Cold Spray (CS) is a low-temperature coating process in which solid powders are accelerated in a De Laval nozzle up to supersonic speed; upon the impact the particles plastically deform and adhere to the substrate without melting. Dissimilar to other thermal spray processes, CS does not involve the melting of the powder, leading to avoid tensile residual stresses and high temperature oxidation. Nowadays different materials can be sprayed, and it can be adopted for corrosion protection, additive manufacturing, and repair, but new fields are continuously explored. Currently, the repair is important not only for the cost efficiency of not scraping the component, but also for the environmental advantages. Thus, a part of this thesis is devoted in the investigation of CS for repairing of localized damages. The affected area must be prepared by machining. Then the geometry of the machined cavity is studied to choose the most suitable repair approach. Choice of the deposition strategy is supported by a developed numerical simulation code which simulates the deposit profile and its growth. The functionality of the code is assessed and validated through a cross-comparison of numerical predictions and experimental results for different geometries and conditions. The necessity of some adjustments has been identified. The other topic of the present work is the employment of CS for fabrication of corrosion protective coating for railway axles. Corrosion could be detrimental for mechanical components, especially for railway axles. In most cases, cracks initiate and propagate from corrosion pits. A zinc-aluminium powder mix has been chosen and sprayed by means of CS. The effectiveness of the coating has been assessed by conducting the relevant corrosion tests on numerous combinations of samples following the relevant standards. In addition, the effect of the coating on the fatigue life of the sprayed parts is investigated by applying the coating on axles and conducting long cycle fatigue tests. The results show the efficiency of coating and its galvanic protective effect and absence of any negative effect on the fatigue life of the coated axle under standard prescribed conditions.
Il Cold Spray (CS) è un processo di deposizione a bassa temperatura in cui polveri solide sono accelerate in un ugello di De Laval raggiungendo velocità supersoniche; all’impatto le particelle si deformano plasticamente e aderiscono al substrato senza fondersi. Diversamente da altri processi termici, nel CS non si ha fusione, ciò permette di non avere stress residui di tensione e processi di ossidazione ad elevate temperature. Attualmente si possono spruzzare diversi materiali ed è possibile utilizzare questa tecnologia per proteggere dalla corrosione, per riparare e in additive manufacturing, ma nuove applicazioni sono continuamente sperimentate. Attualmente, riparare i componenti è importante non solo per il risparmio economico insito alla mancata sostituzione della parte, ma lo è anche per i vantaggi ambientali. Una parte di questa tesi è quindi destinata all’applicazione del CS per riparare danni localizzati. L’area soggetta a danneggiamento deve essere processata. La geometria della cavità lavorata è stata quindi studiata per scegliere il miglior approccio per la riparazione. La scelta della strategia di deposizione è stata supportata da un software che è stato sviluppato, il quale simula il profilo del deposito e la sua crescita. L’ efficacia del codice è stata verificata e validata attraverso il confronto fra i risultati previsti numericamente e quelli ottenuti sperimentalmente per differenti geometrie e condizioni. Si è verificata la necessità di introdurre degli adeguamenti. L’altro aspetto analizzato in questo lavoro è l’utilizzo del CS nel rivestimento di assili ferroviari con lo scopo di proteggere dalla corrosione. Tale fenomeno può essere pericoloso per i componenti meccanici, specialmente per gli assili ferroviari. Tipicamente le cricche iniziano in corrispondenza di alveoli di corrosione. Una polvere ottenuta unendo zinco ed alluminiò è stata spruzzata utilizzando CS. L’efficacia del rivestimento è stata verificata su diversi campioni mediante test corrosivi standardizzati. Inoltre, è stato studiate l’effetto che il rivestimento ha sulla vita a fatica delle parti spruzzate applicando il rivestimento su assili ferroviari e testandoli a fatica. I risultati mostrano l’efficacia del rivestimento, della protezione galvanica ad esso associato e dell’assenza di effetti negativi sulla vita a fatica degli assili spruzzati seguendo le prescritte condizioni standardizzate.
Evaluation of cold spray application for local repair and protective coating
Romanenghi, Luca
2020/2021
Abstract
Cold Spray (CS) is a low-temperature coating process in which solid powders are accelerated in a De Laval nozzle up to supersonic speed; upon the impact the particles plastically deform and adhere to the substrate without melting. Dissimilar to other thermal spray processes, CS does not involve the melting of the powder, leading to avoid tensile residual stresses and high temperature oxidation. Nowadays different materials can be sprayed, and it can be adopted for corrosion protection, additive manufacturing, and repair, but new fields are continuously explored. Currently, the repair is important not only for the cost efficiency of not scraping the component, but also for the environmental advantages. Thus, a part of this thesis is devoted in the investigation of CS for repairing of localized damages. The affected area must be prepared by machining. Then the geometry of the machined cavity is studied to choose the most suitable repair approach. Choice of the deposition strategy is supported by a developed numerical simulation code which simulates the deposit profile and its growth. The functionality of the code is assessed and validated through a cross-comparison of numerical predictions and experimental results for different geometries and conditions. The necessity of some adjustments has been identified. The other topic of the present work is the employment of CS for fabrication of corrosion protective coating for railway axles. Corrosion could be detrimental for mechanical components, especially for railway axles. In most cases, cracks initiate and propagate from corrosion pits. A zinc-aluminium powder mix has been chosen and sprayed by means of CS. The effectiveness of the coating has been assessed by conducting the relevant corrosion tests on numerous combinations of samples following the relevant standards. In addition, the effect of the coating on the fatigue life of the sprayed parts is investigated by applying the coating on axles and conducting long cycle fatigue tests. The results show the efficiency of coating and its galvanic protective effect and absence of any negative effect on the fatigue life of the coated axle under standard prescribed conditions.| File | Dimensione | Formato | |
|---|---|---|---|
|
Thesis_Romanenghi_Luca.pdf
non accessibile
Dimensione
8.77 MB
Formato
Adobe PDF
|
8.77 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/187134