Ultrasonic machining, Thermal modeling, Plasma technology

Ultrasonic Machining (USM)

Sound waves are mechanical vibrations in a solid or in a fluid. Ultrasounds are the same, but at a frequency higher than the range audible to humans; the lowest ultrasonic frequency is normally taken as 20 kHz (20000 cycles per second).

The tool tip vibrates in the vertical direction very fast, at a frequency > 20 kHz. It must be small to avoid too high inertia. Typical amplitudes at the tool tip range from about 15 to 50 microns (low amplitude, fast oscillation). An ultrasonic system operating at 20 kHz and 50 microns is moving with a cyclic acceleration of 20000 g.

Ultrasonic can be used in:

• machining
• inspection
• welding
• cleaning

Process

• The electric current feeds an ELECTRONIC OSCILLATOR that transforms the low frequency into the high frequency (between 20 and 40 kHz).
• A MAGNETOSTRICTIVE or PIEZOELECTRIC TRANSDUCER converts the direct current from the frequency into longitudinal vibration in equal frequency.
• The vibration generated (amplitude 10^-5-10^-6 mm) is transmitted to the tool through a cone concentrator (HORN) which has the task of giving the maximum amplitude of vibration at its tip.
• The tool is in contact with the flow of ABRASIVE fed by a pump and the vertical movement of vibration, added to the static force applied externally, generates removal of material on the surface of the workpiece.

MATERIAL REMOVAL MECHANISM

The removal is due to several mechanisms:

1. FREE IMPACT: at high speed between the abrasive grains and the workpiece, causing fragile fractures. The abrasive is accelerated, hits the material and cools fractures.
2. LOCALIZED HAMMERING: The grain is hammered into the material by the tool that is acting like a hammer.
3. SLIDING EROSION: just on the vertical surfaces (not dominant mechanism).
4. CAVITATION AND CHEMICAL EROSION: due to the cutting fluid acting on the work surface (not dominant).

The tool doesn't hit the workpiece, so fragile materials can be worked by ultrasonic machining.

The material of the workpiece is fractured, but also the abrasive grains will be fractured, so the slurry must be removable to have a refresh of grains that are usually oxides or carbides (hard stones).

Also a vertical feed movement is needed to go deeper into the workpiece, and it is obtained by simply applying a vertical force (no servo control).

The problem is that the removal mechanisms and the MRR are different between horizontal surfaces and vertical surfaces.

Hardness is what opposes to penetration, so we have two hardnesses that matter.

We would like to have a tool with high hardness, but it means it will also be fragile.

So for the tool we should get to a compromise between hard to minimize tool wear.

→ ductile

A lot of problems under “μ” parameter.

So z and V must be ESTIMATED

Theoretical MRR

• d = average abrasive particle diameter
• F = average indentation force
• A = oscillation amplitude
• C = concentration of abrasive in the flow
• H_w = workpiece Brinnel hardness
• f = frequency (cycles per second)
• λ = ratio between penetrations

Tool/workpiece indentation ratio:

Does this formula work? Yes, until a given force

Increasing the force F over a certain threshold, the abrasive grains break, d is reduced and the MRR is lowered → reduced penetration if the whole force is too high. The actual ratio decreases.

A higher penetration of the abrasive grains in the tool (ductile tool) means a lower particle fragmentation which is a cause for lower process efficiency. In other words, softer tools allow larger feed forces.

MRR need depends on oscillation amplitude frequency

Advantages and Performances

• The USM/RUM process has no thermal nor chemical alterations on the machined surface.
• The hammering effect of the abrasive particles on the workpiece surface induces compressive residual stresses which usually increases the fatigue resistance of the machined pieces.
• Materials do not have to be conductors.
• Complex shapes and holes can be machined with good quality and tolerances.
• Common values of surface roughness: Ra = 0.5μm to 0.76μm for USM
• With highly accurate processes and low machining noise, even much better tolerances and roughness can be achieved (0.1 to 0.05 μm).
• Using abrasive particles #120-180 mesh, tolerances up to ± 0.08 mm
• #260-320 mesh, tolerances of ± 0.05 mm
• With RUM processes even tolerances up to some microns can be achieved.

Ultrasonic Welding

Ultrasonic welding is used to weld metals or thermoplastics at the solid state, without melting the material.

The vibration is generated by a similar system (vertical motion for metals, whereas vibration circularly for plastics).

The static force is always vertical.

The upper tool has no regular surface to guarantee very high grip between the upper tool and the sheet (high friction).

The top sheet has to move as an unique rigid body with the upper pin.

THERMAL MODELING OF PROCESSES

A model is a description of a phenomena. Thermal modeling of processes regards reproducing thermal phenomena as a function of input (energy source, process parameters, materials, geometry...)

Thermal models can be physical or empirical.

• INPUT
  • Beam source
  • Process parameters
  • Materials
  • Geometry
  • Boundary conditions
• OUTPUT
  • Temperature (direct)
  • Heat flux
  • Isothermal lines (welding, HAZ)

HEAT TRANSFER REVIEW

Heat transfer happens when there is a difference in temperature.

• CONDUCTION: through direct molecular communication without a flow of the material medium.
• CONVECTION: combination of conduction and transfer of thermal energy by fluid circulation or movement of the hot particles in bulk to cooler areas in a moved medium.
• RADIATION: transfer of heat through electromagnetic radiation from a hot body to a colder one.

Example

Steel QCD/gold and diode laser heat treatment (10⁸ W/m²)

• Steel - less able to let heat flow - higher T
• Gold - higher thermal diffusivity
• Heat flows faster and better
• α_gold > α_steel
• D_gold > D_steel

On the surface: T(0,t) = T₀ + ^q″″₀⁄_K √(πt)

The deeper material needs time for the heat to reach it → a sort of delay in the change of T.

2 thermal distance ≈ 0.05 m

Where T tends to the initial temperature

If 2D ≈ 0.05m

D ≈ 0.025m

Cooling Phase

What happens when I switch off the flux, the diode laser? Have we got an analytical solution for cooling down? Yes.

Switching off is like having another source with an opposite flux (same intensity, opposite sign), so now I can simulate the cooling phase.

Taper

- Taper is measured through the UNEVENESS u.

In plasma cutting, the inclination can take different values at the two surfaces, it is not symmetric.

And so also quality is not symmetric.

Also, the amount of bars would be different. The most credible reason is that this is due to the rotation of the gas: clockwise or counterclockwise will make a difference.

It's necessary to use a 5-axis machine to compensate the taper if you want to obtain vertical walls. The 2 more axis are needed.

PAC = symmetric walls

HDP = asymmetric walls, only one good side depending on cutting direction and cutting speed.

Kerf Width

Relevant parameters:

• Feed rate f
• Current I
• Beam collimation
• Nozzle diameter
• Stand-off distance (voltage)

If V ↑ → transferred energy ↑

→ removed rate ↑

→ Kerf width ↑

Burrs

Burrs are made of resolidified material on the bottom of the kerf (generally more on one side in HDP).

They depend on:

• Cutting speed f
• Arc current I
• Gas type
• Material and thickness