AM Material Selection & Properties 2 — Questions and Answers
Question 1: What is the primary reason titanium alloys are used in aircraft engine compressor blades?
- High strength-to-weight ratio and heat resistance (Correct answer)
- Low cost and easy machinability
- High electrical conductivity
- Excellent corrosion resistance in fuel systems
Correct answer: High strength-to-weight ratio and heat resistance
Titanium alloys offer an exceptional strength-to-weight ratio combined with good heat resistance, making them ideal for compressor blades.
Question 2: Which heat treatment process is used to relieve internal stresses in metal without significantly changing its hardness?
- Annealing
- Stress relieving (Correct answer)
- Normalizing
- Case hardening
Correct answer: Stress relieving
Stress relieving heats metal to a temperature below the transformation range and cools it slowly to reduce residual stresses without major hardness changes.
Question 3: What does the term 'fatigue strength' refer to in aircraft materials?
- Maximum load before permanent deformation
- Stress level a material can withstand for an infinite number of cycles (Correct answer)
- Resistance to surface scratching
- Energy absorbed before fracture
Correct answer: Stress level a material can withstand for an infinite number of cycles
Fatigue strength is the stress level below which a material can endure an indefinitely large number of loading cycles without failing.
Question 4: Which aluminum series alloy is most commonly used for aircraft structural skins due to its balanced strength and corrosion resistance?
- 1xxx series
- 2xxx series
- 6xxx series
- 7xxx series (Correct answer)
Correct answer: 7xxx series
7xxx series aluminum alloys (alloyed with zinc) offer the highest strength of aluminum alloys and are widely used in aircraft structural applications.
Question 5: What is galvanic corrosion and when is it most likely to occur in aircraft?
- Uniform surface oxidation caused by moisture
- Electrochemical corrosion when dissimilar metals contact in the presence of an electrolyte (Correct answer)
- Corrosion caused by high-temperature oxidation
- Pitting caused by chloride ions alone
Correct answer: Electrochemical corrosion when dissimilar metals contact in the presence of an electrolyte
Galvanic corrosion occurs when dissimilar metals are in electrical contact in the presence of an electrolyte, creating a current that corrodes the less noble metal.
Question 6: What property is measured by the Rockwell hardness test?
- Resistance to plastic deformation under indentation (Correct answer)
- Tensile strength at yield point
- Impact energy absorption
- Fatigue life under cyclic loading
Correct answer: Resistance to plastic deformation under indentation
The Rockwell hardness test measures a material's resistance to permanent indentation under a specific load, indicating surface hardness.
Question 7: Which composite material configuration provides the highest stiffness in a specific direction?
- Woven fabric with ±45° plies
- Chopped mat random-orientation composite
- Unidirectional laminate with all fibers aligned 0° (Correct answer)
- Quasi-isotropic layup
Correct answer: Unidirectional laminate with all fibers aligned 0°
A unidirectional 0° laminate concentrates all fiber reinforcement in one direction, maximizing stiffness and strength along that axis.
AM Material Selection & Properties 2
What is the primary reason titanium alloys are used in aircraft engine compressor blades?