Materials Science and Engineering, A 134 ( 1991 ) 1115- l 119 l 115

Rapid solidification of aluminiurn by the Hydro Aluminium process

Ole Johan Sorheim and Lars Auran Hydro A luminium R & D Centre, P.O.B. 219, N-6601 Sunndalsora (Norway)

Abstract

A proprietary method to produce low-cost particular rapid solidified (RS) material for subsequent precompaction and extrusion is described.

Production facilities for both granular production and precompaction/extrusion have been devel- oped and are now available for test runs. At Hydro Aluminium R & D Centre, Sunndalsora, a pilot plant for production of RS needles with annual capacity of up to 200 tons is operable. Cooling rates averaging as high as 10 ~-4 K/s can be achieved, and recent work on alloy development offers improved properties with respect to fatigue- and mechanical-strength and rigidity at both room- and elevated temperatures.

These property profiles and corresponding total production process cost estimates for RS aluminium show promise for future exploitations both in the low-cost high-volume automotive and the non- automotive market.

1. Introduction

In the last few decades, extensive research work has been carried out in several laboratories worldwide to exploit commercially the potential of RS production processes.

A decade ago, work done in the laboratory, mainly in the U.S., had already documented exist- ing improvements in the properties of aluminium alloys which had been rapidly solidified. A number of methods had been explored, new alloys developed and, for aluminium, new appli- cations suggested. At that time information on the relative merits of the various methods employed, production capacity and cost and potential market surveys was lacking.

To obtain an insight into these and other strate- gic questions, Hydro Aluminium (at that time Ardal og Sunndal Verk A/S) together with Norwegian and German industrial partners, about 10 years ago started a five-year project in which ewtluation of production methods, con- solidation techniques, alloy evaluation and market survey were areas of key interest.

This work was further carried on through a Scandinavian project from 1986 to 1989.

On the bases of conclusions and results from the above work, Hydro Aluminium's RS strategy since two years back has been to exploit an in- house developed process--based on the Hydro

Aluminium melt cleaning rotor--towards "large volume low-cost" markets.

A pilot RS aluminium production plant has been designed and set up at Sunndalsora, with the following objectives:

(1) investigation, understanding, evaluation and development of the production process;

(2) providing RS needles for ongoing research on consolidation of RS needles at Sunndalsora.

This part of the project involves alloy develop- ment and marketing, and covers both automotive and non-automotive alloys.

The present article presents and discusses some aspects of the pilot plant design, process total flowroute, product and material properties, areas of application etc. mainly from a technical/ economical point of view.

2. Plant/process description

2.1. General

An outline of the total process from melt, via needle production, precompaction and extrusion is shown schematically in Fig. 1.

Some further comments are given below.

2.2. Needle product ion

The needle production plant consists of five main units, these being induction and resistance

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PROCESS OUTLINE preheating

J r ' precomp.actlon ] melt ] ~ production [SSm" I extru . . . . I

treatment ["'lip (needles) 1 % ~ I / I I ' ~ preheating

| p . . . . p~ction

Fig. 1. P rocess flow diagram.

extrusion (industrial)

HA rotor for production of needles Fig. 2. H y d r o A lumin ium rotat ing crucible.

melting furnaces including a melt cleaning unit, a melt transfer system, a rotor, central control panel and a circular tank system for collecting the as-cast needles.

The capacity of the melting units is 5-600 kg per batch which is sufficient for further work on consolidation, alloy development and premarket- ing.

The metal may be melted down under a pro- tective atmosphere. After melting, alloying and melt cleaning the batch is transferred to a graphite crucible, into which the rotor pump is submerged during casting. The operation of the whole plant is directed from a central control panel.

Rotation of the pump section in the melt brings the melt up in the perforated barrel, and forces the melt through 1 mm diameter holes (Fig. 2). The melt droplets solidify in air.

Typical operational speed is 1000-2000 r.p.m. The Hydro Aluminium rotor produces needle- shaped particles at a solidification rate of about

CONSOLIDATION OF RS GRANULES

preheat preeornpaction heat extrusion

preheat extrusion

Fig. 3. P r ecom pac t i on / ex t ru s ion system.

102-105 K s -1 The cooling rate, size and shape of particles may be varied through changes in rotor design, metal level/pressure, rotating speed, etc., but typically size and shape varies in length from 3-8 mm and in thickness from 0.1-0.6 mm.

The Hydro Aluminium rotor is placed in the centre of a cylindrical tank, which means that as- cast needles are collected in the bottom of the tank and pneumatically transported to storage tanks, thus securing a clean high-quality end pro- duct.

