T6 Hardness Research Articles

Corrosion and hardness properties of retrogression‐formed and warm‐formed AA7075 sheet

AbstractTo achieve combined formability and strength in high‐strength precipitation‐hardened aluminum automotive sheets, different forming path used with fast and slow forming technology has been proposed. This study reports hardness and corrosion properties from two elevated‐temperature forming techniques, retrogression forming (RF) and warm forming (WF), employed on AA7075 alloy sheets. Parameters such as optimal pre‐aging, re‐aging, WF, and RF temperature and time were determined. In the retrogression‐formed (R‐F) sheets, initial retrogression treatment of peak‐aged (i.e., T6) temper resulted in significant loss of properties, but final re‐aging step recovered most. When traditional peak‐aging treatment was used to re‐age R‐F sheets, their T6 hardness and corrosion properties were restored to 95%–97% and 86%, respectively, while re‐aging R‐F sheets using industrial post‐forming paint‐bake treatment restored their T6 hardness and corrosion properties to 87%–92% and 78%–79%. In the warm‐formed (W‐F) sheets, increasing pre‐aging temperature and time resulted in increasing properties, and properties of the best W‐F samples matched the R‐F samples. In addition, shorter‐time forming did not significantly reduce properties of formed part, encouraging the use fast‐forming technology. Overall, these findings show that sheets subjected to RF and WF techniques can possess T6 properties in formed parts.

A Combined Numerical and Experimental Approach to Evaluate the Hardness of AA Aluminum 6063 T6 with A Ni-P Coating

The need to improve the surface properties of materials, has led to the development of parts coated with hard materials deposited on substrates. In this work, by using the finite element method the AA Aluminium 6063 T6 hardness with an error less of the 5 % of the Brinell hardness (HBN), in comparison with the experimental data supplied, was determined. For this, the modelling of a spherical indentation test was carried out, in which the indentation loads and the projected contact radius are obtained from several potential adjustments. Then, using the Lessage-Pertúz model the hardness of the Ni-P coating was determined, obtaining a value of 4.86 GPa.

Effects of Sn/In additions on natural and artificial ageing of Al–Mg–Si alloys

ABSTRACTThe effect of Sn or In addition on natural ageing and subsequent artificial ageing of Al–Mg–Si alloys was investigated. In addition not only retarded the natural ageing but also improved the T6 hardness. Although Sn addition retarded the natural ageing as well, precipitation kinetics was delayed. In addition, slightly refined the β′′ phase distribution. But β′′ phase was coarse with addition of Sn. During artificial ageing, the vacancies released from binding complexes promote the formation of β′′. The detrimental effects of Sn on artificial ageing could be attributed to the reduction of the amount of Sn solutes and of available Mg solutes forming β′′.

Effect of swaging and rolling post sintering treatments on the corrosion behaviour of Alumix 321 PM alloy

In this work, two post sintering hot deformation processes were applied on Alumix 321 PM alloy which is a commercially available aluminum powder alloy which can be considered equivalent to the wrought AA6061 alloy. The powder was compacted, sintered, and then either hot swaged or hot rolled. The results show that both hardness and density were improved as a result of the applied post sintering treatment. Theoretical density (TD) of 99.1% and 99.39% were recorded for the hot rolled and hot swaged alloys, respectively. T6 hardness improves to 106±6 HRE and 96±2 HRE for hot rolled and hot swaged alloys, respectively. The performance was assessed in terms of corrosion behaviour by testing the Alumix 321 PM alloy in as sintered, and as hot deformed conditions after exposure to 3.5wt% NaCl solution. Electrochemical techniques were used to study the effect of hot deformation on corrosion behaviour. Corrosion current density decreases as a result of hot swaging and hot rolling and in both cases the values were less than those of the wrought counterpart, AA6061 alloy. The stair step polarization was applied; the results obtained support that the technique seems promising to bridge the gap between traditional and accelerated corrosion test methods. Pitting was the main corrosion mechanism for the wrought alloy while, pitting, crevice, and intergranular corrosion were observed for the PM alloys.

The Effect of Natural Pre-Ageing on the Mechanical Properties of Rheo-High Pressure Die Cast Aluminium Alloy 2139

Near-net shape casting of wrought aluminium alloys has proven to be difficult due to hot tearing. The Council for Scientific and Industrial Research (CSIR) has successfully processed wrought aluminium alloy 2139 into plate castings using the Rheo-high pressure die casting process (R-HPDC). Alloy 2139 is a Ag-containing aluminium alloy from the Al-Cu-Mg 2xxx series family. The addition of Ag enhances the age hardening response through the formation of co-clusters that act as precursors to the formation of plate-like Ω precipitates. These co-clusters typically form during natural ageing and 12-24 h of natural pre-ageing is normally specified before artificial ageing in Ag-containing Al-Cu-Mg alloys. The T6 hardness and tensile properties of R-HPDC 2139 alloy were investigated with and without natural pre-ageing. It is shown that there is no significant difference in both peak hardness and tensile properties in R-HPDC alloy 2139 with and without natural pre-ageing. The possible precipitation phenomena in both cases are discussed.

