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1 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 247 EVALUATION OF RESULTS OF COMPRESSION AND TENSILE ANALYSIS OF COMPACTS ON THE BASE OF 1 µm ALUMINIUM POWDER Donič T., Martikán M. University of Žilina, Faculty of Mechanical Engineering, Centre of Technological Plastometry, Univerzitná 1, Žilina, Slovak Republic tibor.donic@fstroj.uniza.sk, milan.martikan@fstroj.uniza.sk HODNOTENIE VÝSLEDKOV TLAKOVÝCH A ŤAHOVÝCH ANALÝZ KOMPAKTOV NA BÁZE HLINÍKOVÉHO PRÁŠKU S VEĽKOSŤOU ČASTÍC 1 µm Donič T., Martikán M. Žilinská Univerzita v Žiline, Strojnícka Fakulta, Katedra Aplikovanej Mechaniky, Centrum Technologickej Plastometrie,Univerzitná 1, Žilina, Slovenská Republika tibor.donic@fstroj.uniza.sk, milan.martikan@fstroj.uniza.sk Abstrakt V príspevku je prezentovaná kompaktácia hliníkového prášku s veľkosťou častíc 1 µm metódou dopredného pretlačovania pri rôznych termodynamických podmienkach. Prezentovaný je experimentálny systém obojstranného lisovania s následným dopredným pretlačovaním pri rôznych redukčných pomeroch. Použité sú metódy plánovaného experimentu, ktoré sú spojené s experimentálnymi analýzami variácii teplôt extrúzie deformačných pomerov a vlastných redukcií. Východisková hustota polotovarov kompaktov bola získaná po procese obojstranného lisovania. Kompresné skúšky boli realizované pri izbových teplotách bez špeciálnej úpravy kontaktných plôch a bez maziva. Pri kompresných skúškach nebola hodnotená zmena pôvodného valcovitého tvaru experimentálnych vzoriek. Z funkčných závislostí F = f( h) bola kalkulovaná funkčná závislosť σ = f(φ), kde φ = ln h/h 0. Najväčšie deformačné energie boli identifikované pri kompaktoch, ktoré boli získané pri teplotách 300 a 450 C. Prezentované sú tiež funkčné závislosti σ = f( h), Hollomonove exponenciálne funkcie ako aj výsledné deformačné energie v procese skúšok. Podrobne boli analyzované exponenty deformačného spevnenia pri ťahových skúškach kompaktov na báze 1µm hliníkového prášku. Exponenty deformačného spevnenia boli jednoznačne funkciou difúznych procesov, ktoré v procese kompaktácie veľmi pravdepodobne boli iniciované. Exponenty deformačného spevnenia boli na úrovniach od n=0,63 kompakt získaný 300 C po n=0,9 pre kompakt získaný pri 450 C. Ťahové skúšky ako aj kompresné skúšky boli realizované pri statických podmienkach zaťažovania a sú uvedené veľmi podrobne. Uvedené je tiež hodnotenie metalografické a REM analýza, ktorá umožňovala analýzu lomových plôch. Abstract This article presents the issue of the compaction of 1 µm aluminium powder by the forward extrusion method at different thermodynamic conditions. Experimental systems for twosided pressing and subsequent forward extrusion process at different reduce ratios are presented in detail. The methods of the planned experiment are conducted under experimental analyses and result in variations of various extrusion temperatures, deformation rates, and reduce ratios. Initial, precompact density factors of the compacts were obtained by a two-sided pressing