2.3. Precompaction/extrusion The cast needles are consolidated either by

direct extrusion of the loose preheated needles in R & D presses, or they are precompacted to billet and heated before extrusion. This is shown in Fig. 3, where both methods are outlined.

Direct extrusion of the granules requires extru- sion ratios higher than 16: 1. Only 50% of the container capacity is utilized, due to the packing density of the granules. The deformation resis- tance is large--extrusion therefore requires break-through pressures of 800 MPa and more. Direct extrusion of the granules therefore requires large presses with high specific force.

Because of this fact, and as a result of the market need of larger product dimensions, in the last couple of years a proprietary method for pre- compaction of needles to extrusion billets has been developed and tested.

At present, precompaction equipment for 150-180 mm diameter billets for subsequent induction heating and extrusion in industrial extrusion presses are available. This method gives a 100% dense product and may be handled just like a direct chill (DC) cast extrusion billet. This precompaction method is still in the development stage. Further details will be released at a later time.

3. Results

3.1. General In general, the Hydro Aiuminium RS produc-

tion route has shown promise when it comes to material properties such as strength at elevated temperatures, fatigue strength, thermal expansion coefficient, Young's modulus, etc.

Comments on some alloy types with their respective properties and applications are given in the following sections.

3. 2. Alloy types A number of alloy systems have been studied

in the R & D programme carried out by Hydro Aluminium. The different systems are divided into three main alloy types, as seen in Table 1, these being

(1) automotive alloys (engine applications); (2) high-strength non-automotive alloys; (3) thermally stable non-heat-treatable alloys

(non-automotive). Additional alloy systems combining strength,

thermal stability and good electrical conductivity have also shown promising results.

3.2.1. Automotive alloys When high fatigue strength at elevated tem-

perature and/or low thermal expansion and to some extent increased stiffness are needed, as in engine components, silicon and nickel may be used as alloying elements.

Although the chemistry of IM and RS alloys for automotive applications are similar, the RS process gives improved properties due to refine- ment in structure, and allows beneficial increased additions of silicon plus transition elements (Table 2, Fig. 4).

The relative importance of these properties and which to emphasize differs, of course, for dif- ferent products and applications. Other prop- erties like wear resistance, hardness, thermal

TABLE 2

Property profile ot: Conrod Alloy

]117

MAALE RSAL 138 HA

Rp0.2 (MPa) 2(1 °C 220-260 320-350 150 °C 200-250 310-340 250°C 100 140 ~200

Rm I MPa'= 2(1 °C 230-300 350-380 150°C 210-260 34(I-37(I 250 °C 100-16t) ~ 220

Fatigue (MPa) 2(1 °C 9t)- 120 200 15(1 °C 70-10(1 180 250°C 5t)-70 100

Thermalexpansion ( x 10 ~') 20.3 18.(I (I 5o °c)

Ductility (A %) 20 °C 0.5- 1.5 5 15(1 °C 1-2 8 250 °C 3-5 10

200

Fatigue strength [N/mm = ]

RS-AISi15MnNiCrMg 150 ~ 12)

o~ NiUgCu

""-... 100 = ~ ' ,

1 f '~" AIS'IO- AlSi ONiMgCu ~ ' ~ 0//NiM,IC,, 50 +IO%SiC °'* "% ~ "

(0 16) ...... " ' % ' ~ /

0 ~ I i I i I 100 200 300

Temperature [°C]

Fig. 4. Fatigue properties of RS automotive alloy (Table 2). Rotating bending test at 25 x 1 (Y' cycles.

TABLE 1

Main types of Hydro Aluminium RS alloys--mechanical properties

Alloy type Rp0.2 Rm Ductility (MPa) (MPa) (A %)

Fatigu& Young's Thermal R - 1 modulus expansion (MPa) (GPa) × 10 " at RT

High strength 750 780 7 250 73 23 Forging alloy (automotive) 350 400 4 220 90 17 Temperature-stable alloy b 380 400 5 200 86 21

"Rotating bending x 107 cycles. bNo loss in strength after 1000 h exposure to 300 °C.

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conductivity, creep, forgability, etc., which are not listed in the above tables, but have been evalu- ated, might also be of vital importance. However, for the AI-Si-Ni-based alloys it is clearly in- dicated that their properties could open up new possibilities for application in automotive engines.

Potential areas of application for A1-Si-Ni- based alloys is moving parts in automotive engines such as piston pins, pistons and connect- ing rods.

