Development of a novel heat treatment for high performances L-PBF Ti6Al4V alloy: microstructural and mechanical characterization

This work presents the experimental development of a novel heat treatment for a high performance Laser Powder Bed Fusion Ti6Al4V alloy. Additive manufacturing production processes for titanium alloys are particularly of interest in cutting-edge engineering fields, however, high frequency laser induced thermal cycles generate a brittle as built microstructure. For this reason, heat treatments compliant with near net shape components are needed before their homologation and usage. The experimental campaign focused on the development of a multi-step heat treatment leading to a bilamellar microstructure. In fact, according to literature, such a microstructure should be promising in terms of mechanical properties both under static and cyclic loads. The heat treatment development has asked for the preliminary analyses of samples annealed and aged in laboratory, implementing several cycles, differing for what concerns temperatures, times and cooling rates. Such a characterization has been carried out through optical and electron microscopy analyses, image analyses, hardness and tensile tests. As a result, the most suitable thermal cycle has been selected and performed using industrial equipment on mini bending fatigue samples with different surface conditions. The same tests have been performed on a batch of traditionally treated samples, to provide with a comparison. This master thesis activity has finally led to the definition of a heat treatment resulting into a bilamellar microstructure, promising in terms of fatigue performances with respect to the traditionally treated alloy ones. The industrial implementation of such a heat treatment will require further improvements, particularly for what concerns the post annealing water quench, in order to prevent any surface alteration potentially responsible for the fatigue performances drop. Further development of the research may also include push-pull fatigue tests, crack grow propagation and residual stresses analyses.

Studio del comportamento meccanico e tribologico della lega AlSi10Mg prodotta mediante Laser-based Powder Bed Fusion sottoposta ad un ciclo tecnologico ottimizzato

AlSi10Mg alloy is one of the most widely used alloys for producing structural components by Laser-based Powder Fusion (L-PBF) technology due to the high mechanical and technological properties. The present work aims to characterize mechanically and tribologically the L-PBF AlSi10Mg alloy subjected to both heat treatment and surface modification cycles. Specifically, the effects of three heat treatments on the tribological and mechanical properties of the alloy were analyzed: T5 (artificial aging at 160 °C for 4 h), T6 rapid solution heat treatment (solution heat treatment at 510 °C for 1h and aging at 160 °C for 6 h), and T6 benchmark (solution heat treatment at 540 °C for 1h and aging at 160 °C for 4 h), the latter used as a benchmark. The study highlighted how the better balance between strength and ductility properties induced by the introduction of heat treatments leads to lower wear resistance and not significant variations in the friction coefficient of the alloy. The tribological and mechanical behavior of the alloy coated with two different coating structures, consisting of (i) chemical Ni (Ni-P) and (ii) Ni-P + DLC, was also evaluated. The goal was the identification of a deposition cycle such as to guarantee the optimization of the mechanical and tribological behavior of the alloy. The Ni-P coating provided good wear resistance but an increase in the coefficient of friction. In contrast, using the DLC top coating resulted in excellent tribological performance in wear resistance and friction coefficient. The samples characterized by the Ni-P + DLC multilayer coating were subsequently subjected to mechanical characterization. The results obtained highlighted problems of adhesion and incipient breaking of the material due to the different mechanical behavior of the coating, considerably reducing the mechanical performance of the alloy coated with Ni-P+DLC multilayer solution compared to the specimens in the un-coated condition.

