Microstructure Characterization of Friction Stir Welded Aluminum Alloy 7050 T. W. Nelson 1, J.Q. Su 1, R.J. Steel 1 M.W. Mahoney 2 R.S. Mishra 3 1 Department of Mechanical Engineering, Brigham Young University, Provo, UT 2 Rockwell Scientific, Thousand Oaks, CA 3 Department of Materials Science and Engineering, University of Missouri-Rolla, Rolla, MO
Outline 7XXX aluminum Strengthening mechanisms FSW Weldability Instability Heat affected zone in FSW 7XXX Loss of properties, e.g. mechanical and corrosion Aging capabilities Microstructure Characterization
7XXX Aluminum High strength low density Al-Zn-Mg-Cu Precipitation hardenable T6 temper is peak strength T7 is best combination of strength and corrosion resistance Precipitation kinetics Supersaturated solid solution GP Zones η (MgZn 2 ) HCP η(mgzn 2 ) HCP
Precipitation Strengthening in 7XXX G.P. Zones (Coherent) η (Semicoherent) Strengthening value T6 T7 η (Incoherent) Solid solution Time at elevated temperature
Heat Affected Zone in FSW Weld and surrounding regions naturally age, but HAZ is still the weak link in FSW Microhardness (DPH) 100kg Distance Across Weld (in)
HAZ is Prone to Failure Fracture morphologies all tensile fractures occur in the HAZ Others (Mahoney et. al., and Jata et. al.) report corrosion problems in HAZ
Objective Better understand the evolution of microstructure in FSW 7XXX Despite its many advantages, the thermo-mechanical process of FSW still has detrimental effects: HAZ is susceptible to fracture and corrosion, DXZ is prone to accelerated natural aging and abnormal grain growth during elevated temperature PWHT.
Evaluation of Pre and Post Weld Temper Conditions Alloy Weld Temper Final Temper Sequence of Events O -T6 Anneal, FSW, SHT and Age 7075 T4 T6 T6 -T6 -T6 -T7 SHT, FSW and Age SHT, Age, and FSW SHT, Age, FSW, Overage
Results of Pre and Post FSW Temper Conditions O-T6 produce highest tensile and yield, but poor ductility difficult to perform on large structures Combination of T4 and T4-T6 could be functional but difficult 90000 16 Tensile/Yield Strength (Ksi) 80000 70000 60000 50000 40000 30000 20000 10000 Tensile Strenth Yield Strength Elongation (%) 14 12 10 8 6 4 2 Elongation (%) 0 0 T4-T4 T6-T6 T4-T6 T6-T7 T0-T6 T0-T0 Heat treatment
Results of Pre and Post FSW Temper Conditions Artificial aging response across welds O-T6 homogenized hardness T4-T6 and T6-T7 overages HAZ T4-T4 left long enough, quassi homogenizes weld and HAZ Hardness (HV 100g) 160 150 140 130 120 110 100 90 80 70 60 50 40 0 10 20 30 40 50 60 70 80 Distance (mm) T0 T0-T6 T4-T4 T4-T6 T6-T6 T6-T7
Microhardness Comparison Hardness (HV 100g) 260 240 220 200 180 160 140 Base Metal HAZ/TMAZ DXZ 7075AC-1000 hrs T6-T7 (22hr@100C) T4-T6 (10hr@120C) T4-T6 (60 hr@100c) T6-T7 (132 hr@100c) 120 100 0 20 40 60 80 100 120 Distance Across Weld
Summary of Pre/Post FSW Heat Treatments Pre-weld Temper (Ti) Reduction (%) PWHT and Properties of FSW TMT 7075 Rolling Temp. ( C) Post-weld Temper (Tf) Yield Strength Mpa (Ksi) Ultimate Strength MPa (Ksi) Elongation (%) T7 No TMT Processing None 264 (38.3) 407 (59) 7.1 T4 21% 190 24hr@120C) 328 (47.6) 449 (65.1) 3.4 T4 21% 190 24hr@120 C + 13hr@163 C 321 (46.6) 415 (60.1) 3.4 T6 20% 190 132hr@100 C 334 (48.4) 461 (66.9) 3.91 T6 No TMT Processing 132hr@100 C 306 (44.4) 435 (63.1) 3.94 T4 20% 190 69hr@100 C 335 (48.6) 451 (65.4) 5.26 T4 No TMT Processing 69hr@100 C 356 (51.6) 487 (70.6) 5.87
TEM Characterization of FSW 7050 Regions of analysis Base metal (BM) Heat affected zone (HAZ) Thermal-mechanical affected zone Adjacent HAZ (TMAZI) Adjacent DXZ (TMAZII) Dynamically recrystallized zone (DXZ) BM HAZ TMAZ DXZ I II
Base Metal Characterization pancake grain structure with some subgrains on the order of 1-5 μm Small difference in diffraction contrast indicates presence of mainly low angle boundaries Distribution of fine intragranular η (20-50nm) with grain boundary η and/or Mg 3 Zn 3 Al 3 Small PFZ ~25 nm wide 1 μm 1 μm 200 nm
HAZ Microstructure in FSW 7050 Density of precipitates has decreased as a result of coarsening Grain boundary precipitates have also coarsened nearly continuous PFZ increased to ~100 nm 1 μm 200 nm
HAZ Microstructure in FSW 7050 Precipitate morphologies in the HAZ Dark field images indicating coarsening of η and η 200 nm 200 nm 200 nm
