Electron Beam Melting

EBM (Electron Beam Melting) can be described as the ‘rapid prototyping’ for metals. It is better known as ‘rapid manufacturing’ method. The parts are manufactured by having the metal powder melted layer by layer through a beam of electron in high vacuum. The parts produced acquire strength, solidity, and are void-free as well. The electrons have a very high speed; around 5 to 8 times the light speed. The bombardment of these electrons takes place on the work material’s surface. This generates heat which is enough for melting the part’s surface and causing it to vaporize locally. Vacuum is required for the operation of EBM. This means that the size of work piece is directly proportional to vacuum used. This technique works on composites, ceramics, non-metals, and as stated above, metals.

CRYSTAL SYSTEM

Crystalline Materials:

• A crystalline material is one in which the atoms are situated in a repeating (or) periodic array over large atomic distances.

 

 

Non Crystalline Materials:

• Materials that do not crystallize are called non-crystalline (or) Amorphous materials

 

Space Lattice:

• Lattice is the regular geometrical arrangement of points in crystal space.

 

• The atoms arrange themselves in distinct pattern in space is called a Space Lattice.
• Atoms in crystalline materials are arranged in a regular 3 – Dimensional repeating pattern known as Lattice Structure.
• They are divided by network of lines in to equal volumes, the points of intersection are known as Lattice Points.

 

Unit Cell:

• It is the smallest portion of the lattice which repeated in all directions.
• 3D visualization of 14 Space Lattices are known as Bravai’s Space Lattice.
• If a unit cell contains lattice points only at it’s corners, then it is called Primitive Unit Cell (or) Simple Unit Cell.
• Three edge length x,y, & z and three interaxial angles α, β, & γ are termed as Lattice Parameters.

 

Crystal System:

• It is a scheme by which crystal structures are classified according to unit cell geometry.

 

Types of Crystal Systems:

• Cubic
• Tetragonal
• Hexagonal
• Orthorhombic
• Rhombohedral
• Monoclinic
• Triclinic

 

Crystal Systems

Simple Crystal Structure:

Body Centered Cubic Structure (BCC)

• Unit cell contains 2 atoms
• Lattice Constant a= 4r / √3, where r is atomic radius
• Atomic packing factor APF = 0.68
• Metals are Vanadium, Molybdenum, Titanium, Tungsten

 

Face Centered Cubic (FCC)

• Unit cell contains 4 atoms
• Lattice Constant a= 4r / √2, where r is atomic radius
• Atomic packing factor APF = 0.72
• FCC structures can be plastic deformed at severe rates
• Metals are Copper, Aluminum, Phosphorous, Nickel, Cobalt etc

Hexagonal Closed Packed Structure (HCP)

• Unit cell contains 3 atoms
• Axial ratio c/a, where ‘c’ is Distance between base planes, ‘a’ is Width of Hexagon
• Axial Ratio varies from 1.58 for Beryllium to 1.88 for Cadmium (Therefore  a=2.9787, c=5.617)
• Atomic packing factor APF = 0.74
• Metals are Zinc, Cadmium, Beryllium, Magnesium etc

 

 

Crystallographic Planes and Directions

The Layers of atoms in the planes along which atoms are arranged is known as “Atomic” (or) “Crystallographic planes”.

Miller Indices:

Miller Indices is a system of notation that denotes the orientation of the faces of a crystal and the planes and directions of atoms within that crystal.

Miller Indices for Planes:

1. The (110) surface

 

Intercepts :   a , a , ∞

Fractional intercepts :   1 , 1 , ∞

Miller Indices :   (110)

 

2. The (111) surface

 

Intercepts :   a , a , a

Fractional intercepts :   1 , 1 , 1

Miller Indices :   (111)

The (100), (110) and (111) surfaces considered above are the so-called low index surfaces of a cubic crystal system.

 

3. The (210) surface

 

Intercepts :   ½ a , a , ∞

Fractional intercepts :   ½ , 1 , ∞

Miller Indices :   (210)