1) Title of the invention
Melting and Heat Treatment Practice for Magnesium Alloy Casting GA9 to Improve Mechanical Properties for Aerospace and Automobile Applications.
2) Field of invention
The present invention is a melting and heat treatment process designed to achieve desired mechanical properties by controlling chemistry with use of prefabricated alloy, degassing process and the aging (precipitation) process in the GA9 alloy.
3) Use of invention
The melting and heat treatment process is used to improve mechanical properties of magnesium alloy GA9 higher than the specification requirements. The GA9 alloy casting is used in structural components like housing and casings of engine components for automotive and aerospace applications.
4) Prior art
The magnesium casting alloy GA9 was cast with composition in the range of 7.7-8.9% Al, 0.1 to 1 % Zn and 0.1 to 0.6 % Mn. The gases dissolved in the molten metal during melting were degassed with a hexachloroethane based degasser before casting. The degasser was added 0.15 % by weight. of the total charge in the crucible. The castings were subjected to solutionising heat treatment as follows.
Solutionising: at 410 ± 5 °C for 6 hrs. ±5 minutes Solutionising: at 350 ± 5°Cfor 2 hrs. ±5 minutes Solutionising: at 410 ±5 °C for 10 hrs. ±5 minutes followed by air cooling.
5) Draw backs of prior art
After following the melting practice and the heat treatment process described in the previous paragraph, the mechanical properties obtained were not consistent and most of the time were not meeting the GA9 specification requirements. The mechanical properties requirement given in GA9 specification are tabulated below-

6) Comparison between prior art and present invention
The present invention uses a prefabricated GA9 alloy with an optimized chemistry from lower range of the GA9 specification instead of directly melting the alloy (without prefabrication) with a composition from entire range given in the specification. The degasser is added with 0.3% by weight of the total charge instead of Q.15 %. In the present invention solutionising and aging heat treatment is done on the casting instead of only solutionising heat treatment to achieve mechanical properties given in the specification.

7) Aim of the invention
The aim of the present invention is to consistently meet or exceed the mechanical properties requirement of the magnesium casting alloy QA9.
8) Summary of the present invention
The present invention is development and optimization of melting and heat treatment process for GA9 castings. The alloy finds application in automotive and aerospace industries as structural members. For the said applications, the component needs to have excellent mechanical properties. However, the melting and heat treatment process parameters defined for the alloy is unable to consistently achieve the specification requirements.
This shortcoming is addressed in the present invention by the re-melting of prefabricated alloy with optimum chemistry (Al 8.1-8.3 %, Zn 0.45-0.55 % and Mn 0.2-0.4 %) followed by degassing with 0.3 % degasser and solutionising and aging heat treatment.

10) Statement of invention
The melting procedure and heat treatment process developed in the present invention for the magnesium casting alloy GA9 yield consistent mechanical properties which exceeds the GA9 specification requirement by the re-melting of the prefabricated alloy with optimum chemistry (Al 8.1-3.3 %, Zn 0.45-0.55 % and Mn Q.2-0.4 %) followed by degassing with 0.3 % degasser and solutionising and aging heat treatment.
11) Detailed description of invention
The detailed melting procedure of Magnesium-alloys GA9 in an oil fired furnace of capacity 100 kg for pouring aeronautical grade castings and heat treatment process. The complete process is divided into three major parts, PART A, PART B and PART C.
PART A: Fabrication of the alloy of specific composition.
PART B: Melting of the alloy fabricated in PART A and pouring of castings.
PART C: Heat treatment of castings and test bars cast in PART B.

PROCEDURE
Heat and clean mild steel crucible.
Charge items 'a' and 'b' from materials listed in previous paragraph along with flux into the crucible. Keep items 'c' and 'd' along the edge of the furnace for Preheating. After melting items 'a' and 'b' , charge preheated items 'c' and 'd' and sprinkle flux on the top of the melt to cover any exposed areas. Raise temperature of the melt to around 750°C and hold the temperature for 10 minutes.
Clean:off dross and slag using skimmer. Mix the metal thoroughly by puddling appropriately to homogenize chemical composition. Let the melt settle down and cover the melt with flux.
Lift out steel crucible from furnace, push back flux cover from lip and pour ingots at temperature around 720PC. Sprinkle flowers of sulphur on the open liquid metal surface of ingots immediately after completion of pouring in each ingot. Discard the last two ingots.
PART B: Melting of the alloy fabricated in PART A and pouring of castings
PROCEDURE
Heat and clean mild steel crucible. Charge alloy fabricated in PART A (preferably from same batch) along with flux, into the: crucible. Meltdown. Sprinkle flux on the top of the melt to cover any exposed areas. Raise temperature of the melt to around 750°C.
Degas the molten metal at 750°C by plunging 0.3 % by weight of the prefabricated alloy hexachloroethane based degasser. Mix the metal thoroughly by puddling the melt appropriately for uniform degassing. Let the melt settle down.
Raise the temperature to about 780°C. Clean-off dross and slag using skimmer. Cover the metal bath with 1" layer of flux. Allow the flux to melt completely and give a thorough flux wash to the metal.
Lift out steel crucible from furnace, push back flux cover from lip and pour castings at temperature around 720°C. Sprinkle flowers of sulphur on the open metal stream. Leave back some amount of metal in the crucible.

Heat treatment
Heat treatment for GA9 alloys shall be done as folloyvs. Three step solution heat treatment
1. Solutionising at 410°C for 6 hours ± 5 minutes. Furnace cool to 350 °C
2. Solutionising at 350°Cfor 2 hours ± 5 minutes. Raise temperature to .410 °C
3. Solutionising at 410°C for 10 hours ± 5 minutes. Fan cool to room temperature.
Solutionising heat treatment is followed by aging heat treatment at 210°C for 2 hours. All the heat treatment shall be done under protective atmosphere of Sulphur.
Process Validation: The process is validated by assessing Tensile properties. The standards followed for conducting tensile test is ASTM E8/E8M. The average mechanical properties achieved by following above melting procedure and heat treatment is tabulated below.
12) Drawing and Detailed Examples
The process is validated by assessing tensile properties of two consecutive melts (Melt-1 & Melt-2). In each melt two test bars were tested for mechanical properties. The results of tensile tests in solutionised and aged condition are tabulated below-
Claims "We claim"
1. The melting and heat treatment process developed for the alloy GA9 is a new, the
details of which is hitherto not available in open literature. 2.- The process has been developed by Foundry and Forge Division of HAL, Bangalore and
the parameters are optimised for GA9 Mg-alloy castings. 3. The use of prefabricated alloy is a new concept to achieve close tolerances over
chemistry. . 4. The chemistry of the prefabricated GA9 Mg alloy is optimized.
5. By re-melting of prefabricated alloy with optimum chemistry followed by degassing using optimum amount of degasser and solutionising & aging heat treatment, can achieve excellent mechanical properties.
6. The amount and type of degasser used is optimized.
7. The aging heat treatment process is optimized and applicable only for the melting process (chemistry) developed.
8. The process will be applicable for various 'Housing' and 'Casing' applications of most of the Aerospace and Automobile machine components.