2 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 248 process. Compression tests are realized at cold one dimensional pressing without special treatment of contacts planes and without use of lubricant. There wasn`t evaluated change of original cylindric shape of experimental specimens. Function relation σ = f(φ), where φ = ln h/h 0 was calculated from function relations F = f( h). The highest deformation energies were identificated on compacts produced at tepmeratures of 300 and 450 C. Functionalities σ = f( h), Hollomon`s exponential functions as well as total deformation energy are presented too. Deformation hardening exponents at tensile test of compacts on basis of 1µm aluminium powder were analyzed. Deformation hardening exponents were unambiguously function of diffusion processes which were very probably initiated at process of compactation. Deformation hardening exponents had value from n=0,63 for compact produced at 300 C to n=0,9 for compact produced at 450 C. Tensile tests were realized at static conditions and are like the compression tests presented in detail. Metallographic and REM analyses allowed the structures of fractured surfaces to be evaluated. Keywords: Compression test, Tensile test, Forward extrusion, Aluminium powder, Workhardening exponent, Fracture analysis, Hollomon`s function Abstrakt V príspevku je prezentovaná kompaktácia hliníkového prášku s veľkosťou častíc 1 µm metódou dopredného pretlačovania pri rôznych termodynamických podmienkach. Prezentovaný je experimentálny systém obojstranného lisovania s následným dopredným pretlačovaním pri rôznych redukčných pomeroch. Použité sú metódy plánovaného experimentu, ktoré sú spojené s experimentálnymi analýzami variácii teplôt extrúzie deformačných pomerov a vlastných redukcií. Východisková hustota polotovarov kompaktov bola získaná po procese obojstranného lisovania. Kompresné skúšky boli realizované pri izbových teplotách bez špeciálnej úpravy kontaktných plôch a bez maziva. Pri kompresných skúškach nebola hodnotená zmena pôvodného valcovitého tvaru experimentálnych vzoriek. Z funkčných závislostí F = f( h) bola kalkulovaná funkčná závislosť σ = f(φ), kde φ = ln h/h 0. Najväčšie deformačné energie boli identifikované pri kompaktoch, ktoré boli získané pri teplotách 300 a 450 C. Prezentované sú tiež funkčné závislosti σ = f( h), Hollomonove exponenciálne funkcie ako aj výsledné deformačné energie v procese skúšok. Podrobne boli analyzované exponenty deformačného spevnenia pri ťahových skúškach kompaktov na báze 1µm hliníkového prášku. Exponenty deformačného spevnenia boli jednoznačne funkciou difúznych procesov, ktoré v procese kompaktácie veľmi pravdepodobne boli iniciované. Exponenty deformačného spevnenia boli na úrovniach od n=0,63 kompakt získaný 300 C po n=0,9 pre kompakt získaný pri 450 C. Ťahové skúšky ako aj kompresné skúšky boli realizované pri statických podmienkach zaťažovania a sú uvedené veľmi podrobne. Uvedené je tiež hodnotenie metalografické a REM analýza, ktorá umožňovala analýzu lomových plôch. 1. Introduction Technology of forward extrusion is one of the basic procedures of bulk forming, in which as a consequence of pressure punch, a deformated metal is extruded through a reducing part of a forming tool in the direction of the force. The advantage of this method as applied to the compaction of powder substances is due to the intense shear stresses of the reducing part.

3 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 249 Reasoned assumption is that these extreme shared stresses have a destruction effect on the oxide layers covering the basic aluminium matrix. Thus at optimal thermodynamic conditions of the compaction process, the auto-diffusion process of aluminium atoms can be induced, consequently producing the metal bond between particles. Aspects of forward extrusion for compaction of aluminium powder with partical size of 1 µm are presented in detail in [1, 2]. 2. Experimental systems and equipment Experimental specimens prepared by a two-sided pressing and consequential forward extrusion process are presented in detail with results noted on the conferences [1, 2]. Compression and tensile tests were realized at static conditions on the electro hydraulic testing system BPM TESA which is presented in Fig. 1. Fig.1 Electro hydraulic testing system BPM TESA Compression tests were conducted on nonstandard specimens with height h = 3 mm and diameter d = 2,9 mm. These specimens were obtained by the forward extrusion process at four different temperatures, specifically 300, 350, 400, 450 C. The pure aluminium Al 99,5% specimens were used for comparison having the same shape and size similar to the powder specimens. Functionality F = f( h) was analysed in detail. Analysis of this functionality was achieved through A/D converter and software Microsoft Excel. The limit level of reshaping was a moment or possibly stage of compression when destruction occurred. Measured and subsequently calculated important parameters were chosen from select specimens of the individual temperature groups of the forward extrusion process which were able to absorb the greatest load. The energetic demand was subsequently defined by integral calculus and thus determined the energy needed to define specific levels of deformation. This information was subsequently useful in considerations about dispersion strenghtening of aluminium powder undergoing compaction by the forward extrusion process [3, 4]. 3. The Compression Test The elementary scheme of the process of experimental specimens during compression is presented on Fig. 2. Experimental data of compacts prepared at 450 C were obtained by compression tests in comparison with pure metallurgical aluminium and are showed in tab. 1 a tab. 2.

