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 1950’s • 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 casting’s 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 casting’s application • Alloy selection is based on characteristics and properties of the alloys in 9 categories