Chemical composition modification of casting aluminium alloys for engine applications

The research activities were focused on evaluating the effect of Mo addition to mechanical properties and microstructure of A354 aluminium casting alloy. Samples, with increasing amount of Mo, were produced and heat treated. After heat treatment and exposition to high temperatures samples underwent microstructural and chemical analyses, hardness and tensile tests. The collected data led to the optimization of both casting parameters, for obtaining a homogeneous Mo distribution in the alloy, and heat treatment parameters, allowing the formation of Mo based strengthening precipitates stable at high temperature. Microstructural and chemical analyses highlighted how Mo addition in percentage superior to 0.1% wt. can modify the silicon eutectic morphology and hinder the formation of iron based β intermetallics. High temperature exposure curves, instead, showed that after long exposition hardness is slightly influenced by heat treatment while the effect of Mo addition superior to 0,3% is negligible. Tensile tests confirmed that the addition of 0.3%wt Mo induces an increase of about 10% of ultimate tensile strength after high temperature exposition (250°C for 100h) while heat treatments have slight influence on mechanical behaviour. These results could be exploited for developing innovative heat treatment sequence able to reduce residual stresses in castings produced with A354 modified with Mo.

Investigation of gold-based catalysts for liquid phase oxidation of glucose to glucaric acid

Glucaric acid (GA) is one of the building block chemicals derived from sugar biomass with higher added value. Nowadays, GA is produced by oxidation of glucose (Glu) with either stoichiometric oxidants (HNO3), or by means of electrochemical or biochemical synthesis. However, these processes show drawbacks from either the environmental or economic viewpoint. For this reason, gold nanoparticles (Au NPs) supported on activated carbon (AC) have been studied as catalysts for the oxidation of Glu, using O2 as oxidant in the presence of a base. Using sol immobilization technique, Au NPs have been supported on AC following different experimental procedures. UV-Vis spectroscopy, XRD, TEM and TG analysis were utilized in the characterization of the catalysts. The operational conditions were optimized obtaining 24% of yield of GA, 37% to GO and 27% to byproducts in 1 h, 1000 rpm, 10 bar of O2 and Glu:Au:NaOH molar ratio of 1000:1:3000. Under such conditions, catalysts show relatively high Glu conversion (≥82%) with different GA yields. GO+GA yield is around 58-61%. Therefore, the oxidation reaction was performed at 15 min where Au/AC PVA0 reached the highest yield of GA (16%) and Au/AC PVA2.4 gave the lowest (8%). It is evident that the presence of PVA influences to a higher degree the reaction rate than the Au NPs size. Hence, the effect of different heat treatments where applied for the removal of PVA: washing with water at 60℃ or heat treatment (120-250℃) with Air/H2. Washing treatment and heat treatment at 120℃ with Air/H2 may have resulted in the mildest treatments for the removal of PVA. Finally, two different supports have been used in order to study the effect of metal-support interaction in the immobilization of Au NPs: ZrO2 and AC. Au/AC catalyst demonstrated a higher conversion of GO to GA at short reaction times (15.1% yield GA) compared to Au/ZrO2 (2.4% yield GA).

Outdoor bronze conservation: assessment of protective treatments by accelerated aging and of treatment removal procedures by laser cleaning

