tmr, 200X.

 

Residual stress: Stresses in both the hoop and axial directions drop sharply within the first 20 µm below the surface.  The stress penetration in both directions is around 40~50 µm below the surface.  The hoop stress is mainly tensile at the surface, but becomes compressive about 10 µm below the surface with the peak compressive stress in the range from -150 to -300 MPa.  The axial stress is less tensile on the surface for the feed of 0.075 mm/rev and even becomes compressive for the feed of 0.05 mm/rev.  The peak compressive stress in the axial direction is higher than that in the hoop direction and is about –400 MPa for all the four conditions. 

 
 
 
 
 
 
 
 
Residual stress in the hoop direction is tensile and is generally within the range of 200~400 MPa.  For the feeds of 0.075 and 0.1 mm/rev, the magnitude of the axial stress is less than that of the hoop stress and ranges from -200 to 200 MPa.  For the feed of 0.05 mm/rev, the compressive axial stress has a higher magnitude than that of the hoop stress and ranges from -250 to -300 MPa.  As Tmr increases from room temperature to 200 °C, the hoop stress also increases by about 50~100 MPa.  

 

 

LAM of Heat Resistant Alloys (Waspaloy) 

 

Heat-resistant alloys are designated to provide unique strength and/or corrosion properties at elevated temperatures (540 °C, or above).  Major attributes include properties such as high strength, high creep resistance, resistance to softening, or resistance to metal loss at high temperature from oxidation, sulfidation, or carburization.  Waspaloy is a nickel-base superalloy, which is primarily used in aircraft turbine engines as forged turbine and compressor disks.  Other applications include turbine cases, shafts, and blades.  The combination of good tensile and fatigue properties of Waspaloy at intermediate temperatures has made it attractive for disk applications in both turbine and compressor sections.  

Cutting force

The cutting forces and specific cutting energy of Waspaloy during LAM decreased by about 20% as Tmr increased to 400°C.  

Tool wear

Notch wear and the flank wear are the primary tool wear modes. Notch wear decreased to about a half and the flank wear decreases by about 40~60%.  

The tool life during LAM increased by about 50% as compared to conventional machining.

Surface Integrity

The surface finish improves as the material removal temperature increases.  On average, arithmetic average Ra is about 0.6~0.8 μm for a fresh tool insert in LAM with Tmr above 300°C, while it is over 1 μm in conventional machining.  

The surface texture improves greatly as the material removal temperature increases.

 Microstructure

No sign of phase compositional change, white layer, or other defects can be observed in the microstructure produced by LAM from the machined surface to about 400 µm below. This indicates that no thermal damage is induced during LAM.  

The grains produced by LAM near the machined surface tend to be smaller and more uniformly distributed than those of the original unaffected area, which will provide a higher fatigue resistance than the as-received microstructure.

Hardness

Vickers microhardness measurements were taken on the same specimens below the machined surface using a square-base pyramidal diamond indenter with a force load of 10 g.  

Compared with conventional cutting, LAM relieves the strong surface work hardening effect induced by machining.    

 


 

 

LAM of Wear Resistant Alloys (high chromium white cast irons) 

 

The machinability of high chromium wear resistant materials is poor due to their high hardness with a large amount of hard chromium carbides.  This study is focused on improving the machinability of high chromium wear resistant materials with different microstructures and hardness levels via laser-assisted machining (LAM).  A laser pre-scan process is designed to preheat the workpiece before LAM to overcome the laser power constraint.  A transient, three-dimensional LAM thermal model is expanded to include the laser pre-scan process, and is validated through experiments using an infrared camera.  The machinability of highly alloyed wear resistant materials of 27% and 35% chromium content is evaluated in terms of tool wear, cutting forces, and surface integrity through LAM experiments using CBN tools.  With increasing material removal temperature from room temperature to 400°C, the benefit of LAM is demonstrated by 28% decrease in specific cutting energy, 50% improvement in surface roughness and a 100% increase in CBN tool life over conventional machining.

Thermal modeling

A laser pre-scan process was designed to preheat the workpiece before LAM and a 3D thermal model was used to correctly predict the temperature inside the workpiece during the two-step process of laser pre-scan and LAM.  Tmr of 400°C was found to be the optimum temperature during LAM 

Tool Wear

Extensive tool wear tests conducted under both LAM and conventional cutting conditions revealed LAM doubled the tool life for the cutting of 27%Cr at various cutting speeds. 

For the harder material 35%Cr, LAM improved the tool life by a factor of 10.

Surface finish

LAM produced the parts of a consistently smooth surface finish with Ra of 0.8 µm. 

Surface roughness produced by conventional cutting was more than 1 µm and many small metal powders were observed to be stuck on the machined surface. 

Microstructure

Compared with conventional machining, no microstructural change was introduced by the two-step LAM process.  

Hardness 

The subsurface microhardness remains relatively constant within 1 mm below the machined surface after LAM and conventional cutting for both materials.  

Optimal design

Empirical tool life formula derived from the tool wear experiments was shown to be a useful tool to design the LAM process using CBN tools and improve the machinability of high chromium white irons.