Chapter 6 Overview

1
Chapter 6 Overview

• Four major alloy groups most common
• Aluminum, magnesium, zinc and ZA
• These alloys have a range of properties and
  characteristics
• Properties are slightly less but overlapping with
  sheet steels
• Properties are greater than but somewhat
  overlapping with high strength plastic resins

2
Chapter 6 Objectives

• Correctly identify the most common alloy from
  each major group
• Identify the alloy with the highest strength
• Identify the major alloying ingredients from an
  alloy specification
• List nine important criteria used to select an
  alloy for a particular job

3
New Terms

• Tensile strength
• Maximum stress achieved when pulling a test
  specimen to failure in the direction of its
  length
• Yield strength
• Level of strength at which elastic strain becomes
  plastic strain

4
New Terms

• Elongation
• Amount of permanent extension in the vicinity of
  the fracture in the tension test
• Modulus of elasticity
• Slope of the elastic portion of the stress-strain
  curve in mechanical testing

5
Mechanical Properties

• Properties include tensile strength (ultimate),
  yield strength, elongation (ductility), and
  modulus of elasticity (MOE)
• Predicts how alloy will react to stressed
  condition
• Strong alloy high values of tensile and yield
  strengths and MOE, and low values of elongation
• Weak alloy low strengths and MOE, and higher
  values of elongation

6
Aluminum Alloy Characteristics

• Has a specific gravity of 2.7 lightweight
• As a base, it has 3 primary alloying ingredients
  silicon, copper and magnesium
• All the other ingredients can be called
  impurities
• In some cases impurities must be controlled at
  specific levels, in other cases the level of
  impurity may be an economic compromise

7
Aluminum Product Applications

• 380 aluminum alloy most commonly used
• Lawn mower housings, electronics chassis, engine
  components, home appliances, tools
• 383 and 384 for intricate components, improved
  die filling, improved resistance to hot cracking
• 360 improved corrosion resistance, superior
  strength

8
Aluminum Product Applications

• 443 greatest ductility
• 413 excellent pressure tightness, highly fluid
  and useful for intricate detail
• 390 greatest wear resistance
• 518 very good corrosion resistance and
  ductility used in marine and aircraft hardware
  and also in escalators

9
Magnesium Alloy Characteristics

• Has a specific gravity of 1.74 lightest
  commonly used structural metal
• As a base, it has 4 primary alloying ingredients
  
• Aluminum, zinc, manganese and silicon
• All other ingredients are impurities and are
  controlled to maximum limits

10
Magnesium Product Applications

• AZ91D found in drive train automotive components
  as well as handheld and laptop computers
• AM60A has good elongation and toughness used in
  automotive wheels and steering wheels and archery
  equipment
• AS41A has creep strength at elevated temperatures

11
Zinc Alloy Characteristics

• Has a specific gravity of 7.0 one of the
  heavier commonly used structural metal
• As a base, has 3 primary alloying ingredients
• Aluminum, magnesium and copper
• All other ingredients are impurities and are
  controlled to maximum limits
• Sometimes referred to as Zamak
• Highest purity of the die casting alloys

12
Zinc Alloy Product Applications

• 3 zinc specified most frequently for functional
  and hardware castings
• 5 zinc has higher tensile strength, hardness,
  creep resistance automotive locks
• 7 zinc has slightly lower hardness and higher
  ductility higher fluidity than either 3 or 5
  could be a better choice for thinner walls and
  finer detail

13
ZA Alloy Characteristics

• Developed in 1950s
• Alloys of zinc, aluminum and copper
• Superior properties over Zinc Alloys
• Wear resistance
• Creep resistance
• Higher strength
• Lighter weight

14
9 Criteria Alloy Cost

• An important factor in overall product cost
• Cost of alloy should be converted to a volume
  basis
• Aluminum alloys usually have lowest cost per
  cubic inch
• Magnesium and zinc can be competitive because can
  be cast with thinner walls and at reduced volume

15
9 Criteria Process Cost

• An important component of overall product cost
• Alloys run with hot chamber process usually run
  in smaller DCMs and at higher production rates
  than equivalent casting with the cold chamber
  process
• Maintenance and replacement costs can vary
  significantly

16
9 Criteria Structural Properties

• Aluminum alloys have the highest modulus of
  elasticity (MOE)
• Relatively high strength/low density
• Magnesium has lower strength and rigidity
• Competitive with aluminum in some applications
  through strategic placement of reinforcing ribs
• ZA alloys offer highest tensile and yield
  strengths

17
9 Criteria Minimum Weight

• Magnesium alloys are the dominant choice if
  weight must be minimized

18
9 Criteria Impact Strength and Dent Resistance

• Highest among the zinc (Zamak) alloys
• Diminishes sharply as temperature is reduced
  below 32ºF (0ºC)
• Impact resistance of aluminum and magnesium
  alloys varies within each alloy group

19
9 Criteria Surface Finish

• Surface finish best achieved by the zinc and
  magnesium alloys
• Die steel surface quality is essential to casting
  surface quality

20
9 Criteria Corrosion Resistance

• Corrosion resistance varies from alloy to alloy
  and within an alloy group
• Can be improved with low-cost surface treatments

21
9 Criteria Bearing Properties and Wear
Resistance

• Bearing properties and wear resistance
• Good for hydrodynamic bearing applications
• If partial lubrication only, ZA alloys and 390
  aluminum resistant to abrasion and wear

22
9 Criteria Machineability

• Machineability of all alloys excellent
• Magnesium alloys offer best machinability in
  terms of tool life, energy consumption and low
  cutting forces

23
Freezing

• Same as water
• Various metals freeze at a particular temperature
  for that metal
• Time versus temperature chart is slightly
  different than that for elements (pure metals)
  and compounds

24
Quality

• Alloy chemical composition is controlled by an
  ASTM, American Society for Testing and Materials,
  specification
• Each die casting plan has a method for
  maintaining alloy quality
• Begins with purchasing of material, and continues
  through manufacturing process and shipment

25
Quality Cleanliness

• Cleanliness component of alloy quality
• Not as easily checked as chemical composition
• Each time alloy is melted, some material is
  oxidized
• Oxides are impurities in alloy that could affect
  the castings properties if not removed
• Processes to minimize amount of oxidation and
  remove the oxides from the alloy

26
Summary

• Designers consider range of issues when creating
  a die casting
• Each alloy has different mechanical properties
  tensile strength, yield strength, elongation, and
  MOE
• The alloy chosen must be appropriate to the die
  castings application
• Alloy selection is based on characteristics and
  properties of the alloys in 9 categories