4 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 250 Fig.2 Schematic representation of compression test Table 1 Compression test of specimens forward extrusion temperature of 300 C specimen h 0 (mm) h k (mm) Def. Force F max (kn) Stress σ max (MPa) Total Energy A (Nm) n K Table 2 Compression test of specimens forward extrusion temperature of 450 C specimen h 0 (mm) h k (mm) Def. Force F max (kn) Stress σ max (MPa) Total Energy A (Nm) n K Fig.3 Function F = f( h)

5 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 251 The functionalities F = f( h) and true stress strain diagram σ = f(φ) for the compression test of specimen no.29 obtained by the forward extrusion process at temperature of 450 C are presented on Fig. 3 and Fig. 4. Final comparison of the deformation energies at equal levels of deformation dependent on the temperature of the forward extrusion process and pure aluminium is presented on Fig. 5. Fig.4 Function σ = f(φ) Fig.5 Comparison of different types of specimen vs. compression energy 4. The Tensile Test The static tensile test was realized on an electro hydraulic experimental system BPM TESA. Shape and size of test specimens are presented on Fig. 6. Initial measured length l 0 = 42 mm and diameter d 0 = 3 mm. Functionalities F = f( l) a σ = f(φ) for specimens obtained by the forward extrusion processs at 300 and 450 C are presented on Fig. 7 to Fig. 10. l 0 d 0 Fig.6 Elemental dimensions of specimens for static tensile test

6 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 252 Fig.7 Function F = f( l) Fig.8 Function σ = f(φ) Fig.9 Function F = f( l)

7 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 253 Fig.10 Function σ = f(φ) 5. Metallographic analysis of specimens after compression test Fig.11 Details of free surfaces of compressed specimens 1 µm aluminium powder (a 300 C, b 350 C, c 350 C, d 350 C, e 400 C, f 450 C, g Pure Aluminium, h 400 C) System Olympus was used for metallographic analyses which consist of CK40M microscope and digital camera OLYMPUS CAMEDIA Metallographic analysis was

8 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 254 aimed to evaluate the destructed specimens after the compression test. Specimens obtained by the forward extrusion process at temperatures of 300, 350, 400 and 450 C were compared during the compression test predominantly their destructed outer boundaries [5]. Comparison of the surface area of the compressed pure metallurgical aluminium was also realized. Details of the destructed surfaces as well as inside of compressed specimens are presented on fig Fractographic analysis of specimens after tensile test Fracture surfaces after static tensile test were fractographically analysed on REM Tesla BS 343. Details of the fractured surfaces are presented on fig. 12. Fig.12 Details of fracture surfaces after tensile test 1 µm aluminium powder (a 300 C, b detail of a, c 350 C, d detail of c, e 400 C, f detail of e, g 450 C, h detail of g) 7. Conclusion Mechanical tests were oriented on one dimensional stress schemes of load induced by pressure and pull. This provided information and data which allowed us to adopt an attitude

