PROPERTIES OF FREE MACHINING ALUMINIUM ALLOYS AT ELEVATED TEMPERATURES. Ji í Faltus, Petr Homola, Peter Sláma

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1 PROPERTIES OF FREE MACHINING ALUMINIUM ALLOYS AT ELEVATED TEMPERATURES Ji í Faltus, Petr Homola, Peter Sláma VÚK Panenské B e any a.s., Panenské B e any 50, Odolena Voda faltus@vukpb.cz Abstract During laboratory tests of machinability performed in the course of development of new freecutting alloys, it was found that the temperature of material being dry machined can reach values as high as 350 C or more in areas close to cutting tool. During machining it is therefore theoretically possible that in some cases, such as failure of cutting fluid inflow, the temperature of thin wall machined piece may exceed normal temperature considerably. In modern methods of machining using high cutting rates the material may undergo impact loads at elevated temperatures. This type of load can be simulated using the impact bend test, so called Charpy impact test according to European standard EN Unlike common construction AA6082 alloy, the free machining AA6262 and AA6023 aluminium alloys show a significant decrease in notch impact strength KU at elevated temperatures. This drop of KU, caused by melting of disperse phases containing low-melting metals, takes place within a certain relatively narrow transition range of temperature. This decrease in notch impact strength leads to the risk of occasional fracture of parts manufactured from these alloys by cutting process. 1 INTRODUCTION Using laboratory tests of machinability during development of new free-cutting alloys it was found that the temperature of material being dry machined can reach the values as high as 350 C or more in areas close to a cutting tool [1] (fig.1). During machining it is therefore theoretically possible that in some cases, such as failure of cutting fluid inflow, the temperature of thin wall machined piece may exceed normal temperature considerably. max C Machined piece of Al alloy Fig. 1. Turning operation 1

2 From this reason it is of interest to learn the properties of free machining aluminium alloys at elevated temperatures. In modern methods of machining using high cutting rates the material may undergo impact loads at elevated temperatures. This type of load can be simulated with impact bend test, so called Charpy impact test according to European standard EN From this reason it is of interest to learn the properties of free machining aluminium alloys at elevated temperatures. In modern methods of machining using high cutting rates the material may undergo impact loads at elevated temperatures. This type of load can be simulated using an impact bend test, so called Charpy impact test, according to European standard EN CHARPY IMPACT TEST Charpy U-test piece was used in our tests. The impact tests were performed at ambient temperature as well as at 100, 150, 200, 250, 300 and 350 C. Warming to the test temperature was performed by rapid heating, the piece was put in contact with aluminium plate preheated to the test temperature (Fig. 2). a) b) c) Fig. 2 Charpy U-test piece (a), heat-delivery Al surface (b) and testing machine (c) Warming up to the required temperature and immediate break of the sample took a period of max. 1.5 min, so that the strength loss by precipitation (overaging) or recrystallization during the test was negligible. 3 MATERIALS The tests were perfomed on samples of dimensions of 55x10x10 mm with U-notch taken from pressed and drawn rods of diameter of 17.7 mm from AA6082, AA6262 and AA6023 alloys in T8 temper. The rods were manufactured by standard technology in Alcan D ín Extrusions Ltd. Chemical composition of the alloys is given in Table 1. Table 1. Chemical composition (in wt. %) Alloy Melt Si Fe Cu Mn Mg Cr Zn Ti Ni Pb Bi Sn billet AA AA billet

3 billet AA Structure Distribution of constituent phases Al x (Fe,Si,Cr,Mn,Cu) y containing Fe, Si and Cr (in case of AA6262), Fe, Si and Mn (AA6082) and Fe, Si, Mn and Cu (AA6023) is similar in all three alloys. They are arranged in arrays in the pressing direction and are finer at the surface as compared to the central areas. The AA6262 and AA6023 alloys contain disperse phases involving low-melting metals, either Pb and Bi (AA6262) or Sn and Bi in case of AA6023. These phases have array arrangements as well and their size and distribution were similar in both alloys. Phases containing Sn-Bi in the structure of AA6023 are a little coarser than phases containing Pb-Bi in the structure of AA6262 alloy (Fig. 3). a b c Fig. 3 Distribution of constituent phases in the structure in central areas of the rods of AA6082-T8 (a), AA6262-T8 (b) and AA6023-T8 (c) alloy The structure in the central areas of the rods in all examined AA6082, AA6262 and AA6023 alloys was fibrous (Fig. 4). a b c Fig. 4 Unrecrystallized "fibrous" structure of the rods of AA6082-T8 (a), AA6262-T8 (b) and AA6023-T8 (c) alloy Mechanical properties Initial mechanical properties of rods of diameter of 17.7 mm from tested AA6262, AA6023 and AA6082 alloys are given in Table 2. The properties were mutually similar for all materials and the average values occur within a technological scatter. Table 2. Mechanical properties of rods of diameter of 17.7 mm 3

