Tensile behavior of friction-stri-welded Al 6061-T651

In the present study, tensile behavior of friction-stir-welded Al 6061-T651 with varying welding parameters, including rotating and welding speeds, was examined. The 4-mm-thick Al 6061-T651 alloy plates were FSW with varying tool rotating speeds, 1000, 1400, 1600, 2000, and 2500 rpm, and welding speeds, 0.1, 0.2, 0.3, to 0.4 mpm (m/min). Tensile specimens were prepared with the tensile direction perpendicular to the welding direction, so that the weld zone is located in the middle of the specimen. It was found that the tensile elongation of friction-stir-welded Al 6061-T651 decreased with decreasing welding speed or increasing rotating speed. The yield and ultimate tensile strength were also affected, but to a significantly lesser degree, with varying welding parameters. The micrographic and fractographic observations strongly suggested that the change in tensile behavior of friction-stri-welded Al 6061-T651 was largely related to the clustering of coarse Mg₂Si precipitates, due to the whirling and hurling action by severe plastic flow in the weld zone. Low welding speed or high rotating speed tended to encourage the plastic flow per unit time and consequently the clustering of coarse precipitates.     

Corrosive wear of SiC Whisker-and particulate-reinforced 6061 aluminum alloy composites

Wear tests on SiC whisker- and SiC particulate-reinforced 6061-T6 aluminum matrix composites (SiCw/Al and SiCp/Al), fabricated using a high pressure infiltration method, were performed in laboratory air, ion-exchanged water and a 3 pct NaCl aqueous solution using a block-on-ring type apparatus. The effects of environment, applied load, and rotational (sliding) speed on the wear prop-erties against a sintered alumina block were evaluated. Electrochemical measurements in ion-ex-changed water and a 3 pct NaCl aqueous solution were also made under the same conditions as the wear tests. A comparison was made with the properties of the matrix aluminum alloy 6061-T6. The SiC-reinforced composites exhibited better wear resistance compared with the monolithic 6061 Al alloy even in a 3 pct NaCl aqueous solution. Increase in the wear resistance depended on the shape, size, and volume fraction of the SiC reinforcement. Good correlation was obtained between corrosion resistance and corrosion wear. The ratios of wear volume due to the corrosive effect to noncorrosive wear were 23 to 83 pct, depending on the wear conditions.     

Tensile behavior of friction-stir-welded A356-T6/Al 6061-T651 bi-alloy plate

Abnormally low tensile ductility has often been reported for the friction-stir-welded (FSWed) dissimilar metals. The mechanism(s) for such a low tensile ductility has, however, not been established. In the present study, the tensile behavior of FSWed A356-T6/Al 6061-T651 bi-alloy plate was studied to understand the underlying mechanism for the reduced tensile ductility with the friction stir welding of dissimilar metals based on thorough micrographic and fractographic observations. The present study also demonstrated that the tensile ductility of the friction-stir-welded A356-T6/Al 6061-T651 bi-alloy specimen was substantially lower than that of the weighted mean value of the uni-alloy counterparts, including A356-T6 and Al 6061-T651 alloys. Interestingly, a relatively large number of acicular shaped Si particles were observed locally in the FSWed bi-alloy specimens compared to the dominantly globular shaped particles in the FSWed uni-alloy counterpart. Moreover, these acicular shaped Si particles were found to be mostly aligned parallel to the tool-rotating direction. Such agglomerated areas of the preferentially oriented, acicular Si particles in the present study appeared to serve as initiation sites for the tensile fracture and eventually caused low tensile ductility.     

Fracture behavior of thixoformed 357-T5 Al alloys

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost threefold increase in impact energy value was, for example, o served with increasing volume fraction of primary α from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary α and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thix oformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tesile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 375-T5 alloy was discussed.     

Fracture behavior of thixoformed 357-T5 Al alloys

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost threefold increase in impact energy value was, for example, observed with increasing volume fraction of primary α from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary α and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thixoformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tensile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 357-T5 alloy was discussed.     