9 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 255 toward technological technique for compaction of the aluminium powder with predominant grain size of 1 µm at different temperatures during the extrusion process. Four temperatures at which the static forward extrusion at quasi isothermal conditions were realized was intended to provide direct and indirect information about possibilities of the difussion processes on aluminium powder particle surfaces. Subsequently, this most probably induced metallurgical bonds. Results of static tensile test without the use of a lubricant and in the absence of any special geometrical treatment of contact surfaces helped identify differences between individual series of the specimens. These were obtained by the forward extrusion process at temperatures of 300, 350, 400 and 450 C. Tests were realized at room temperature. Work hardening-exponent was the lowest in the compression test at 300 C n 300 = 0,32. On the other hand it was n 450 = 0,81 at temperature of 450 C. Coefficient K in Hollomon`s function is another very important indicator. Value of K 300 was from 277 to 1012 at temperature of 300 C and analogically K 450 was from 428 to 3971 at temperature of 450 C. Values of the engineering strain A 16 obtained from static tensile test varied from 9 to 16%, specifically A 16(300) = 10,47% and A 16(450) = 13,54% and were dependent upon the forward extrusion process temperature. It was remarkable that the work-hardening exponents have a decreasing tendency with increasing extrusion temperature in the static tensile tests. Specifically, these mean work-hardening exponents were observed: n 300 = 0,34 and n 450 = 0,75. This information indicates in certain terms the possible predisposition of analysed compacts for a certain kind of load at which the compacts obtained by the forward extrusion process of aluminium powder have markedly better mechanical properties than the other kind from the outer load. In our case the use of the compression stress scheme is more advantageous than the tensile stress scheme. These facts also indirectly point out the very possible and considerable direction anisotropy of mechanical properties of compacts on the base of 1µm pure aluminium powder. Metallographic analyses of specimens after the destruction compression test - temperature of extrusion 300 and 350 C unequivocally confirmed that both the spread of microfractures and fatal destruction of specimens have initial points in the center of the specimens. The elevated temperatures of compaction have a very positive influence on the destrucion process of specimens. Only microfractures emerged in the undersurface layers, realized through non-continuous oxide layers Al 2 O 3 and were induced by extreme concentration of compression stresses. Oxide layers affect the dispersoid elements, strengthening the base aluminium matrix. The results of compression analyses were confirmed by REM analyses of fractures obtained after tensile test. The fractured surface has an interparticle character at the forward extrusion temperatures of 300 and 350 C and it is possible to place them among low energetic fractures with pit morphology. Elevated temperatures of compaction, specifically 400 and 450 C cause transparticle damage. Fractured surfaces show a facet which underwent plastic deformation without the presence of pit morphology. These facts confirm the presence of intensive diffusion bonds between individual aluminium powder particles at these elevated temperatures and the forward extrusion conditions of this experiment. Acknowledgemnts The experiments were realized within a research plans number supported by the Agency APVV of the Slovak Republic. Literature [1] Donič T., Martikán M.: Hodnotenie termomechanických apektov kompaktovania prášku na báze hliníka dopredným pretlačovaním, FORMING 2005, 12. medzinárodná vedecká

10 Acta Metallurgica Slovaca, 14, 2008, 2 ( ) 256 konferencia, Lednice, Česká Republika, p , Vydavateľ: VŠB Technická univerzita Ostrava, [2] Fogagnolo J.B., et al.: The effects of mechanical alloying on the compressibility of aluminium matrix composite powder, Material Science and Engineering, Volume 355, Issues 1-2, 2003, s. [3] Donič T., Martikán M.: Tvorba jemnozrnne štrukturovaných hliníkových profilov metódou dopredného pretlačovania, Acta Metallurgica Slovaca, Košice, Mimoriadne číslo 2/2005, ročník 11, str [4] Fogagnolo J.B., Robert R.H., Torralba J.M.: The effect of mechanical alloying on the extrusion process of AA6061 alloy reinforced with Si 3 N 4, J. Braz. Soc. Mech. Sci & Eng., 2003, vol. 25, no 2, s. [5] Botta Fieho W. J., et al.consolidation of partially amorphous aluminium alloy powders by severe plastic deformation, Material Science and Engineering, Volume , 2004, s.