Thesis
Feasibility study of a hybrid process for improving cold incremental sheet forming of titanium alloy
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2024
- Thesis identifier
- T16854
- Person Identifier (Local)
- 201861797
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Titanium is used for high value engineering components where high strength-to-weight ratio and corrosion resistance is a critical requirement. However, the production of complex components made from titanium alloys is limited due to their limited formability at low temperature, which necessitates thermal activation to increase their formability and ductility. High temperature forming is expensive. To justify the use of titanium alloys in industry the benefits gained must be sufficient to justify the high cost. A potential route for increasing the use of titanium alloys in industry is to improve their formability at ambient room temperature. Incremental sheet forming (ISF) is an innovative metal forming technology which uses a computer numerical controlled (CNC) tool to incrementally form sheet parts in place of a dedicated punch and die combination. ISF can increase the forming limits of sheet metal, meaning high temperature forming may no longer be necessary to process titanium alloys. However, sheet thinning and springback during room temperature ISF are two major drawbacks which compromise performance and can result in complete failure of the part. To counter thinning, the introduction of a pre-ISF additive step to thicken the titanium preform in areas of predicted thinning was investigated. This novel hybrid technology has been developed with the aim of improving thickness homogeneity in ISF titanium parts. The main gap in knowledge was the non availability of data and research on a pre-ISF additive step, and its impact on the geometric accuracy and part quality of titanium sheet parts formed by ISF. In the present research, laser metal deposition (LMD) was used to locally thicken a commercially pure titanium (CP-Ti) grade 2 sheet. No further surface preparation was performed on the tailored material following LMD. The mechanical behaviour of the tailored material was investigated using room temperature uniaxial testing. In-situ digital image correlation (DIC) measured the strain distribution across the surface of the material throughout. The goal was to examine the impact the additive pre-ISF step had on the room temperature formability of the LMD tailored CP-Ti sheet. The uniaxial tensile testing found that isotropic mechanical properties were obtained within the sheet plane in contrast to the anisotropic properties of the as received (AR) material and build height appeared to have little influence on strength. The in-situ DIC evaluation analysis showed strain to concentrate between the LMD tracks. Microstructural analysis showed distinct regions through the cross-section of the LMD thickened material with a heat affected zone (HAZ) at the interface between the top LMD layer and the non-transformed substrate material. Grain growth and intragranular misorientation in the LMD deposit was observed. A finite element analysis (FEA) simulation of the uniaxial tensile test was modelled to generate a FE material model for the anisotropic CP-Ti sheet. Building a material model of the tailored sheet and simulating the ISF process was not possible within the project time span, rather it was suggested for future research. ISF with a back support die was performed on a LMD thickened CP-Ti sheet. A GOM Atos metrology technique was used to generate a 3dimensional representation of both a LMD tailored preform and the sheet following ISF, as well as a previously formed AR CP-Ti part to compare. The results showed thickening the sheet in areas of predicted thinning helped achieve greater thickness homogeneity across the sheet following room temperature ISF. However, nucleation and development of cracks in the forming direction followed by catastrophic failure was observed in the 60° angled section of the LMD tailored sheet during ISF, earlier than the AR CP-Ti sheet. Some wrinkling was seen in the preform in response to LMD however fixtures and a post-LMD residual stress relief heat treatment ensured springback was limited. Further exploration of the LMD thickened material with a room temperature Nakajima test showed lower strain limit under the equi-biaxial strain state. Lack of surface preparation was considered as a likely cause of premature failure. This was briefly investigated, and more thorough analysis was recommended. Despite some issues, this technique shows potential for the room temperature forming of titanium parts and further investigations should be performed in the future to explore its capability. The literature review and experimental investigation contained in this thesis has identified and provided data and research on a pre-ISF additive step, fulfilling the above identified knowledge gap. Based on the research results, suggestions for further studies have been proposed and the major industrial relevance have been discussed.
- Advisor / supervisor
- Yakushina, Evgenia
- Blackwell, Paul
- Resource Type
- DOI
Beziehungen
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PDF of thesis T16854 | 2024-03-19 | Öffentlich | Herunterladen |