Aluminium Alloy AlSi7Mg Research Articles

A Comparative Study of Self-Piercing Riveting and Friction Self-Piercing Riveting of Cast Aluminum Alloy Al–Si7Mg

Abstract Cast aluminum alloys are promising materials that can simplify the manufacturing process of automobile body structures. However, the low ductility of cast aluminum poses significant challenges to existing riveting technologies. In the present work, dissimilar AA6061-T6 aluminum alloy and Al–Si7Mg cast aluminum were joined by self-piercing riveting (SPR) and friction self-piercing riveting (F-SPR) processes to reveal the effect of friction heat on rivetability of low-ductility cast aluminum alloys. The joint macro-morphology, microstructure, peak tooling force, microhardness distribution, tensile-shear, and cross-tension performance of the two processes were comparatively studied. Results indicated that the in-situ softening effect of friction heat in the F-SPR process could effectively improve the ductility of cast aluminum, avoid cracking, and reduce the tooling force by 53%, compared to the SPR process. The severe plastic deformation and friction heat induced by rivet rotation results in refined equiaxed grains of aluminum near the rivets and solid-state bonding between aluminum sheets in the rivet cavity. The F-SPR joints are superior to SPR joints in both tensile-shear and cross-tension performance due to the avoidance of cracking, increase of mechanical interlocking, and solid-state bonding of interfaces. Significantly, when Al–Si7Mg is placed on the lower layer, the peak tensile-shear and cross-tension loads of the F-SPR joints are 7.2% and 45.5% higher than the corresponding SPR joints, respectively.

Effect of Platform Temperature and Post-Processing Heat Treatment on the Fatigue Life of Additively Manufactured AlSi7Mg Alloy

The effect of platform temperature combined with a T5 heat treatment on the fatigue life of additively manufactured aluminum alloy AlSi7Mg was characterized and understood. High-cycle fatigue tests were carried out on samples built with four platform temperatures (35 °C, 60 °C, 80 °C and 200 °C) and post-processing heat treatment strategies (F and T5). Microstructural and fractographic observations combined with microhardness measurements were performed. A log-normal statistical distribution regressed with 90% B-basis probabilities of survival revealed that specimens produced on a platform maintained at 80 °C and post-processed with a T5 heat treatment presented the highest fatigue life among the conditions tested. Precipitation of silicon within the aluminum cells during the T5 heat treatment is the proposed explanation for the improved fatigue life of the T5 samples. In the as-built condition, specimens produced at 200 °C were found to be less resistant to fatigue than the specimens built at lower temperatures. The coarser microstructure and lowest microhardness resulting from high-temperature manufacturing explain this reduced fatigue strength. All fatigue cracks initiated from manufacturing discontinuities. This led to a fatigue life prediction model based upon linear elastic fracture mechanics. The model was fitted to the experimental results of the F and T5 samples separately. With the exception of the 35 °C—T5 specimens, the predicted fatigue lives agree with the experimental results and literature.

Experimental Investigation of Second-Harmonic Lamb Wave Generation in Additively Manufactured Aluminum

The correlation between the nonlinear acousto-ultrasonic response and the progressive accumulation of fatigue damage is investigated for an additively manufactured aluminum alloy AlSi7Mg and compared with the behavior of a conventional wrought aluminum alloy 6060-T5. A dual transducer and wedge setup is employed to excite a 30-cycle Hann-windowed tone burst at a center frequency of 500 kHz in plate-like specimens that are 7.2 mm thick. This choice of frequency-thickness is designed to excite the symmetric Lamb mode s1, which, in turn, generates a second-harmonic s2 mode in the presence of distributed material nonlinearity. This s1-s2 mode pair satisfies the conditions for internal resonance, thereby leading to a cumulative build-up of amplitude for the second-harmonic s2 mode with increasing propagation distance. Measurements of a nonlinearity parameter β derived from the second-harmonic amplitude are plotted against propagation distance at various fractions of fatigue life under constant amplitude loading, for three different stress levels corresponding to low-cycle fatigue (LCF), high-cycle fatigue (HCF), and an intermediate case. The results show both qualitative and quantitative differences between LCF and HCF, and between the additively manufactured specimens and the wrought alloy. The potential use of this nonlinearity parameter for monitoring the early stages of fatigue damage accumulation, and hence for predicting the residual fatigue life, is discussed, as well as the potential for quality control of the additive manufacturing (AM) process.