Microstructure and Low-Cycle Fatigue of a Friction-Stir-Welded 6061 Aluminum Alloy

Strain-controlled low-cycle fatigue (LCF) tests and microstructural evaluation were performed on a friction-stir-welded 6061Al-T651 alloy with varying welding parameters. Friction stir welding (FSW) resulted in fine recrystallized grains with uniformly distributed dispersoids and dissolution of primary strengthening precipitates β″ in the nugget zone (NZ). Two low-hardness zones (LHZs) appeared in the heat-affected zone (HAZ) adjacent to the border between the thermomechanically-affected zone (TMAZ) and HAZ, with the width decreasing with increasing welding speed. No obvious effect of the rotational rate on the LHZs was observed. Cyclic hardening of the friction-stir-welded joints was appreciably stronger than that of base metal (BM), and it also exhibited a two-stage character where cyclic hardening of the friction-stir-welded 6061Al-T651 alloy at higher strain amplitudes was initially stronger followed by an almost linear increase of cyclic stress amplitudes on the semilog scale. Fatigue life, cyclic yield strength, cyclic strain hardening exponent, and cyclic strength coefficient all increased with increasing welding speed, but were nearly independent of the rotational rate. Most friction-stir-welded joints failed along the LHZs and exhibited a shear fracture mode. Fatigue crack initiation was observed to occur from the specimen surface, and crack propagation was mainly characterized by the characteristic fatigue striations. Some distinctive tiremark patterns arising from the interaction between the hard dispersoids/inclusions and the relatively soft matrix in the LHZ under cyclic loading were observed to be present in-between the fatigue striations.     

Effect of N addition on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The effect of N addition on the microstructure, tensile, and corrosion behaviors of CD4MCU (Fe-25Cr-5Ni-2.8Cu-2Mo) cast duplex stainless steel was examined in the present study. The slow strain rate tests were also conducted at a nominal strain rate of 1 × 10⁻⁶/s in air and 3.5 pct NaCl+5 pct H₂SO₄ solution for studying the stress corrosion cracking (SCC) behavior. It was observed that the volume fraction of austenitic phase in CD4MCU alloy varied from 38 to 59 pct with increasing nitrogen content from 0 to 0.27 wt. pct. The tensile behavior of CD4MCU cast duplex stainless steels, which tended to vary significantly with different N contents, appeared to be strongly related to the volume changes in ferritic and austenitic phases, rather than the intrinsic N effect. The improvement in the resistance to general corrosion in 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution was notable with 0.13 pct N addition. The further improvement was not significant with further N addition. The resistance to SCC of CD4MCU cast duplex stainless steels in 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution, however, increased continuously with increasing N content. The enhancement in the SCC resistance was believed to be related to the volume fraction of globular austenitic colonies, which tended to act as barriers for the development of initial pitting cracks in the ferritic phase into the sharp ones.     

Effect of different Cr contents on tensile and corrosion behaviors of 0.13 pct N-containing CD4MCU cast duplex stainless steels

The tensile and corrosion behaviors of 0.13 pct N-containing CD4MCU cast duplex stainless steels with different Cr contents ranging from 23 to 28 pct were examined in the present study. The polarization tests were conducted in 3.5 pct NaCl + 5 pct H₂SO₄ aqueous solution for general corrosion resistance, and the in-situ slow strain rate (SSR) tests were also conducted in air and 3.5 pct NaCl + 5 pct H₂SO₄ aqueous solution to quantify the resistance to stress corrosion cracking (SCC) of the three materials. A substantial microstructural change in 0.13 pct N-containing CD4MCU cast duplex stainless steel was observed with different Cr contents, which in turn affected the tensile and corrosion behaviors significantly. Tensile behavior of 0.13 pct N-containing CD4MCU cast duplex stainless steel, for example, varied in a nonlinear manner with different Cr contents due to the volume change of hard ferritic phase and the presence of the second precipitates of soft austenitic phase in the ferrite matrix. The beneficial effect of Cr for improving the general corrosion and the SCC resistances was largely overshadowed by this variation in microstructural characteristics. The relationship between the microstructural evolution and the tensile and corrosion behavior of 0.13 pct N-containing CD4MCU cast duplex stainless steels with different Cr contents was discussed based on the optical microscopy and scanning electron microscopy (SEM) micrographic and fractographic observations.     

Tensile behavior of friction-stir-welded AZ31-H24 Mg alloy

In the present study, tensile behavior of friction-stir-welded AZ31 (Mg-3.6Al-1Zn-0.6Mn in wt pct)-H24 Mg alloy was investigated. It was found that the tensile property, particularly tensile elongation, of AZ31-H24 alloy was significantly degraded with friction stir welding (FSW). The tensile fracture always occurred at the boundary between the thermomechanically affected zone (TMAZ) and the stir zone (SZ) on the advancing side. The fractographic examination on the tensile-fractured AZ31-H24 alloy specimen showed a mixed mode of cleavage and dimpled rupture. The AES analysis suggested that the significant reduction in tensile elongation of friction-stir-welded AZ31-H24 Mg alloy was attributable to the entrapped oxides along the boundary between the TMAZ and SZ.     