4 Alloy Rp0.2 Rm [MPa] [MPa] A [%] Z [%] E [GPa] HV30 HB AA6082-T AA6262-T AA6023-T RESULTS AND DISCUSSION Results of impact tests by Charpy method at ambient and elevated temperatures, used for measuring notch impact strength KU, are given in Fig AA6082-T8 AA6262-T8 AA6023-T8 Charpy impact energy [J] (Notch toughness) Temperature [ C] Fig. 5 Temperature dependence of Charpy impact energy (KU) for AA6082, AA6262 and AA6023 alloys The results could be summarized as follows: a) The tested alloys differ substantially in their dependence of notch impact strength KU on the temperature in the range from 20 to 350 C. While the AA6082 alloy exhibits practically constant notch impact strength up to 200 C and its growth above this temperature, in case of free machining AA6262 and, especially, AA6023 alloy a strength decrease with increasing temperature is observed. b) In case of both free machining alloys, AA6262 and AA6023, the decrease of impact strength with increasing temperature is slow at first, however, at a certain critical temperature, T k, a sharp decrease occurs. The critical temperature is 250 C for the AA6262 alloy, whereas for the AA6023 alloy it is lower, only 150 C (Fig. 6). c) Certain transition ranges can be defined, i.e., temperature intervals within which the notch impact strength KU of an alloy decreases sharply. This temperature transition range is C for the AA6262 alloy. Within this temperature range, KU decreases from 6 J to 4

5 approximately 4 J. In case of the AA6023 alloy, the transition temperature range is lower by approx. 100 C, being C. Within this temperature range the notch impact strength of the alloy drops from 5.5 J to 2 J (Fig. 6). 9 8 Trans.area AA C Trans. area AA C Charpy impact energy [J] (Notch toughness) AA6262-T8 AA6023-T Temperature [ C] Fig. 6 Charpy impact energy (KU) versus temperature curves for AA6082 and AA6262 with denotation of the transition ranges d) The different dependence of KU on the temperature is related to different phase composition in the structure of alloys tested. Whereas the AA6082 alloy does not contain any low-melting phases, the structure of AA6262 alloy contains disperse phases comprising lowmelting metals lead and bismuth and that of AA6023 alloy dispersion phases comprising lowmelting metals tin and bismuth. These metals form diverse variants of low-melting phases. It is important to mention that the phases in the AA6262 alloy, containing Pb and Bi, exhibit, with only one exception, higher melting points than phases containing Sn and Bi in the AA6023 alloy (Table 3). These results were confirmed by DTA measurements [4]. These analyses confirm a melting of the phases in the AA6023 and AA6262 alloys at temperature of 189 C and 257 C, respectively. Table 3. Low-melting phases in AA6262 and AA6023 alloys, their melting points and presence in the structure [2, 3, 5, 6] Alloy Phases Melting point Presence in structure Pb 327 C High Bi 271 C Low AA6262 (43,5%)Pb-Bi 125 C Very low (99,5%)Bi-Mg 3 Bi C Very high (97,8%)Pb-Mg 2 Pb 253 C Low Sn 232 C Mddle AA6023 Bi 271 C Low (43%)Sn-Bi 139 C Low 5

6 (99,5%)Bi-Mg 3 Bi C High (98%)Sn-Mg 2 Sn 200 C Very high Fracture properties from impact tests Fracture properties from impact tests were studied by methods of both light microscopy (LM) and scanning electron microscopy (SEM) using DSM 940 and FEI Quanta 200 FEG microscopes. The results of the experiments could be summarized as follows: a) At ambient temperature, the fractures of all three studied alloys exhibited a ductile dimple rupture. In the AA6082-T8 alloy, the bottoms of dimples were either empty or complex intermetallic phases were present containing Fe, Si, Mn, in some cases also Mg. In the AA6262 alloy, there were predominantly phases containing Pb, Bi and Mg observed at bottom of the dimples, and in case of the AA6023 alloy the phases containing Sn, Bi and Mg (see Fig.7). 20 C - SE mode a) b) c) 20 C - BSE mode Phases Pb and/or Bi (+Mg) Phases Sn and/or Bi (+Mg) d) e) f) Fig. 7 Fracture properties from Charpy impact tests of AA6082-T8 (a, d), AA6262-T8 (b, e) and AA6023-T8 piece (c, f) at ambient temperature (Magnification: 500 x) b) The appearance of fracture changed at elevated temperatures. The AA6082 alloy exhibited a pronounced ductile fracture at C. In AA6262 and AA6023 alloys the fracture quality was similar as that at ambient temperature. However, in contrast to fractures at ambient temperature, the branched fractures with many side cracks were observed at elevated temperatures. The number of side cracks in the AA6023 alloy was higher as compared with AA6262 alloy (Fig. 8, 9). Small area of the branched Big area of the branched 6