Fatigue crack initiation and damage mechanisms during ultrasonic fatigue testing of cast aluminium alloy AlSi7Mg

During the last years, an increased interest in in situ-damage characterization during ultrasonic fatigue testing evolved. Different non-contact in situ-surveillance systems have been developed or applied to the present ultrasonic fatigue testing systems, e.g. analysis of the nonlinearity parameter and the resonant frequency [1, 2] as well as thermal measurements [3–5]. In addition, Heinz et al. [6] performed noncontact strain measurements during ultrasonic fatigue testing. The aim of the present investigations is to evaluate the fatigue crack initiation in the cast aluminium alloy AlSi7Mg (A356) in sand and die cast condition under high cycle and very high cycle fatigue loading conditions. Therefore, a combined analysis of the development of the specimen temperature, the resonant frequency fres and the nonlinearity parameter rel was performed. For such investigations it is essential to keep the ambient temperature strictly constant and the increase of temperature due to the resonant conditions as low as possible. Therefore, a spot cooler using a vortex tube was applied to the ultrasonic fatigue testing system. With this special cooling device it was possible to keep the ambient temperature constant. An infrared thermo-camera system was used for the in situ-examination of the specimen temperature and the surveillance of the ambient temperature during ultrasonic fatigue testing. The determination of the resonant frequency and the nonlinearity parameter was realized online during fatigue tests according to Mayer et al. [2]. After failure, the fracture surfaces of the specimens were investigated intensively by scanning electron microscopy (SEM). According to Krewerth et al. [5], the correlation of the results from thermographic and fractographic investigations can shed some light on the crack initiation point. Figure 1a shows the S-N curves of the fatigue tests on the aluminum specimens produced by sand casting (batch 1) and die-casting (batch 2) in hot isostatically pressed condition, respectively. Specimens of both batches failed due to casting porosity. However, the casting porosity in the sand cast material was significantly higher leading to a lower fatigue strength. Several specimens were investigated by thermographic measurements accompanied by simultaneous analysis of the resonant frequency as well as the nonlinearity parameter. Exemplarily, specimen 1 (sand casting, see Fig. 1a) is explained here in more detail. The development of the resonant frequency (fres), the nonlinearity parameter ( rel), the ambient temperature Tamb, the maximum temperature Tmax and the temperature variation T is shown in fig. 1b. This specimen failed due to an internal defect at Nf = 2.16 × 106 cycles. The dashed black

Investigation of the structure and mechanical properties of castings of alloy AlSi7Mg, cast irons GG15 and GG25 and steel GX120Mn12, modified by nanosized powders

The influence of nanopowders on the structure and mechanical properties was investigated in castings of aluminium alloy AlSi7Mg, grey cast irons GG15 and GG25 and steel GX120Mn12. Mixtures of SiC, TiCN and Y2O3 nanopowders were used. These mixtures or nanocompositions were coated with Cu, Cr, Ni and Fe metal protectors in order to improve wetting of the metal melt. Introducing small amounts of nanocompositions (less than 0.1 wt%) into the melt was found to refine the microstructure and improve the mechanical properties of the castings.

A Comparative Study on Infrared Thermography during Ultrasonic Fatigue Testing of Cast Steel 42CrMo4 and Cast Aluminium Alloy AlSi7Mg

The present paper illustrates a comparison of infrared thermography during ultrasonic fatigue testing of cast steel 42CrMo4 and cast aluminium alloy AlSi7Mg. Against the background of different material properties (e.g. mechanical properties as well as thermal properties) the benefit of this non-destructive material testing method in terms of determining the crack initiation point and time during fatigue testing as well as crack propagation is evaluated and discussed. Moreover, correlations between fractography and infrared thermography are performed for both materials.

Comparison of Interfacial Reactions Between AlSi7Mg and Alumina Filter After Casting and Spark Plasma Sintering**

AbstractSo‐called “small impingement tests” were performed with alumina ceramic foam filters and melt of the aluminum alloy AlSi7Mg (A356). A second set of samples was produced using the “spark plasma sintering” (SPS). After solidification, the resulting interface formation between metal melt and ceramic was compared to evaluate differences between the impingement test that is very close to industrial metal melt filtration conditions and the powder metallurgical SPS process without convection of the liquid metal. The melt generation and interface formation during SPS was characterized by high heating rates of up to 1500 K min−1, holding times of 1 min up to 60 min and fast cooling. Using energy and wavelength dispersive X‐ray spectroscopy, indications for spinel (MgAl2O4) were found at the original interface between AlSi7Mg melt and the alumina filter. Thermodynamic calculations were conducted and spinel formation was confirmed. It was demonstrated that SPS could serve as a fast and reliable tool to facilitate the reactions that occur at metal melt/ceramic interfaces during filtration.