Environmental factors affecting localized corrosion of 7075-t7351 aluminum alloy plate

An earlier paper on the mechanism of localized corrosion of 7075 alloy plate in 3.5 pct NaCl solution showed that pitting of an anodic zone in the vicinity of grain boundaries caused intergranular corrosion of 7075-T651 (peak aged condition). It also showed that overaging to the T7351 temper reduced the difference between pitting potentials of the grain boundaries and grain interiors so that intergranular corrosion could not be sustained. This paper shows that certain changes in the environment strongly affect the difference between pitting potentials of the grain interior and the anodic grain boundary of 7075-T7351. Consequently, the susceptibility for intergranular corrosion is substantially greater in certain environments. Nitrates are most detrimental. The effects of sulfates and pH in chloride + nitrate solutions were relatively small.     

Effect of Loading History on Stress Corrosion Cracking of 7075-T651 Aluminum Alloy in Saline Aqueous Environment

An experimental study of stress corrosion cracking (SCC) was conducted on 7075-T651 aluminum alloy in a chromate-inhibited, acidic 3.5 pct sodium chloride aqueous solution using compact tension specimens with a thickness of 3.8 mm under permanent immersion conditions. The effects of loading magnitude, overload, underload, and two-step high-low sequence loading on incubation time and crack growth behavior were investigated. The results show that the SCC process consists of three stages: incubation, transient crack growth, and stable crack growth. The incubation time is highly dependent on the load level. Tensile overload or compressive underload applied prior to SCC significantly altered the initiation time of corrosion cracking. Transition from a high to a low loading magnitude resulted in a second incubation but much shorter or disappearing transient stage. The stable crack growth rate is independent of stress intensity factor in the range of 10 to 22 MPa ?{\textm} . \sqrt {\text{m}} .     

Effect of different mo contents on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The tensile and corrosion behaviors of CD4MCU cast duplex stainless steels with different Mo contents of 0, 2, and 4 pct, respectively, were examined in the present study. The polarization and the in-situ slow-strain-rate (SSR) tests were conducted in a 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution to quantify the resistances to pitting corrosion and stress corrosion cracking (SCC) with different Mo contents. The addition of Mo, which is a strong ferrite stabilizer, affected the microstructure of the present alloy and, eventually, the tensile and corrosion behaviors in a complex manner. The tensile properties of CD4MCU cast duplex stainless steel, for example, were found to be determined by the volume fraction of hard ferritic phase, the presence of the second precipitates of soft austenitic phase in the ferrite matrix, and the shape of the austenitic phase. The addition of 2 pct Mo was detrimental to the corrosion properties of CD4MCU cast duplex stainless steel due to the significant increase in the volume fraction of ferritic phase. With the addition of 4 pct Mo, however, the resistances to pitting corrosion and SCC recovered to those of the specimen without Mo. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mo contents was discussed based on the micrographic and fractographic observations.     

Applied Stress Affecting the Environmentally Assisted Cracking

Stress corrosion cracking (SCC) is affected by the mode of applied stress, i.e., tension, compression, or torsion. The cracking is measured in terms of initiation time to nucleate a crack or time to failure. In a simple uniaxial loading under tension or compression, it is observed that the initiation time can vary in orders of magnitude depending on the alloy and the environment. Fracture can be intergranular or transgranular or mixed mode. Factors that affect SCC are solubility of the metal into surrounding chemical solution, and diffusion rate (like hydrogen into a tensile region) of an aggressive element into the metal and liquid metallic elements in the grain boundaries. Strain hardening exponent that affects the local internal stresses and their gradients can affect the diffusion kinetics. We examine two environments (Ga and 3.5 pct NaCl) for the same alloy 7075-T651, under constant uniaxial tension and compression load. These two cases provide us application to two different governing mechanisms namely liquid metal embrittlement (7075-Ga) and hydrogen-assisted cracking (7075-NaCl). We note that, in spite of the differences in their mechanisms, both systems show similar behavior in the applied K vs crack initiation time plots. One common theme among them is the transport mechanism of a solute element to a tensile-stress region to initiate fracture.     

