HOT WORKING ALUMINUM BASE ALLOYS

Abstract: Discloses hot working by rolling or forging of mechanically alloyed aluminum-base alloys containing 5 to 35 volume percent of an aluminum transition metal intermetallic phase, e.g. Al₃Ti which is insoluble in the solid aluminum matrix. Hot working is carried out at a temperature above about 370°C.

Description: The present invention is concerned with hot working of aluminum-base alloys and, more particularly, with hot working by forging, rolling and the like aluminum-base alloys having an ultra-fine hard dispersed transition-metal-intermetallic phase in the microstructure, this intermetallic dispersed phase being of such a character that it cannot be solubilized in the aluminum matrix below the melting point of the matrix. It is known to produce dispersion hardened aluminum-base alloys by powder metallurgical methods and, more particularly, to use the process known as mechanical alloying in the production of such alloys. Generally, a mechanically alloyed (or otherwise formed) aluminum powder containing a dispersoid is hot compressed in a vacuum and consolidated and formed by extrusion. A problem exists in producing useful shapes from the dispersion hardened aluminum bar stock provided by extrusion when the bar stock contains significant amounts of dispersed, transition metal, intermetallic phase insoluble in the solid aluminum matrix. Ordinarily a cheap, generally applicable metallurgical solution to providing useful shapes from extruded or otherwise formed bar stock is hot working by forging, rolling or the like. In such processes, unlike extrusion, metal is free to expand in more than one direction. Generally speaking such forging, rolling and the like is done hot because at high temperatures metal is weaker and has good ductility. At high temperatures precipitated strengthening phases dissolve; matrices change from one phase to another, e.g. ferrite to austenite; and generally workability as indicated by tensile elongation is enhanced. An exception exists in the case of mechanically alloyed dispersion-hardened aluminum containing insoluble intermetallic dispersoid. It has been observed in mechanically alloyed aluminum-base alloys containing Al₃Ti dispersant that, as the test temperature rises, while the strength of dispersion-hardened aluminum alloy decreases, the ductility as measured by elongation in tensile testing, also decreases. The ductility of two- (or multi-) phase alloys is most commonly discussed in the art in terms of the volume fraction of the hard phases. Previous theoretical as well as experimental studies have demonstrated that at a given temperature, particularly at room temperature, alloy ductility (as evidenced by the elongation to fracture during a tensile test) decreases sharply as the volume fraction of the hard phase increases. From previous empirical work, a simple relationship has been developed relating ductility and hard-phase volume fraction: In this equation k is an empirical constant (whose value depends upon the characteristics of the matrix alloy), and f is the volume fraction of the hard phase. The above relationship has been shown to hold approximately true at room temperature for a variety of dual or multi-phase alloys, including Al-SiC composites. Applicants have discovered that in aluminum alloys made by mechanical alloying and containing dispersed hard phase made of an aluminum-titanium metal intermetallic compound (e.g., Al₃Ti) which is essentially insoluble below the solidus of the aluminum matrix, the tensile elongation at all temperatures is in excess of what would previously have been expected in mechanical alloyed aluminum alloys at least over the range of 15 to 35, advantageously 15 to 30, volume percent of intermetallic phase. Even more unexpectedly, applicants have discovered that at temperatures in excess of 370°C, e.g. 427°C and higher, but below the solidus temperature of the matrix, alloys prepared by mechanical alloying and containing 15-35 volume percent Al₃Ti in an aluminum matrix along with dispersed Al₄C₃ and Al₂O₃ have tensile elongations in excess of 5% and are therefore amenable to hot working. In contrast, work done by applicants' former colleagues on mechanically alloyed aluminum-base alloys containing titanium and reported to Wright Aeronautical Laboratories as published in Technical Report AFML-TR-79-4210 showed tensile elongation decreasing with temperature to 2.5% and 1-3% at 343°C for alloys containing 4.13 and 10 volume percent Al₃Ti dispersant respectively. Based upon the knowledge of mechanically alloyed aluminum alloy systems available at that time, the occurrence of anomalously high ductility at temperatures higher than 343°C was completely unknown to those of normal skill in the art. Moreover, applicants have discovered that the present worked alloys retain good strength, ductility and stable micro-structure. Based on this discovery, the invention aims to provide a hot working process for a dispersion-hardened aluminum alloy made by mechanical alloying wherein the hard phase is present in an amount of 15 to 35 volume percent and comprises an aluminum-titanium or aluminium-titanium-niobium metal inter-metallic compound, essentially insoluble in the aluminous matrix at temperatures below the solidus temperature of the matrix. The invention includes the hot worked alloy product. The method of the present invention comprises forming a hot-worked metal object by hot working, by a process in which the metal is free to expand in more than one direction, a mechanically alloyed aluminum-base alloy consisting of an aluminum matrix having dispersed therein 15 to 35 volume % of an aluminum-titanium or aluminum-titanium-niobium intermetallic compound insoluble below the solidus temperature of the aluminum matrix, carbide phases, principally aluminum carbide, in an amount up to 14 vol. % and oxidic phases in an amount up to 5 vol. % of the alloy, the balance, apart from impurities, being aluminum, said hot working being conducted in the temperature interval between 370°C and the solidus temperature of the aluminum matrix. The invention also includes the resultant hot-worked alloy which exhibits a unique combination of strength, modulus, ductility and stability over a range of temperatures up to about 95% of the melting temperature (0.95 Tm). The aluminum-base alloys to be hot worked in accordance with the present invention are made by mechanical alloying following generally procedures as described in U.S. Patent Nos. 3,740,210, 4,668, 470 and 4,668,282 using stearic acid as a process control agent. The levels of carbide and oxide set forth in the preceding paragraph generally derive from the levels of process control agent normally used in mechanical alloying with or without intentional inclusion of oxide, e.g. alumina or yttria or carbon in a mechanically alloyed charge. For example, up to about 5 volume percent carbide and 2 volume percent oxide are the usual amounts of these phases encountered when stearic acid is employed as the process control agent with no other non-metallic additions to the charge. Those skilled in the art will appreciate that, although levels above 5 volume percent carbide and 2 volume percent oxide can be present in hot worked alloys of the invention, one can expect decreased alloy ductility at such high levels. Compositions of hot worked aluminum-base alloys are set forth in Table 1. COMPOSITIONS OF MA Al-Ti BASED ALLOYS Alloy No. Composition (Wt. %) Al Ti C O Other 1 Bal. 6.0 2.20 0.75 -- 2 Bal. 8.7 2.60 0.85 -- 3 Bal. 9.7 1.50 0.60 -- 4 Bal. 9.7 1.58 0.55 2.10 Nb 5 Bal. 12.3 1.50 0.85 -- Those skilled in the art will appreciate that the percent by weight compositions set forth in Table 1 can be converted to approximate percent by volume of phases such as Al₂O₃, Al₄C₃, Al₃Ti and the like by simple formulas such as: Wt. % 0 x 1.7 = Vol. % Al₂O₃ Wt. % C x 3.71 = Vol. % Al₄C₃ Wt. % Ti x 2.5 = Vol. % Al₃Ti The alloys in Table 1 contain roughly 15 to 31 volume percent of aluminum-titanium or aluminum-titanium-niobium intermetallic phase, specifically in alloys 1-3 and 5 the phase being Al₃Ti in the range of 15 to 31 volume percent. In alloy 4 the intermetallic phase is a combination made up principally of Al₃Ti along with aluminides and/or other compounds of niobium. Those skilled in the art will appreciate that the "intermetallic phase" may be a single phase or more than one phase, no specific limitation being implied by the singularity of the term "intermetallic phase". After mechanical alloying, alloys 1-5 were consolidated and extruded at about 400°C using an extrusion ratio of about 15 to 1. Tensile characteristics of the as-extruded alloys are set forth in Table 2. MECHANICAL PROPERTIES OF MA Al-Ti BASED ALLOYS Alloy No. T UTS YS e f E 1 24 467.6 379.4 14.0 88.9 427 NA. N.A. N.A. 2 24 471.1 375.9 12.0 98.0 427 N.A. N.A. N.A. 3 24 487.2 464.8 7.1 96.6 427 112.0 100.8 8.3 4 24 530.6 478.1 8.9 100.1 427 N.A. N.A. N.A. 5 24 441.3 372.3 10.0 100.0 427 N.A. N.A. N.A. All of the alloys set forth in Table 1 were successfully hot rolled in the temperature range of about 400°C to about 510°C from 50 x 100 mm thick bar to sheet about 1.5 mm thick and about 90 to 100 mm wide. In sheet form, these alloys retained excellent combinations of strength, ductility and modulus indicative of stable microstructures as exemplified for alloys 3 and 5 by the data given in Table 3. TENSILE PROPERTIES OF MA Al-Ti ALLOYS IN SHEET FORM Alloy No. T UTS YS e f E 3 24 441 413 11.0 93.1 150 343 308 6.2 -- 315 196 167 4.3 -- 427 112 102 12.1 -- 5 24 465 430 9.0 100.0 150 350 321 4.9 -- 315 202 179 3.2 -- 427 120 109 10.3 -- Carbide phases that may be present in addition to aluminium carbide include titanium carbide, carbides of other alloy ingredients and chemical modifications of aluminum, titanium and other carbides. The term "oxidic phase" is intended to include not only aluminum oxide formed by reaction between aluminum and oxygen in the stearic acid process control agent during mechanical alloying but also small amounts not exceeding 5 volume percent of other oxide, e.g. yttria, yttrium-aluminum-garnet or alumina which might be added to or formed while processing a mechanical alloying charge. While specific embodiments of the invention are described herein, those skilled in the art will understand that the invention is not limited to these embodiments.

