Relationships between weld parameters, hardness distribution and temperature history in alloy 7050 friction stir welds A. P. Reynolds* 1 , W. Tang 1 , Z. Khandkar 1 , J. A. Khan 1 and K. Lindner 1,2 Aluminium alloy 7050 was friction stir welded using three different ratios of tool rotation rate to weld travel speed. Welds were made using travel speeds of between 0 . 85 and 5 . 1 mm s –1 . Weld power and torque were recorded for each weld. An FEM simulation was used to calculate the time–temperature history for a subset of the welds. For each weld the hardness distribution with and without post-weld heat treatment was determined. The hardness distributions within the welds are rationalised based on the friction stir welding parameters and the resulting temperature histories. The analysis provides a basis for manipulation of weld parameters to achieve desired properties. Keywords: Friction stir welding, 7xxx series alloys, HAZ Introduction Since its invention at The Welding Institute 1 the friction stir welding (FSW) process has proven to be a superior technique for joining of a variety of aluminium alloys. In particular, for the precipitation hardenable 2xxx and 7xxx alloys, which may be quite difficult to weld via fusion techniques, it is of great utility. 2 While copious data have been generated on the properties of FSWs in many alloys, few systematic studies of weld parameter effects on properties in 7xxx alloys have been produced: most published work has been performed on single welds. In order to correlate weld hardness with weld parameters, it is necessary to understand the range of metallurgical reactions possible for the alloy in question. There is a substantial body of literature regarding the aging of 7050 (e.g. Ref. 3); however, much of it deals with industrial aging practice and overaging of the alloy at temperatures associated with industrial aging prac- tice. The timescales for precipitation aging are generally much longer than the transient heat treatments pro- duced by welding processes (e.g. thousands of seconds compared to tens of seconds). Also, the temperatures used for precipitation aging are substantially lower than those encountered in the heat affected zone (HAZ) hardness minima and the weld nuggets of the welds under consideration here. More data are available for alloy 7075 than for 7050. Fortuitously, the kinetics and thermodynamics of precipitation in 7075 and 7050 are similar even thought the absolute values of hardness attained in the two alloys may differ. 3 In general, the strengthening precipitates precipitation/dissolution sequences are similar in many 7xxx alloys and it is well established that the primary strengthening precipitate in 7050-T7451 alloy is the coherent g9 phase. 4 Examination of the literature reveals the following regarding pre- cipitate stability in the 7xxx series alloys: 5,6 (i) dissolution of the strengthening g9 phase occurs at T.190uC (ii) the incoherent g phase precipitates between approximately 215 and 250uC. This phase contributes much less to strengthening than does g9. Near 250uC, g begins to coarsen rapidly (iii) g phase begins to dissolve at T.320uC (iv) there is a maximum in the formation rate of the high temperature, non-strengthening, incoher- ent M-phase at approximately 350uC. Hence, the solute will be most rapidly depleted from the matrix at this temperature. Further, there is pertinent information regarding the thermal conditions associated with HAZ formation in welding of 7075. Mahoney et al. 7 found the minimum HAZ hardness in a 7075 friction stir weld in a region where the maximum temperatures were in the range of 300–350uC. Hwang and Chou 8 performed weld simula- tion of alloy 7075 and found that the minimum strength resulted from a weld thermal cycle with a peak tem- perature of 377uC. This was not necessarily the tem- perature which would result in the absolute minimum hardness as a continuum of peak temperatures was not examined (adjacent temperatures were 288 and 445uC). Hwang and Chou ascribed the low strength at 377uC to rapid formation of coarse g. Temperatures above 377uC were considered partial solution treatments with sub- sequent natural aging leading to higher strength while those below 377uC resulted in less dissolution of g9 and, hence, higher strength. Based on the work of Archambault and Godard, 6 it seems likely that the 1 USC Dept. of Mechanical Engineering, 300 Main Street, Room A224, Columbia, SC 29208, USA 2 University of Karlsruhe, Karlsruhe, Germany *Corresponding author, email apr@sc.edu ß 2005 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 9 December 2003; accepted 9 April 2004 DOI 10.1179/174329305X37024 Science and Technology of Welding and Joining 2005 VOL 10 NO 2 190