A STUDY OF CASTING CHARACTERISTICS FOR DIE-CAST ALUMINUM ALLOY

Transcription

1 ME8109: Casting And Solidification of Material A STUDY OF CASTING CHARACTERISTICS FOR DIE-CAST ALUMINUM ALLOY Department of Mechanical & Industrial Engineering Graduate Program in Mechanical Engineering Ryerson University ME8109: Casting And Solidification of Material Submitted To: Dr. C.RAVINDRAN Prepared by: Anwar Hussain 1

2 CONTENTS: INTRODUCTION CAST ABILITY TYPE OF STANDARD DIE CAST ALUMINUM ALLOYS CHEMICAL COMPOSITION PHYSICAL AND MECHANICAL PROPERTIES CHARACTERISTICS OF DIE CAST ALUMINUM ALLOYS COMPARISON DIFFERENT DIE CAST MATERIAL WITH DIE CAST AL-ALLOY DIE SOLDERING: SLUDGE FORMATION EFFECT OF HOLDING TEMPERATURES FOR SLUDGE FORMATION SLUDGE FORMATION FACTORS FLUDITY FACTORS WHICH EFFECT ON FLUDITY POROSITY 2

3 INTRODUCTION Die casting technology is one of the mainly applied manufacturing practice In the die casting process the molten metal is injected under pressure into the hardened steel dies. Different casting processes require the alloy to have different casting characteristics. We will define here the different casting characteristics of die cast AL- Alloy 1) Cast ability 2) sludge Formation 3) Alloy fluidity 4) Machine ability 5) Die soldering 6) Porosity 7) Macro segregation 3

4 CASTABILITY: Cast ability is the property of an alloy to be cast without formation of defects such as cracks, segregations, pores or mis runs. Alloy dependent the phenomena that determine castability, fluidity, macro segregation, hot tearing and porosity. Cast ability of aluminum alloys can also defined such as solidification path, dendrite coherency, solidification shrinkage and inter dendrite permeability. The influence of alloy composition on fluidity, macro segregation, hot tearing and porosity will be described. coherency, fluidity, permeability and shrinkage will be presented 4

5 TYPE OF ALUMINUM DIE CASTING ALLOYS Aluminum die casting alloys are lightweight, offer good corrosion resistance, ease of casting, good mechanical properties and dimensional stability. Although a variety of aluminum alloys can be die cast from primary or recycled metal, designers select a standard alloy listed below. A Selected for best corrosion resistance and pressure tightness. A The most common and cost effective of AL- die casting alloys. Provides the best combination of utility and cost. A383 & A These alloys are a modification of 380. Both provide better die filling but with a moderate sacrifice in mechanical properties such as toughness. A Selected for special applications where high strength, fluidity and wearresistance and bearing properties are required. A413 (A13) -- Used for maximum pressure tightness and fluidity. 5

6 CHEMICAL COMPOSITION OF DIE CAST ALUMINUM ALLOYS The following table will provide a general idea of the differences in different Aluminum Die Cast alloys. This table will help us technical comparison in Al. alloys ( ALLOY CHEMICAL COMPOSITION) ALLOY COMPOSITION (% max or range) A360 A380 A383 A384 A390 A413 (A13) Silicon Iron Copper Manganese Magnesium Nickel Zinc Tin Titanium Total others Aluminum BAL. BAL. BAL. BAL. BAL. BAL. 6

7 PHYSICAL AND MECHANICAL PROPERTIES ALLOY A360 A380 A383 A384 A390 A413 PROPERTIES (A13) Ultimate tensile strength (ksi) Tensile yield strength (ksi) Elongation (% in 2" G.L.) Hardness (HB) Shear strength (ksi) Charpy impact strength(ft.lb) Fatigue strength (ksi) Density (lb./in.3) Melting range (of) approx Specific heat (Btu/lb.o F) Coefficient of thermal expansion Thermal conductivity (Btu/ft hr. of) Electrical conductivity (% IACS) Modulus of elasticity (106 psi)

8 CHARACTERISTICS OF DIE CAST ALUMINUM ALLOYS (1-most desirable; 4 least desirable) CHARACTERISTICS ALLOY A360 A380 A383 A384 A390 A413 (A13) Resistance to Hot Cracking Pressure Tightness Polishing Fluidity Corrosion Resistance Machine-ability Strength at Elev. Temp Anti-Die Soldering Tend Electroplating Anodizing Appearance

9 COMPARISON THE PROPERTIES OF DIFFERENT DIE CAST MATERIAL WITH DIE CAST AL-ALLOY Aluminum Brass Magnesium Zinc Tensile strength, psi x Yield strength, psi x 100 (0.2 pct offset) Shear strength, psi x Fatigue strength, psi x Elongation, pct in 2 in Hardness (Brinell) Specific gravity Weight, lb/cu. in Melting point (liquid), F Thermal conductivity, CG Thermal expansion, in./in./ F x Electrical conductivity, Modulus of elasticity, psi x Impact strength (Charpy), ft/lb

10 10

11 11

12 DIE SOLDERING: sticks to the surface of the die material and remains there after the ejection of the cast part. Die soldering is referred to the phenomenon that molten aluminum Die Soldering is the result of an interface reaction between molten aluminum and the die material during the impact of the high-velocity molten aluminum onto the die surface and the intimate contact between alloy and die at high temperature When molten aluminum enters in the die with a high velocity and destroys the protective film (coating and lubricant) on the die surface. the molten aluminum comes in contact with the virgin die surface. iron in the die dissolves into the molten aluminum and a layer of inter metallic phases is formed. A soldering layer is formed over this intermediate layer at an atomic level, which is difficult to prevent 12