TMAZI Microstructure in FSW 7050 Elongated nature of BM grains still evident Some grain boundary migration evident by precipitate strings along prior gb Highly deformed structure. i.e. most grains contain high dislocation densities Well developed subgrains with interior dislocations Suggests incomplete recovery or continuous in nature Dislocation pinning also observed 1 μm 200 nm 200 nm 500 nm
TMAZI Microstructure in FSW 7050 Precipitate morphologies in the TMAZI Inhomogeneous duplex distribution of precipitates Course (~100nm), and finer (~10 nm) intragranular precipitates very course gb precipitates Hypothesis: Coarsening & partial dissolution of strengthening precipitates during weld TC Very small precipitates form heterogeneously on dislocations during cooling 500 nm 500 nm
TMAZII Microstructure in FSW 7050 Recovered grains containing high density of subgrains Subgrains roughly 1-2 μm contain low density of dislocations Subgrains are separated by low angle grain boundaries 1 μm 500 nm
TMAZII Microstructure in FSW 7050 Precipitate morphologies in TMAZII Very fine (10-20 nm) inhomogeneously distributed precipitates Nearly continuous grain boundary precipitates Precipitation along subgrain boundaries was also observed Hypothesis: Temperature sufficient to completely dissolve strengthening precipitates Heterogeneous re-precipitation preferentially along grain boundaries, subgrain boundaries, and dislocations during cooling 200 nm 200 nm
DXZ Microstructure in FSW 7050 Recrystallized fine (1-4 μm) equiaxed grain structure Grains separated by high angle grain boundaries A range dislocation densities (mostly high) and structure Dislocation loops found in grain with relatively low dislocation densities Grain with high dislocation densities exhibit network structures Implies plastic deformation may have been introduced after dynamic recrystallization Grain in the process of recovery adjacent to grains of high dislocation density 200 nm 200 nm 200 nm 400 nm
DXZ Microstructure in FSW 7050 Precipitation Strengthening precipitates dissolve during weld TC Re precipitate (both hetero and homogeneously) during cooling and NA Precipitate distribution strongly dependant upon dislocation structure Lower densities in grains with lower dislocation densities Larger gb precipitates PFZ ~50-100 nm wide 200 nm 200 nm 200 nm
Summary 7050-T651 alloy contains: mainly intragranular fine η precipitates (less than 50 nm) and coarser η precipitates along grain boundaries a small precipitate-free zone of 25 nm HAZ: The precipitates and precipitate-free zone have been coarsened by a factor of 5 more η phase has been formed TMAZ I: large amount of lattice dislocations were introduced and grain growth occurred strengthening precipitates were severely coarsened and partly dissolved a small amount of very fine η particles were re-precipitated during cooling and NA TMAZ II: recovery structure with subgrains of 1-2 μm in size The temperature in this region possibly reached the solution heat-treatment temperature the strengthening precipitates were completely dissolved and re-precipitated preferentially along grain boundaries, subgrain boundaries and on dislocations during cooling.
Summary (cont.) DXZ: consisted of recrystallized, fine equiaxed grains on the order of 1-4 μm in diameter. Most of the grains contain high dislocation densities with various degrees of recovery from grain to grain The strengthening precipitates have gone into solution and re-precipitated heterogeneously at dislocations and on Al3Zr in the matrix The extent of the precipitation strongly depends on the density of lattice dislocations. Relatively stable GP(II) zones exist in the parent material. GP(II) zones were partially preserved in the HAZ region In the TMAZ and DXZ regions, the GP(I) zones were formed during post weld natural aging No GP(II) zone were detected in these regions.
Hypothesis on DXZ The dynamic recrystallization processing in the DXZ can be considered a continuous dynamic recrystallization (CDR) on the basis of dynamic recovery. Subgrain growth associated with absorption of dislocations into the boundaries is the mechanism of the CDR. Repeated absorption of dislocations into subgrain boundaries is dominant mechanism of increasing misorientation between adjacent subgrains during the CDR processing.