Magnesium with a density of 1.7 is the lightest structural material and is 40% lighter than aluminium. However, magnesium alloys have not been extensively used because of their lower mechanical properties and poorer corrosion resistance compared with aluminium. Bringing these properties up to a higher level will enable magnesium to play an increasing role in structural applications. The main objective of the project is to develop new magnesium based alloys with mechanical properties competitive to the best aluminium alloys at room and elevated temperatures and with better corrosion resistance than conventional magnesium alloys, using the rapid solidification processes.
The magnesium alloying process consisted of 3 steps to produce a final forged profile. The first step was the manufacture of the alloy by melt spun or planar flow cast ribbons at temperatures below 850 C. The second step of consolidation was by extrusion at low speed. At this stage of the procedure the microstructure and the subsequent mechanical properties of the finished product were established. The lower the temperture the stronger the product; subsequent heat treatment was found to be superfluous. The final step involved forging to obtain specific configurations; this process leaves the microstructure unaltered.
The extremely fine grain and dispersoid size of rapid solidification processing led to the use of unusual alloying elements. The most promising combinations were:
magnesium/aluminium with calcium or strontium as ternary alloying elements and further additions of zinc, manganese and rare earths;
magnesium/rare earths/zinc with possible additions of zirconium and calcium.
Thesecombinations gave alloys of outstanding strength (60% greater than the strongest conventional wrought magnesium alloy and twice that of conventional extrusions) giving a product with the strength of conventional aluminium alloy (AA7075) utilized in the aeronautical industry but with a 35% weight reduction. The creep resistance of the various alloys was found to be acceptable with that of the calcium containing magnesium/aluminium based alloys being greater than conventional sand cast magnesium/aluminium (AZ91) products up to 150 C. Rare earth alloys gave the most resistance. Magnesium/aluminium/calcium and magnesium/ aluminium/strontium systems showed corrosion resistance similar to high purity sand cast magnesium/aluminium alloy (AZ91 E T6) but AZ91/calcium products were 3 times more resistant than the conventional alloy, giving the most resistant magnesium alloy ever produced and comparable to die cast aluminium alloys used in the automotive industry.
THE RAPID SOLIDIFICATION TECHNOLOGY ITSELF REPRESENTS AN INNOVATIVE PROCESS WITH INTERESTING ECONOMICAL POTENTIAL. GROWING INTEREST IN THIS FIELD HAS LEAD NORSK HYDRO AND PECHINEY TO SET UP A JOINT PROGRAMME IN THE RAPID SOLIDIFICATION OF MAGNESIUM ALLOYS. NORSK HYDRO AND PECHINEY HAVE BOTH EXPERIENCE IN RSP TECHNOLOGY THROUGH WORK ON ALUMINIUM.
THE EXPECTED INCREASE IN PROPERTIES WILL EXTEND THE USE OF WROUGHT MAGNESIUM ALLOYS TO AREAS WHERE ALUMINIUM ALLOYS UNTIL NOW HAVE CONTROLLED THE MARKET (TRANSPORT, AEROSPACE...)
THE PROGRAM WILL BE RUN AS A JOINT PROJECT BETWEEN NORSK HYDRO AND PECHINEY. IT CAN BE DIVIDED IN SUB-PROJECTS:
1.- PRELIMINARY EXPERIMENTAL WORK AND DETAILED PLANNING.
2.- PRODUCTION OF MASTER ALLOYS.
3.- PRODUCTION OF RAPIDLY SOLIDIFIED ALLOYS (MELT-SPINNING AND PLANAR FLOW CASTING). CONSOLIDATION EXTRUSION.
4.- EVALUATION OF MATERIALS.
THE PROGRAM HAS BEEN PLANNED AS AN INTEGRATED WHOLE IN ORDER TO TAKE THE MAXIMUM ADVANTAGE OF THE INTERACTION AND THE SYNERGY ARISING THEREFROM. THUS SOME OF THE SUB-PROJECTS WILL BE CARRIED OUT BY THE TWO COMPANIES IN COLLABORATION, SOME TASKS WILL BE DIVIDED AND SOME WILL BE CARRIED OUT BY PARALLEL WORK.