Graduation Year

2018

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Nathan Crane, Ph.D.

Co-Major Professor

Rasim Guldiken, Ph.D.

Committee Member

George Nolas, Ph.D.

Committee Member

Craig Lusk, Ph.D.

Committee Member

Thomas Weller, Ph.D.

Committee Member

Wenjun Cai, Ph.D.

Keywords

Binder Jet Printing, Demineralized Bone Matrix, Fine Stainless Steel, Powder-Bed Fusion, Shrinkage, Varied Density

Abstract

Additive Manufacturing (AM), as the name implies, produces three-dimensional objects by progressively adding material rather than cutting away unneeded material. Typically this is done by processing a series of cross-sections obtained from a Computer Aided Design (CAD) model of the object. AM systems can process liquid photopolymers and solid materials in form of powder, wire and laminates to create intricate geometries in a short lead time.

This superb capability of AM methods enables to locally suit the properties of the part for the desired function by incorporating the designed pores in the CAD model [2]. The pore gradient creation opens up a new route to a wide variety of properties such as varied density, optimized strain distribution, tuned thermal conductivity, etc. [3-5]. But, the minimum feature size of the AM systems restricts the lower limit of the pore size. So, as an alternative, temporary space holders are introduced to the base material to introduce stochastic fine pores [6].

Nevertheless, processing powders has been notorious for reduced mechanical properties as a result of the residual voids within the compact. Further, the voids may cause distortion in the part during the post-processing and can lead to a significant deviation from the intended CAD geometry.

The inter-particle voids could impose more problems in fabricating parts with varied density can be even more due to shrinkage variation. In spite of these drawbacks, the high demand of porous structures in industrial and medical applications propels the researches to explore new ways to fabricate the parts with compatible shrinkage.

The first part of the work studies that how processing of fine 316L steel powder would influences on density and shrinkage of the final parts. The agglomeration of the base powder and mixing in temporary space holders were practiced to treat the raw powder. The prepared powder stocks must meet the spreadability criteria to allow printing proceed successfully. After screening the final parts, the processing methodologies that result in nearly same shrinkage level but different final densities are promising to create a multi-level porous part. Of the printed parts in this study, the agglomerates treated at 185 ̊C reached ~95% full density whereas the 25Vol% Nylon mixture led to 30% less density. The similar shrinkage of these parts is a key step toward creating spatially-controlled porosity with compatible shrinkage in all directions.

The second part of the dissertation utilizes powder-based AM to process Demineralized Bone Matrix (DBM) particles for tissue engineering applications. We combined the geometric freedom of AM advantage with biological benefits of natural bone to fabricate scaffolds. The spreading behavior of the granulated bone and polycaprolactone (PCL) composite was studied to identify the spreading device and conditions to enhance the compaction level of powder bed which later on were used in sample creation step.

A novel AM method that projects visible light to selectively treat the powder bed i.e. Large Area Projection Sintering (LAPS), was the first approach assessed to process the bone based implants. The mechanical properties of the samples showed that the LAPS-fabricated composites of 45-55% bone particulates produced strength comparable to cast composites and demineralized cancellous bone as two conventionally fabricated methods. The LAPS had a rather short process time but showed limited capacity in defining fine features as the low absorptivity of visible light by PCL required long heating times.

In order to overcome the challenge and define a cleaner geometry, a CO2 laser was utilized as the second candidate fabrication method. To retain bone biological features, the safe operation window of variables including laser scanning speed, line spacing and the power range were determined. The best power setting was selected to provide adequate strength without burning while maintaining flat layers after processing so that a new layer could be spread. Then the green samples were cured under different heat treatment conditions and, finally, the shrinkage level and mechanical properties of the as-manufactured samples were determined. The mechanical strength analysis suggested that the material ratio and the heat treatment conditions are the two critical factors that enable to tune the final mechanical properties of the parts to match up with that of the host tissue.

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