Claims: A method of producing a hot-worked metal object which comprises forming a hot-worked metal object by hot working, by a process in which the metal is free to expand in more than one direction, a mechanically alloyed aluminum-base alloy consisting of an aluminum matrix having dispersed therein 15 to 35 volume % of an aluminum-titanium or aluminum-titanium-niobium intermetallic compound insoluble below the solidus temperature of the aluminum matrix, carbide phases, principally aluminum carbide, in an amount up to 14 vol. % and oxidic phases in an amount up to 5 vol. % of the alloy, the balance, apart from impurities, being aluminum, said hot working being conducted in the temperature interval between 370°C and the solidus temperature of the aluminum matrix. A method according to claim 1 in which the carbon content of the alloy is at least 1.5% by weight and the oxygen content is at least 0.55% by weight of the alloy. A method according to claim 1 or claim 2 in which the amount of carbide phase does not exceed 5% by volume and the amount of oxide phase does not exceed 2% by volume of the alloy. A method according to any preceding claim in which the hot working is carried out in the temperature range of 400 to 510°C. A method according to any preceding claim wherein the hot working is performed by hot rolling.

Inventors: BENN, RAYMOND CHRISTOPHER, MATTSON, WALTER ERNEST, MIRCHANDANI, PRAKASH KISHINCHAND
Applicants: INCO ALLOYS INTERNATIONAL, INC.
Application year: 1989
Main classification: C22F1
IPC codes: C22C1, B22F3, C22F1, C22C32, C22C21