Heat treatment of thixoformed EN AW 7075 alloy

Heat treatment of thixoformed EN AW 7075 alloy was investigated in the present work. The hardness of the thixoformed alloy is 76±7 HV. This is nearly equivalent to the soft temper for this particular alloy. Zn, Cu and Mg are predominantly in the eutectic phase in the thixoformed state and have no favourable impact on strength. Besides, forming has occurred in the semisolid state with no evidence of plastic deformation. The maximum hardness that can be achieved after a commercial aging cycle, 16 h at 121°C, is 105 HV. The failure of the thixoformed alloy to adequately age harden is attributed to the lack of Zn, Mg and Cu in solution to offer adequate precipitation hardening capacity. It takes at least a 2 h long solution heat treatment at 470°C to solutionise the interglobular eutectic phase and to offer sufficient precipitation hardening capacity. The T6 and T7 hardness values thus obtained are 181 and 170 HV respectively, and demonstrate a remarkable age hardening potential. The superior T6 hardness of the thixoformed alloy with respect to the hot extruded counterpart is attributed to improved aging kinetics in the former, possibly due to a globular matrix morphology which provides relatively shorter diffusion paths.

Heat treatment of twin-belt cast EN AW 7075 alloy

Abstract The potential of twin-belt cast EN AW 7075 sheet for structural sheet applications was investigated with a particular emphasis on its response to the standard industrial T6 heat treatment. The twin belt cast EN AW 7075 strip is dendritic with an α(Al)–Mg(Zn,Cu,Al) 2 eutectic and predominantly Mg 2 Si and Al 12 (Fe,Cr,Mn) 3 Si particles at interdendritic sites. The α(Al)–Mg(Zn,Cu,Al) 2 eutectic has largely transformed into a Mg(Zn,Cu,Al) 2 precipitate-rich network during the break down pass. The coarse intermetallic particles have fragmented while the fine intragranular precipitates have coarsened upon further hot rolling to a gauge of 2 mm. The predominant restoration process during rolling at 350 °C was dynamic recovery with no evidence of recrystallization. A fraction of Mg(Zn,Cu,Al) 2 was replaced with Al 2 CuMg. Both phases dissolved during the solution heat treatment at 470 °C while the hot band has entirely recrystallized. Subsequent ageing at 121 °C for 16 h has produced a T6 hardness of 170 HB, considerably higher than that reported for the conventionally processed counterpart (150 HB). In addition to a remarkable age hardening capacity, twin belt cast EN AW 7075 alloy has enjoyed a very high resistance to age softening as inferred from a nearly T6 hardness after the industrial T73 heat treatment with an additional ageing cycle at 160 °C.

The Influence of Prior Natural Aging on the Subsequent Artificial Aging Response of Aluminium Alloy A356 with Respective Globular and Dendritic Microstructures

Alloy A356 is one of the most popular alloys used for semisolid metal forming. The heat treatment cycles that are currently applied to semisolid processed components are mostly those that are in use for dendritic casting alloys. The assumption has been made that these heat treatments are not necessarily the optimum treatments, as the difference in solidification history and microstructure of SSM processed components should be considered. The objective of this study is to determine whether dendritic A356 behaves in a similar way to globular A356 in terms of its response to artificial aging with or without prior natural aging. The results indicate that the differences in microstructures (globular or dendritic) do not have a noteworthy effect on the heat treatment response. It is also shown that strong linear correlations are found between T4 and T6 hardness and wt% Mg of A356, regardless of the casting technique used.

Impact of pre-ageing on age hardening response of twin-belt cast AlMg1SiCu sheet

The potential of twin-belt cast (TBC) AlMg1SiCu sheet for structural automotive applications was investigated with a particular emphasis on the impact of pre-ageing on its age hardening response. The optimum T6 process for the TBC AlMg1SiCu sheet is identified to be a water-quench from the solution heat treatment at 540 °C and a subsequent ageing treatment at 180 °C. This process gives hardness values as high as 120 HV within several hours when ageing at 180 °C is performed shortly after the solution treatment. The age hardening capacity is impaired, however, when the sheet is stored at room temperature before the artificial ageing cycle. Pre-ageing at 100 and 140 °C is effective in improving the age hardening response of the naturally aged 6061 sheet. Pre-ageing suppresses natural ageing and clustering activities and gives lower T4 yet a much higher T6 hardness.

Metallurgical assessment of an emerging Al–Zn–Mg–Cu P/M alloy

The objective of this work was to conduct a detailed assessment of the microstructure and mechanical properties of an emerging Al–Zn–Mg–Cu powder metallurgy (P/M) alloy known as Alumix 431D. A variety of techniques were considered including optical microscopy, X-ray diffraction, electron-probe micro-analysis, thermal dilatometry, and differential scanning calorimetry as well as apparent hardness, tensile testing, and bending fatigue. Alumix 431D exhibited many of the same attributes found in wrought counter parts such as 7075. A sintered density of approximately 99% of theoretical was achieved, indicating that the alloy was highly responsive to sintering. Once heat treated, a T6 hardness of 86 HRB and a room temperature ultimate tensile strength of 448 MPa were noted. Thermal analyses implied that the precipitation behaviour of Alumix 431D closely mimicked comparable 7XXX series wrought alloys and was largely premised on the precipitation of η-phase variants. Tensile properties of the alloy in a T1 temper were found to be relatively stable at temperatures up to 150 °C and 1000 h of exposure time. Those of T6 specimens degraded under the same exposure conditions to the point where equivalency with the T1 product was noted.