Outdoor bronzes exposed to the environment form naturally a layer called patina, which may be able to protect the metallic substrate. However, since the last century, with the appearance of acid rains, a strong change in the nature and properties of the copper based patinas occurred [1]. Studies and general observations have established that bronze corrosion patinas created by acid rain are not only disfiguring in terms of loss of detail and homogeneity, but are also unstable [2]. The unstable patina is partially leached away by rainwater. This leaching is represented by green streaking on bronze monuments [3]. Because of the instability of the patina, conservation techniques are usually required. On a bronze object exposed to the outdoor environment, there are different actions of the rainfall and other atmospheric agents as a function of the monument shape. In fact, we recognize sheltered and unsheltered areas as regards exposure to rainwater [4]. As a consequence of these different actions, two main patina types are formed on monuments exposed to the outdoor environment. These patinas have different electrochemical, morphological and compositional characteristics [1]. In the case of sheltered areas, the patina contains mainly copper products, stratified above a layer strongly enriched in insoluble Sn oxides, located at the interface with the uncorroded metal. Moreover, different colors of the patina result from the exposure geometry. The surface color may be pale green for unsheltered areas, and green and mat black for sheltered areas [4]. Thus, in real outdoor bronze monuments, the corrosion behavior is strongly influenced by the exposure geometry. This must be taken into account when designing conservation procedures, since the patina is in most cases the support on which corrosion inhibitors are applied. Presently, for protecting outdoor bronzes against atmospheric corrosion, inhibitors and protective treatments are used. BTA and its derivatives, which are the most common inhibitors used for copper and its alloy, were found to be toxic for the environment and human health [5, 6]. Moreover, it has been demonstrated that BTA is efficient when applied on bare copper but not as efficient when applied on bare bronze [7]. Thus it was necessary to find alternative compounds. Silane-based inhibitors (already successfully tested on copper and other metallic substrates [8]), were taken into consideration as a non-toxic, environmentally friendly alternative to BTA derivatives for bronze protection. The purpose of this thesis was based on the assessment of the efficiency of a selected compound, to protect the bronze against corrosion, which is the 3-mercapto-propyl-trimethoxy-silane (PropS-SH). It was selected thanks to the collaboration with the Corrosion Studies Centre “Aldo Daccò” at the Università di Ferrara. Since previous studies [9, 10, 11] demonstrated that the addition of nanoparticles to silane-based inhibitors leads to an increase of the protective efficiency, we also wanted to evaluate the influence of the addition of CeO2, La2O3, TiO2 nanoparticles on the protective efficiency of 3-mercapto-propyl-trimethoxy-silane, applied on pre-patinated bronze surfaces. This study is the first section of the thesis. Since restorers have to work on patinated bronzes and not on bare metal (except for contemporary art), it is important to be able to recreate the patina, under laboratory conditions, either in sheltered or unsheltered conditions to test the coating and to obtain reliable results. Therefore, at the University of Bologna, different devices have been designed to simulate the real outdoor conditions and to create a patina which is representative of real application conditions of inhibitor or protective treatments. In particular, accelerated ageing devices by wet & dry (simulating the action of stagnant rain in sheltered areas [12]) and by dropping (simulating the leaching action of the rain in unsheltered areas [1]) tests were used. In the present work, we used the dropping test as a method to produce pre-patinated bronze surfaces for the application of a candidate inhibitor as well as for evaluating its protective efficiency on aged bronze (unsheltered areas). In this thesis, gilded bronzes were also studied. When they are exposed to the outside environment, a corrosion phenomenon appears which is due to the electrochemical couple gold/copper where copper is the anode. In the presence of an electrolyte, this phenomenon results in the formation of corrosion products than will cause a blistering of the gold (or a break-up and loss of the film in some cases). Moreover, because of the diffusion of the copper salts to the surface, aggregates and a greenish film will be formed on the surface of the sample [13]. By coating gilded samples with PropS-SH and PropS-SH containing nano-particles and carrying out accelerated ageing by the dropping test, a discussion is possible on the effectiveness of this coating, either with nano-particles or not, against the corrosion process. This part is the section 2 of this thesis. Finally, a discussion about laser treatment aiming at the assessment of reversibility/re-applicability of the PropS-SH coating can be found in section 3 of this thesis. Because the protective layer loses its efficiency with time, it is necessary to find a way of removing the silane layer, before applying a new one on the “bare” patina. One request is to minimize the damages that a laser treatment would create on the patina. Therefore, different laser fluences (energy/surface) were applied on the sample surface during the treatment process in order to find the best range of fluence. In particular, we made a characterization of surfaces before and after removal of PropS-SH (applied on a naturally patinated surface, and subsequently aged by natural exposure) with laser methods. The laser removal treatment was done by the CNR Institute of Applied Physics “Nello Carrara” of Sesto Fiorentino in Florence. In all the three sections of the thesis, a range of non-destructive spectroscopic methods (Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), μ-Raman spectroscopy, X-Ray diffractometry (XRD)) were used for characterizing the corroded surfaces. AAS (Atomic Absorption Spectroscopy) was used to analyze the ageing solutions from the dropping test in sections 1 and 2.