Mechanical properties and microstructure of Al-Li-Cu-Mg-Zr die forgings

Four forgings of Al 8090 alloy were evaluated for strength, toughness, and stress corrosion resistance. A microstructural evaluation was also conducted. The forgings easily met the strength requirements for Al 7075-T73 but only just met the 7 pct minimum elongation requirement. The stress corrosion threshold was less than 35 MPa in the short transverse orientation, a value that is far below the 310 MPa minimum often required for Al 7075-T73. Fracture in areas which received low forging reductions was intergranular, while fracture in more heavily forged areas was also along subgrain boundaries. Possible mechanisms for the low toughness and stress corrosion resistance of Al-Li alloys are discussed. Formerly Senior Staff Research Engineer, Kaiser luminum & Chemical Corporation, Pleasanton, CA 94566,     

Effect of Repair Welding on Electrochemical Corrosion and Stress Corrosion Cracking Behavior of TIG Welded AA2219 Aluminum Alloy in 3.5 Wt Pct NaCl Solution

The stress corrosion cracking (SCC) behavior of AA2219 aluminum alloy in the as-welded (AW) and repair-welded (RW) conditions was examined and compared with that of the base metal (BM) in 3.5 wt pct NaCl solution using the slow strain rate technique (SSRT). The reduction in ductility was used as a parameter to evaluate the SCC susceptibility of both BM and welded joints. The results show that the ductility ratio (ε _NaCl/(ε _air)) of the BM was close to one (0.97) and reduced to 0.9 for the AW joint. This value further reduced to 0.77 after carrying out one repair welding operation. However, the RW specimen exhibited higher ductility than the single-weld specimens even in 3.5 wt pct NaCl solution. SSRT results obtained using pre-exposed samples followed by post-test metallographic observations clearly showed localized pitting corrosion along the partially melted zone (PMZ), signifying that the reduction in ductility ratio of both the AW and RW joints was more due to mechanical overload failure, caused by the localized corrosion and a consequent reduction in specimen thickness, than due to SCC. Also, the RW joint exhibited higher ductility than the AW joint both in air and the environment, although SCC index (SI) for the former is lower than that of the latter. Fractographic examination of the failed samples, in general, revealed a typical ductile cracking morphology for all the base and welded joints, indicating the good environmental cracking resistance of this alloy. Microstructural examination and polarization tests further demonstrate grain boundary melting along the PMZ, and that provided the necessary electrochemical condition for the preferential cracking on that zone of the weldment.     

Chloride Ion Activity and Susceptibility of Al Alloys 7075-T6 and 5083-H131 to Stress Corrosion Cracking

The influence of chloride ion activity on the susceptibility of aluminum alloys 5083-H131 and 7075-T6 to stress corrosion cracking (SCC) was investigated by conducting slow strain-rate tensile tests at a strain-rate of 10^?7 s^?1 in naturally aerated aqueous solutions with varying NaCl mass fraction (0.001 to 20 pct) and in a 3.5 pct mass fraction NaCl solution with varying strain-rates (10^?8 to 10^?4 s^?1). This study found that both alloys exhibited reduced strengths and failure strains (times) in the solutions compared with laboratory air. The extent of these reductions was greater in alloy 5083 for the conditions examined. The strength and ductility of both alloys decreased with chloride ion activity in a manner that indicates a chemical reaction is responsible. The strength and ductility of both alloys decreased with strain-rate in a sigmoidal manner, but the transition in alloy 7075 occurred at slower strain-rates of approximately two orders of magnitude. It was deduced that the chloride ion interacts chemically with the passivated surface in the potential gradient at the crack tip to cause SCC. While no mechanism of cracking can be eliminated on the basis of these results alone, the results are consistent with the hypothesis that the absorbed hydrogen causes cracking in alloy 7075 while cracking in 5083 is the result of a dissolution mechanism.     

