- Post Doctoral
MIT Unit Affiliation:
- Mechanical Engineering
Post Doc Sponsor / Advisor:
Date PhD Completed:
Top 3 Areas of Expertise:
I completed my PhD in March 2015. My thesis was on coating/additive manufacturing technology called cold spray and its applications for repairing damaged structural parts and also bio-medical engineering. The insights in this dissertation contributed to the fundamental understanding of the bonding mechanism at high impact velocity which is vitally important to understanding the structure-property relationship.
My current research interest is experimental and theoretical aspects of advanced materials and manufacturing. I believe the future of manufacturing will be primarily governed by powders and how we consolidate them into different shapes. My long-term career goal is to design required properties at the micro/nano level in starting powders and consolidate them faster, cheaper and into much larger components while designed properties remain intact. My background in mechanical engineering and materials science is important to increase understanding of this significant interdisciplinary process, integrating materials science, metallurgical engineering, and mechanical engineering.
Expected End Date of Post Doctoral Position:
- Leading a multi-disciplinary team on producing bioactive coatings by cold spray for accelerated bone regeneration.
- Served as a lead scientist from Politecnico di Milano in a collaborative project with 12 other partners. In-situ repair of aeronautical components with improved reliability and mechanical strength was achieved by the development of a new customized and optimized coating. http://corsair-project.eu/,
- Led a collaborative project to evaluate a new working window for cold spray deposition at “under-critical conditions” to enhance porosity and successfully developed a porous Ti6Al4V coating, as strong as bone, for biomedical applications.
Cold spray (CS) is a solid state additive manufacturing process in which powders are accelerated in a de Laval nozzle toward a substrate. If the impact velocity exceeds a threshold value, particles endure plastic deformation and adhere to the surface. Different materials such as metals, ceramics, composites and polymers can be deposited using cold spray, creating a wealth of interesting opportunities towards harvesting particular properties. The goal of this thesis can be divided into two main categories:
The first category is process evaluation and understanding the fundamental features of the cold spray. In this regard, the focus is on the followings:
- assessment of critical and erosion velocities: To reveal the phenomenological characteristic of interface bonding in cold spray, severe and localized plastic deformation and material jet formation at the impacting interface is studied. Six metals and alloys e.g. Ta, Cu, Ni, Al2024, Ti6Al4V and SS304 are examined to cover a wide range of physical, thermal and mechanical properties. General deformation, induced plastic strain, temperature rise in the particle and in particular in the material jet are presented and discussed. The first physics-based model is proposed to predict critical velocity applicable to all metals and alloys.
- The mechanical behavior of cold spray coating under different loading condition and how it differs from the conventional manufacturing processes: Cold spray deposited coatings, unlike bulk materials, show strong dependency on the indentation size scale, different from the typical Nix-Gao behavior. The hardness suddenly decreases in the micro-indentation region in comparison to nano-indentation region. This is related to the existence of macroscopic defects such as interparticle boundaries, subsequent splats boundary cracking and porosities. To interpret the experimental observation, a finite element model is developed. The interparticle damage is taken into consideration. Ductile damage initiation in combination with the linear damage evolution models are used. The deviation of load-displacement curves in the material with inter-particle defects in comparison to bulk material is studied. A load bearing reduction parameter is proposed to evaluate the excursion of load-depth data.
- Two comprehensive reviews: The first review is on different material systems that have been cold sprayed. The review covers different material systems that have been studied up to now with a view on potential innovative applications. This includes metallic, ceramic, metal matrix composite (MMC), polymer (both as substrate and coating) and nano-structured powders. A critical discussion on the future of this technology is provided.The second review is on cold spray to mitigate corrosion. The review includes application of cold spray to provide superior corrosion resistance by depositing more noble materials than substrate as well as sacrificial protection. Special attention is given to understand how cold spray parameters and conditions could affect the resulting corrosion behavior of the coating. The effect of deposition temperature and pressure, particle size, carrier gas, post-treatment and co-deposition of metals and ceramic particles on corrosion behavior is discussed. A few examples of cold spray applications against corrosion in various fields (biomedical, naval, electronics and hot corrosion) are presented. A discussion with a possible future perspective is also given.
The second category is investigating the application of cold spray coating mainly on repairing damaged parts and biomedical engineering.
- Repairing damaged structural parts: a systematic study of defect shape and ability of cold spray to fill it and shape optimization for maximum bond strength is done. Furthermore, repaired part must retain bulk material properties to withstand service loads. Fatigue represents one of the most intricate types of damage to which structural parts are subjected in service. The effect of cold spray deposition on fatigue life of coated specimens is studied. A model to predict the fatigue limit of the cold-sprayed specimen is proposed and has good agreement with experimental results. The enhancement of fatigue life by means of hybrid treatments is also investigated. Shot peening which is a surface treatment that can enhance the fatigue life by inducing near-surface compressive residual stresses is considered. The effect of severe shot peening (SSP) and conventional shot peening (SP) as pre- and post-treatment on mechanical and fatigue behavior of cold-sprayed aluminum alloy 6082 is experimentally evaluated. It is found that conventional and severe shot peening are most efficient to improve fatigue behavior if they are performed prior to cold spray deposition. The best results are obtained for SSP+CS which increases the fatigue strength up to 26 % in comparison to as-received condition.
- Biomedical application of CS coating: guidelines are proposed for one-step deposition of porous coatings without any subsequent heat treatment. Experiments are carried out at under-critical impact conditions, using rather coarse powders and fast gun traverse speeds. The effect of substrate material and spray parameters on the strength and porosity of the coating is investigated. Thick, macro rough, sufficiently high strength coatings, suitable for implant applications, with porosity of up to about 30% was successfully deposited using marginally low impact conditions.
Top 5 Awards and honors (name of award, date received):
5 Recent Papers:
A. Moridi, SM Hassani-Gangaraj, M Guagliano, M Dao. (2014) Cold spray coating: a review of the material systems and future perspectives, Surface Engineering, 30, 369-395.
A. Moridi, SM Hassani-Gangaraj, S. Vezzú, L. Trško, M. Guagliano, (2015) Fatigue behavior of cold spray coating: The effect of conventional and severe shot peening as pre/post treatment Surface and Coating Technology, article in press.
A. Moridi, SM Hassani-Gangaraj, M Guagliano. (2013) A hybrid approach to determine critical and erosion velocities in the cold spray process. Applied Surface Science, 273, 617-624.
A. Moridi, M. Azadi, G.H. Farrahi, (2014) Thermo-mechanical stress analysis of thermal barrier coating system considering thickness and roughness effects, Surface and Coating Technology, 243, 91-99.
SM Hassani-Gangaraj, A. Moridi, M Guagliano, A Ghidini. (2014) Nitriding duration reduction without sacrificing mechanical characteristics and fatigue behavior: The beneficial effect of surface nano-crystallization by prior severe shot peening. Materials & Design 55, 492-498.