Application of in situ thermography for evaluating the high-cycle and very high-cycle fatigue behaviour of cast aluminium alloy AlSi7Mg (T6)

The present paper illustrates the application of infrared thermal measurements for the investigation of crack initiation point and crack propagation in the high-cycle and the very high-cycle fatigue range of cast AlSi7Mg alloy (A356). The influence of casting defects, their location, size and amount was studied both by fractography and thermography. Besides internal and surface fatigue crack initiation as a further crack initiation type multiple fatigue crack initiation was observed via in situ thermography which can be well correlated with the results from fractography obtained by SEM investigations. In addition, crack propagation was studied by the development of the temperature measured via thermography. Moreover, the frequency influence on high-cycle fatigue behaviour was investigated. The presented results demonstrate well that the combination of fractography and thermography can give a significant contribution to the knowledge of crack initiation and propagation in the VHCF regime.

Combustion synthesis and development of Ti–O–C aluminium composites

Aluminium matrix composite reinforced with Ti compounds was successfully fabricated by SHS combustion synthesis and squeeze casting course. Prepared samples from mixture containing Ti, C and Al 2O 3 fibres were heated in microwave reactor to ignite synthesis and produce porous preform for subsequent infiltrating with liquid metal. Studies showed that synthesizing temperature has been remarkably increased by applying higher magnetron power and addition of graphite. Synthesis of specimens prepared from preliminary ball milled Ti and C powders proceeded at the highest propagation wave velocity. Microwave heating of metal Ti powder in the stream of CO 2 resulted in formation of corrugated precipitates composed of titanium oxide with carbon inclusions TiO(C) and Ti 2O 3. The produced preforms were impregnated by squeeze casting with the aluminium alloy AlSi7Mg. Proper interface with slight reduction of Ti oxide between the reinforcement and the matrix was developed. Subsequently, the samples were annealed at 500 and 1000 °C. Annealing at the lower temperature induced creation of Ti 3O 2(C) and Al 2O 3. This process was continued at 1000 °C, and additionally some Ti(Al 0,8Si 0,2) 3 pellets appeared in the matrix. With prolonged annealing, oxygen was completely removed from Ti compound and oval grains of Ti(C) were created, enveloped with Al 2O 3. In the matrix, larger and numerous Ti 3AlSi 5 pellets were formed. Hardness examination showed that the best strengthening effect was achieved after annealing at 1000 °C.

Thixo-forging and simulation of complex parts of aluminum alloy AlSi7Mg

In order to investigate the formability of metal material in semisolid state, a series of experiments were carried out by thixo-forging the complex part of aluminum alloy AlSi7Mg. Through changing the upper punch, aluminum parts with different upper-cup dimensions can be successfully produced. The numerical simulation was conducted for investigating the forming limits of AlSi7Mg during thixo-forging. It is found that the simulation result is in good agreement with the experiment one.

Investigation on Forming Defects During Thixo-forging of Aluminum Alloy AlSi7Mg

Investigation on Forming Defects During Thixo-forging of Aluminum Alloy AlSi7Mg

Fatigue behaviour of graphite and interpenetrating graphite–aluminium composite up to 10 9 load cycles

Fatigue behaviour of polycrystalline, isotropic graphite FU2590 (porosity 13 vol%) and of FU2590 infiltrated with the aluminium alloy AlSi7Mg (FU2590/AlSi7Mg) is investigated. The interpenetrating graphite–aluminium composite is produced by squeeze casting infiltration, where the open porosity of FU2590 (10–11 vol%) is infiltrated with AlSi7Mg. Fully reversed bending fatigue tests at cycling frequency 25 Hz and fully reversed tension–compression experiments at 20 kHz are performed. Fatigue data of FU2590 measured with both methods are comparable when presented vs. the normalized maximum stress, i.e. maximum tension stress of a cycle divided by the bending or tensile strength, respectively. The mean endurance limit of FU2590 at 10 9 cycles under tension–compression loading is at 23.0 ± 0.9 MPa (normalized maximum stress 0.65 ± 0.03), and 26.1 ± 1.5 MPa (normalized maximum stress 0.55 ± 0.03) is found for FU2590/AlSi7Mg. The increased endurance limit of FU2590/AlSi7Mg is mainly caused by the 15% higher stiffness compared with FU2590. In reversed bending experiments, at cycles to failure below 10 7, the composite shows about 30% higher cyclic strength. The beneficial influence of infiltration on static properties is more pronounced, and 50% higher bending strength and 35% higher tensile strength are determined. In the graphite, fatigue cracks predominantly initiate at porosity. In the composite, cracks initiate at pores, which are filled with AlSi7Mg. Interface failures and failures of the aluminium alloy limit the beneficial influence of infiltration on the cyclic properties.