Hydro Aluminium have ongoing cooperating projects with both French and German industry. Kolbenschmidt AG, Germany is now running an acceptance test for an alloy for piston application based on 55 mm diameter forging stock.

3.2.2. Thermally stable alloys These alloys are developed for application at

higher temperatures (200-350 °C). Their stability and strength at elevated temperatures are based on fine and stable dispersoids based on transition metal elements.

Alloy 223 (AI-2Fe-2Mn-3Ni + Mg, Zr) repre- sents here a typical composition with moderate additions of these transition metal elements to give a non-ageable alloy system with stable fine- grain structure.

Examples of improvements in stability and strength at elevated temperatures are shown in Fig.5 and are compared with an existing high- temperature ingot alloy AA 2618.

The highest temperature to which conven- tional IM or cast aluminium alloys can be exposed for a long time without losing strength is in the range of 125-200 °C, depending on alloy and temper. The RS A1-Fe-Mn-Ni-MM types of alloys have moved this barrier to 300-350°C. This opens up new perspectives for applications. Examples of possible products are "cold" com- ponents in turbocharger and building structure applications (fire resistance).

4. Cost considerations

The reason for selecting the modified rotating crucible method (the Hydro Aluminium produc- tion process)--out of several methods evaluated in the early 1980s--was primarily the potential low production cost.

In contrast to early market philosophy in U.S.A.--looking towards small, cost-insensitive aircraft and spacecraft applications--Hydro Alu-

Rp0.2 [MPa]

400

300

200

100

RSAL AI-8Fe-4MM 200 IVT before test DC AA 2618 AI.Mgl.5.Cu2.2-Nil.2.Fel.0 100 h/T before test

RSAL AI-4Mn.4Ni,-0.6Mg-O.6Zr 200 h/T before test

,, ", .

DC AA 60 \ AI-MgO 6 Sil ' / ~ , ~ ~ i \ 10 000 h/T before test ~

DC AI-SMg-0.SMn N,, 1 000 h/T before test %

0 100 200 300 400

Temperature [ °C ]

Fig. 5. Thermally stable RS alloys compared with some con- ventional DC cast alloys.

minium's strategy is to go for the high-volume low-cost European market.

As pointed out above the production of RS needles is the first step in a three-step process route. To make rapidly solidified aluminium cost competitive, it is necessary to minimize the costs of all three steps.

Addressing the production step first, the favourable production cost for needles is based on the following characteristics:

( 1 ) high production capacity; (2) 100% yield (no sieving); (3) small consumable cost (gas etc.); (4) simple handling and storage (no explosion

hazards); (5) simple reliable process. The cost of producing the needles is about one

third of the total variable production cost (ex metal) of as-extruded material.

For the low-cost market, consolidation by canning and the HIP process is too expensive. Several alternative processes have been studied and evaluated over the last 10 years in joint Norwegian programmes.

Precompacted material from the propriatory method looks promising and is now being tested in industrial extrusion presses.

The largest single cost factor of the variable production cost of extruded RS material is the extrusion step.

Typical for such temperature-stable RS mater- ial, the deformation resistance at the extrusion temperature is high, consequently the extrusion speed must be kept low to prevent overheating. Partly responsible for the low production output and consequent high cost today is an insufficient specific pressure in the majority of soft alloy pro- duction presses.

As the market develops, the extrusion cost may be brought down by acquiring dedicated extrusion presses with improved die material and use of liquid nitrogen cooled dies.

Even with the present extrusion technology, the aim of bringing the cost of extruded RS alu- minium forging stock to well within a factor two of comparable conventional DC cast material should be within reach.

5. Conclusion

Hydro Aluminium has chosen to use in-house

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developed technology to produce rapidly solidi- fied aluminium for the low-cost high-volume European market.

Marketing and product development will be based on production from a pilot plant at the R & D Centre at Sunndalsora. A pilot plant with a capacity of some hundred tons a year is now operational at the R & D Centre.

The property profiles and corresponding pro- duction cost estimates for RSAL shows promise for future exploitations both in automotive and non-automotive markets.

The actual European market for this new material is at present hard to quantify. The cost/ benefit factor will for each potential application (product) determine future volumes. As an example, just now a company in Germany is running acceptance tests for an alloy for piston applications.

To further reduce the cost of extruded alu- minium material, most is to be gained by reducing the extrusion cost by looking at entirely new con- cepts or acquiring dedicated direct or indirect extrusion presses.