13 DIE SOLDERING: There are several classes of process parameters that influence die soldering. These are under : Temperature of the metal and die. Nature and constituents of casting alloy and inter metallic layers. Die Lubrication and coating. Nature of the die and operating parameters. THE MECHANISM OF DIE SOLDERING OCCURS IN SIX STAGES: STAGE I ==> Erosion of grain boundaries on the die surface STAGE II ==> Pitting of the die surface STAGE III ==> Formation of iron-aluminum compounds STAGE IV ==> Formation of pyramid shaped inter metallic phases STAGE V ==> Adherence of aluminum onto the pyramids of inter metallic phases 13 STAGE VI ==> Merging and straightening of erosion pits and inter metallic phases.

14 EFFECT OF VARIOUS ELEMENTS ON THE INTERMEDIATE LAYER THICKNESS. The effects of alloy composition on the die soldering for the A 380 type alloys and effects of some key elements on the growth of the intermediate layer between the tool steel surface and the soldered Aluminum. ELEMENTS AMOUNT EFFECT Nickel 0.5% Alloy Layer thickness increases by about 50% at C Manganese 1 3% Same as above Beryllium 0.3 2% Alloy layer reduces by 7% Copper ---- No effect Free Nitrogen % Alloy layer thickness reduces by about 70%. Chromium 2 20% Alloy layer reduction by about 60% Titanium 0.1% Alloy layer decreases by 85% Silicon Alloy layer thickness decreasesas Silicon content increases 14

15 15

16 SLUDGE Sludge is made up of primary crystals containing Al, Si, Fe, Mn, Cr, Mg, etc. and having high melting temperature and high specific gravity. Factors that affect sludge formation: Alloy composition. Melting and holding temperatures Cooling rate Sludge factor (SF) (SF) = (1 x wt % Fe) + (2 x wt% Mn) + (3 x wt % Cr With an increase in cooling rate, the size of the sludge particles and the volume fraction of sludge decrease,sludge formed at the low cooling rate as large platelets, and polyhedral particles. At the high cooling rate, SLUDGE is in the form of platelets and star-like particles 16

17 EFFECT OF HOLDING TEMPERATURES Relation of sludge forming temperature with Fe content: Temperature ( C) = (%Fe)2 TEMPERATURE VERSUS SLUDGE FACTOR FOR SPECIFIC ALLOY 17

18 When the alloys solidify at low cooling rate, holding at 670 C and 720 C does not affect sludge formation. When the alloys solidify at high cooling rate,more sludge was found in A380 alloy afterholding at 670 C 18

19 SLUDGE FORMATION FACTORS: In 380 type alloys, the sludge phases are mainly Fe-rich compounds consisting of Al, Fe, Mn, Cr, Ni. Alloys with higher SF have a higher tendency to form sludge and form larger volume fraction of sludge. The morphology of sludge is influenced by the Fe:Mn: Cr ratio and by the cooling rate. Slow cooling favours formation of sludge. Holding at 670 C (~1240 F) and 720 C (1328 F) does not affect sludge formation in all the alloys solidify slowly. When cooling fast, more sludge formed in A380 alloy after holding at 670 C than at 720 C. 19

20 FLUIDITY: Fluidity is a material s ability to flow into and fill a given cavity, as measured by the dimensions of that cavity under specific conditions. The length of molten liquid metal that can flow through a given mould before freezing Fluidity is heavily dependent on heat flow during solidification. Fluidity of Al- Alloy depends on lubricant coatings, alloying additions, head pressure, and temperature of molten metal. Increasing the solidification range results in decreasing fluidity FACTORS WHICH EFFECT ON FLUDITY: Alloy chemical composition play a important role in die filling that is the fluidity of molten metal. Alloying Elements that lean to form high temperature compounds, e.g., Fe, Mn, Cr, and Mg, tend to decrease fluidity.

21 L f = CASTING CHARACTERISTICS FOR DIE-CAST ALUMINUM ALLOYS FLUDITY LENGTH DEPENDENT VARIABLE (Lf) L f = = (C /H) L f Final length, fluidity a Channel Radius C Specific Heat of Liquid Metal T O Ambient environmental Temperature (room temperature) T Superheat Temperature ρ Density of metal V O Velocity of metal flow H Heat of Fusion of metal h Heat transfer coefficient of metal interface T Time average melt temperature in the fluidity test T m Metal melting temperature T Temperature of superheated metal entering flow channel Critical solid concentration required to stop flow in mushy alloy. 21

22 POROSITY: Hydrogen gas dissolve in the liquid molten aluminum alloy from the atmosphere. Its solubility varies directly with temperature and the square root of pressure. During the cooling and solidification of molten aluminum, dissolved hydrogen in excess of the extremely low solid solubility may precipitate in molecular form, resulting in the formation of primary and / or secondary voids There are two types of hydrogen porosity occur in the die cast. One is the inter-dendritic porosity, which is encountered when hydrogen contents are sufficiently high that hydrogen rejected at the solidification due to high pressures above atmospheric. Secondary (micron-size) porosity occurs when dissolved hydrogen contents are low, and void formation is characteristically critical. 22

23 THANK YOU & Question 23