Cutting Strain and Velocity in Metalworking

Cutting Strain

• Cutting strain is defined by the relationship: ϵ = cot φ + tan(φ − α).

Velocity Diagram in Cutting Zone

• Importance of velocities: Required for power estimation during the cutting process.
• Parameters:
  • VMaterial: Cutting velocity.
  • VTool: Speeds of the tool and material.
  • VChip: Chip velocity, represents material being removed.
  • Vs: Shear velocity, indicating speed related to shear deformation.

Shear Strain Rate

• Equation for shear strain rate: ε̇ = dε/dt = Vs/t_s (where t_s = thickness of shear zone).
• t_s is typically 10% of shear plane length, with a maximum value of 25 microns.

Heat Input

• Heat input (Hin) calculated as: H = Vl/Abv (Joule/mm³), where:
  • Abv: Weld bead area (mm²).
  • v: Velocity (mm/sec).
  • Hm: Heat required for melting.
  • η_t: Melting efficiency.

Duty Cycle

• Defined as the number of minutes a welder can produce a set current in a 10-minute interval.
• Formula for required duty cycle: Ta = (I/I_a) * T.
• Suggested duty cycles: 60% for manual welding and 100% for automatic welding.

Electrode Coating Characteristics

• Functions of electrode coating:
  • Provides a protective atmosphere.
  • Stabilizes the arc.
  • Forms protective slag coating to prevent impurities and oxidation.
  • Reduces spatter and adds alloying elements.
  • Influences arc penetration.

Arc Length & Stability

• Short arc length critical for:
  • Concentrated heat for effective welding.
  • Enhanced stability and protective atmosphere.
• Drawbacks of long arc length include:
  • Increased heat loss, unstable arc, and insufficient weld protection.

Arc Blow

• Phenomenon observed during DC welding of magnetic materials.
• Issues caused by arc blow:
  • Reduced heat penetration.
  • Increased weld spatter.
  • Pinch effect from electromagnetic forces.

Gas Shields in Welding

• Inert gases like argon, helium, and nitrogen are used to shield weld zones from atmospheric gases.

Tungsten Inert Gas (TIG) Welding

• Utilizes a non-consumable tungsten electrode.
• Stable arc maintained with or without filler material.
• Common shielding gases are helium and argon.

Gas Metal Arc Welding (GMAW)/MIG

• Involves an arc between a consumable electrode and the workpiece.
• Electrode continuously supplied in small wire form for melt transfer.
• Features spray transfer under gas protection.

Submerged Arc Welding (SAW)

• Method suitable for joining thick materials in one pass.
• Uses solid granular flux to submerge the arc, minimizing heat loss and spatter.
• Utilizes high currents (200-2000A) and fast speeds (5m/min).

Hot Working of Metals

• Plastic deformation of metals at temperatures above recrystallization, typically above 0.5Tm.
• Advantages include significant shaping, lower energy requirements, and isotropic properties.
• Disadvantages involve heat energy needs, costs of tooling, and poor surface finish.

Rolling Process

• Reduces thickness or alters the cross-section of materials via compressive forces of rollers.
• Types:
  • Hot Rolling: Above recrystallization temperature; less accurate dimension and surface quality.
  • Cold Rolling: Below recrystallization temperature; better surface finish and mechanical strength.

Punching and Blank Force Calculations

• Maximum force in punching described by the equation: F_max = Ltτ.
• F_max can also be estimated for small holes: F_max = (πdt*σ)/3.

Drawing Process

• Plastic deformation technique transforming flat materials into three-dimensional shapes with depth exceeding thickness.