MetalicaRap:Physics Principles
Principles
Following EBM metal vaporisation loss of vacuum leading to loss of beam quality
cal given metal melting speed based on this senario;
100KV at 11.5mA Beam power 1.15kW/mm2 or 115 kW/cm2. 5-15µ deep beam energy beam spot dia 7µ.
85% energy transfer from beam to metal powder
Treat as cylinder. 15µ*7µ*7µ x10-12cm^3 or 600µ3. 600 x<math>10^{-12}</math><math>cm^3</math> at 115k W/cm2 in to 40µ^2 spot same as 40*10^-8cm2. Is 40x10-8cm2*100K= .04 W in to metal volume , how many joules to vaporize this volume; 5x104 J/cm3 for 600x10-12cm3 600x10-12cm3* 5x104 J/cm3= 3x10-5 Joules getting .04 joule in a second so it will take 3x10-5/.04= 75x10-5 Seconds or 750µS to vaporize cylinder, heat conduction problem will remain cold process?
Problem: Pressure from evaporated material
Example from “Practial manufacturing walk through” will add in soon!
Vaporizing Titanium at with .04 W into metal volume evaporating at the price of <math>5*10^4 J/cm^3</math> evaporates
<math>\frac{0.04W}{5*10^4 J/cm^3}=8*10^{-7} cm^3</math>
With the density of metallic Ti 4.5 g/cm3 the above volume amounts to 3.6x10-6 g. And with a molar mass of Ti 204.37 g/mol it is 1.76x10-8 mol. Now: ONE mole of a gas takes a volume of 24 Liter at a pressure of 1 atmosphere or 101,325 Pa = 1013.25 millibar. Relative to this an amount of 1.76x10-8 mol vaporized titanium in a 1000 L (1 m3) volume will per second contribute to the pressure with the following value;
<math>=\Delta P /S =\frac{1.76*10^{-8} mol/s}{1 mol}*\frac{24L}{1000L}*1013.25 mBar =4.43*10^{-7} mBar/s </math>
Thus the pump has to remove 4.3x10-7 millibars of pressure per second. Maybe this is not at all a problem. I don’t know about pumping capacities
. I just wanted to put attention to it.
PS: According to my tables the heat of evaporation of Ti is not 5x104 but 4x104 J/cm3 (8.953 MJ/kg times 4.5x103 kg/m3
Discussion
Problem: Pressure from evaporated material
Not problem with tubo-pump but could be a problem with oil diffusion pump? what are typical oil diffusion pump pumping rates?.
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