7 fracture with side cracks fracture with many side cracks a) b) c) Fig. 8 Fracture properties from Charpy impact tests of AA6082-T8 (a), AA6262-T8 (b) and AA6023-T8 (c) on longitudinal section of the piece at 300 C (Notch is on the right) ** 300 C - SE mode a) b) c) 300 C - BSE mode Side cracks Side cracks d) e) f) Fig. 9 Fracture properties from Charpy impact tests of AA6082-T8 (a, d), AA6262-T8 (b, e) and AA6023-T8 piece (c, f) at 300 C (Magnification: 500 x) 5 CONCLUSIONS a) Unlike common construction AA6082 alloy, the free machining AA6262 and AA6023 aluminium alloys show a significant decrease in notch impact strength (impact energy KU) at elevated temperatures. This drop of KU, caused by melting of disperse phases containing low-melting metals, takes place within a certain relatively narrow transition range of temperature. 7

8 b) In AA6262 alloy, this transition range of "warm embrittlement" is between temperatures from 250 to 300 C, in case of the AA6023 alloy it was assessed between temperatures 150 and 200 C. It is important to mention that the notch impact strength KU of the AA6262 alloy drops from approximately J to the value of 4 J, which is much less as compared to the AA6023 alloy, showing a decrease from 6 to 2 J. c) From theoretical considerations on machining it implies that a temperature of a thin part machined using a high cutting speed may considerably rise above ambient temperature in case of insufficient cooling. Such an increase may exceed the temperature of embrittlement of free-cutting aluminium alloys and leads to damage of the piece during machining process. Since the transition temperature of embrittlement is lower and the embrittlement itself more intense for the alloy AA6023 (Stanal 32) as compared with the AA6262 alloy, the probability of fracture occurrence in parts made of the AA6023 alloy is higher than in case of the AA6262 alloy. d) The lower transition temperature of embrittlement and the deeper decrease of KU in case of alloy AA6023 as compared to AA6262 are related to three main aspects: i) Melting temperatures of low-melting phases containing Sn and Bi in the AA6023 alloy are lower than those of phases containing Pb and Bi in the AA6262 alloy. ii) Owing to the lower surface tension, tin wets more the surrounding matrix as compared to lead. iii) Since a phenomena of so called "separated eutectics" occurs in the Al-Sn system, tin, as compared to lead, exhibits a higher tendency to be interdendritic separated in the form of thin layers that may more impair the coherence of material at elevated temperatures during subsequent melting. Acknowledgments This work was done as a part of project of Ministry of Education, Youth and Sports of Czech Republic, Research Centre 1M The authors acknowledge also the company Alcan D in Extrusions, Ltd. for providing the experimental material. References [1] MÁDL, J. KOUTNÝ, V. RÁZEK, V.: Study of the properties of the lead-free machinable aluminium alloys the type Al-Cu-Mg, U223/2002/002 January 2001 [2] HANSEN, M.: Constitution of Binary Alloys, McGraw-Hill Book Com., 1958, p. 317, 324, 336,911 and p. 918 [3] FALTUS, J. BENDÍKOVÁ, E. UHLÍ, J.: Influence of chemical composition on structure and properties of lead-free machinable AA6023 (Al-Mg-Si-Sn-Bi) alloy, Acta Metallurgica Slovaca, 13, 2007, p.597 [4] Personal communication of Dr. A. Bigot from Voreppe, France, May 2008 [5] FALTUS, J.: Properties of free machining aluminium alloys at elevated temperatures I, Research report No. 01/2008 [6] FALTUS, J.: Properties of free machining aluminium alloys at elevated temperatures II, Research report No. 14/2008 8

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