Indirect Hybrid Manufacturing – Integrating 3D Sand-Printing for Metal Casting
Open Access
- Author:
- Martinez Lepp, Daniel
- Graduate Program:
- Additive Manufacturing and Design
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- October 25, 2019
- Committee Members:
- Guhaprasanna Manogharan, Thesis Advisor/Co-Advisor
Robert Carl Voigt, Committee Member
Timothy W. Simpson, Program Head/Chair - Keywords:
- Additive Manufacturing
Metal Casting
3D Sand-Printing
Computed Tomography Scanning
Mechanical Testing
Thermal Properties
Aluminum
Gray Iron - Abstract:
- Additive manufacturing (AM) has been demonstrated to be a transformative technology for rapid manufacturing from three-dimensional digital models. Due to the disruptive nature of the technology, its integration into manufacturing settings is still difficult to assess. Factors like the large upfront investment required and limited number of certified materials (specially for metal components) available represent challenges for its adoption in industry at a larger scale. One manufacturing sector where these difficulties are lessened is the metal casting industry. The American Foundry Society (AFS) estimates that the metal casting industry amounts to 33.7 billion USD and within the United States, employs directly about 200,000 people. Binder jetting technology and specifically 3D sand-printing (3DSP) offers the possibility of leveraging the freedom of design of AM using comparatively low-cost materials (i.e., silica sand vs. metallic powders) to enhance casting performance. By printing the sand mold, the full range of metals and alloys already qualified for conventional casting applications can continue to be used without any need for further certification. Two important opportunities for reducing casting defects arise when using 3DSP. In the first case, the ability to manufacture complex mold geometries previously unfeasible using conventional methods, means that the flow of liquid metal or alloy circulating through the mold can be controlled by designing rigging components that direct the flow in accordance with predetermined conditions. These design criterions can be set by running numerical simulations of the filling process. In this thesis, the effectiveness of novel sprue geometries designed mathematically for alloys of different flow and solidification behavior is examined. The use of these sprues is simulated and experimentally tested for alloys at separate ends of the freezing range (i.e., gray cast iron Class 30, a very short-freezing range alloy and aluminum alloy 319, a very long-freezing range alloy). Computed tomography (CT) scanning, scanning electron microscopy and three-point bending tests are used to characterize the results in each sprue case. The casting mechanical properties can also be altered by controlling cooling behavior of the casting. Due to the layer-by-layer nature of binder and sand deposition in the 3DSP process, precise control on the thermo-physical properties of the mold such as thermal conductivity, thermal diffusivity, permeability and mechanical strength can be achieved. The second part of this thesis presents experimental procedures used to characterize 3DSP samples obtained from different printing conditions. Permeability and mechanical properties of the mold are examined using AFS standards. Thermal properties are obtained using laboratory techniques to determine thermal conductivity and diffusivity. Casting experiments are monitored to obtain thermal properties using a linear model to solve the backward heat conduction problem. The mechanical and microstructural properties of pure aluminum samples casted using the 3DSP molds are examined using three-point bending and microscopy of etched samples. A framework for integrating these thermomechanical properties into a design loop for metal casting mold design is proposed. Finally, 3DSP samples are examined using micro computed tomography scanning. This technique allows for the creation of a 3D model of the sand mold to determine void spaces and pore interconnectedness. An understanding of the pore network properties of 3DSP samples and how printing conditions affect mold permeability are essential to creating defect-free castings. During mold filling, air present in the mold and gasses generated by the binder burn-out must be evacuated through the mold. Insufficient void space and pore interconnectedness will lead to the appearance of a backpressure within the mold. If this happens, then volumes within the mold will remain unfilled, creating serious casting defects. Samples printed using different printing parameters are examined via CT scanning and permeability simulations are performed to create an understanding of how air flow through the mold occurs.