Stress corrosion cracking of an aluminum alloy under compressive stress

Stress corrosion cracking (SCC) of 7075 aluminum alloy in a 3.5 pet NaCl aqueous solution under compressive stress was investigated using modified WOL notched specimen. The result showed that SCC could occur if an applied compressive displacement was larger than a critical value. Finiteelement analysis indicated that there was a stress concentration and the stress components were negative at the notch tip under the compressive displacement. Since the unloaded displacements were equal but opposite to the loaded ones, no stress relaxation occurred throughout SCC. Thus, the SCC was induced by compressive stress. The threshold stress intensity nucleating SCC from the notch under the compressive applied stress was 27.6 MPa m^1/2, but the corresponding value under tensile stress was 8.3 MPa m^1/2. Besides, the incubation period for SCC under compressive stress was one order of magnitude longer than that under tensile stress in the sameK ₁ The fracture surfaces of SCC under compressive stress were quite different from those under tensile stress. The latter was composed of intergranular but the former was quasi-cleavage characterized by parallel striation pattern. Formerly Student at Beijing University of Iron and Steel Technology     

Effect of Multipasses on Microstructure and Electrochemical Behavior of Weldments

Shielded metal arc welding was applied to AISI 1045 medium carbon steel. The microstructural changes and electrochemical corrosion behavior of the heat-affected zone (HAZ), base metal (BM), and weld zone (WZ) were investigated. The effect of welding passes on microstructural changes of BM, HAZ, and WZ were elucidated using optical microscopy, potentiodynamic Tafel scan, and linear polarization resistance (LPR) methods in plain water and 3.5 pct (w/v) NaCl solution under standard temperature and pressure using corrosion kinetic parameters. From microstructural observations, the variations in ferrite morphology in the BM and WZ showed dissimilar electrochemical corrosion behavior and a corrosion rate than that of HAZ.     

Stress Corrosion Cracking Behavior of Multipass TIG-Welded AA2219 Aluminum Alloy in 3.5?wt?pct NaCl Solution

The stress corrosion cracking (SCC) behavior of the AA2219 aluminum alloy in the single-pass (SP) and multipass (MP) welded conditions was examined and compared with that of the base metal (BM) in 3.5?wt?pct NaCl solution using a slow-strain-rate technique (SSRT). The reduction in ductility was used as a parameter to evaluate the SCC susceptibility of both the BM and welded joints. The results showed that the ductility ratio (?? _NaCl/(?? _air) was 0.97 and 0.96, respectively, for the BM and MP welded joint, and the same was marginally reduced to 0.9 for the SP welded joint. The fractographic examination of the failed samples revealed a typical ductile cracking morphology for all the base and welded joints, indicating the good environmental cracking resistance of this alloy under all welded conditions. To understand the decrease in the ductility of the SP welded joint, preexposure SSRT followed by microstructural observations were made, which showed that the decrease in ductility ratio of the SP welded joint was caused by the electrochemical pitting that assisted the nucleation of cracks in the form of corrosion induced mechanical cracking rather than true SCC failure of the alloy. The microstructural examination and polarization tests demonstrated a clear grain boundary (GB) sensitization of the PMZ, resulting in severe galvanic corrosion of the SP weld joint, which initiated the necessary conditions for the localized corrosion and cracking along the PMZ. The absence of PMZ and a refined fusion zone (FZ) structure because of the lesser heat input and postweld heating effect improved the galvanic corrosion resistance of the MP welded joint greatly, and thus, failure occurred along the FZ.     

Environmentally assisted cracking behavior of peak-aged 7010 aluminum alloy containing scandium

The 7010 Al alloy with and without addition of 0.25 wt pct Sc in peak-aged condition was examined for its environmentally assisted cracking (EAC) behavior. Slow strain rate testing (SSRT) per ASTM standard G129-00 was employed to investigate EAC. The base 7010 Al alloy showed 10 pct elongation, 9.9 pct reduction in area, and 561 MPa ultimate tensile strength (UTS), when tested in air. The ductility of the base alloy dropped to 3 and 3.3 pct in terms of elongation and reduction in area, respectively, when tested in 3.5 pct NaCl solution, showing its high susceptibility to EAC. On the other hand, the 0.25 wt pct Sc containing alloy showed a significant improvement in ductility not only in air but also in 3.5 pct NaCl solution, without any loss in the UTS. Thus, the 0.25 wt pct Sc containing alloy exhibited 13.4 pct elongation, 15.8 pct reduction in area, and 560 MPa UTS in air and 12.5 pct elongation, 16.4 pct reduction in area and 560 MPa UTS in 3.5 pct NaCl solution. The study for the first time shows that the high resistance to EAC of 7010 alloy can be imparted even in peak-aged condition by the addition of 0.25 wt pct Sc.