Additive manufacturing and repair of crack-free high chromium white irons with improved wear resistance
Creators: name: Zhu, Chengbo
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)
classificationCode: 401607
subject: Metals and alloy materials

description: White cast irons with high chromium concentration are widely used to manufacture components in the mining industry due to their good hardness and wear resistance. Mining is a machine-intensive industry and regular maintenance is required to repair the damage or malfunctions of the equipment. To reduce the equipment cost, there has been a demand to repair the damaged components in a quick and cost-effective way. Additive manufacturing (AM) is a promising technique to fulfil this need. 
The research is focused on laser direct energy deposition (L-DED) as this technique has been applied by various industry sectors to repair components made of different alloys. But the adoption of L-DED in the mining industry is relatively slow because high chromium white iron (HCWI) alloy is brittle and has high cracking susceptibility. The aim of this thesis is to additively manufacture and repair crack-free HCWI with improved wear resistance.

HCWI is considered unweldable and there has been little research on L-DED of HCWI. This thesis first explores the optimisation of processing conditions to lower the cracking susceptibility of L-DEDed HCWI. Three substrate pre-heat temperatures and three laser scan speeds were used in the deposition. The result shows that the cracking in L-DED of HCWI is shrinkage cracking that generally initiates from the deposit-substrate interface. It is due to the solidification shrinkage and thermal contraction of the melt pool during rapid cooling. Higher input energy, e.g. higher substrate pre-heat temperature or slow laser scan speed, lowers the cracking susceptibility. In this work, cracking is successfully eliminated in the deposit with a pre-heat temperature of 600°C or with a pre-heat temperature 450°C at the laser scan of speed 400 mm/min. Residual stresses in the substrate were measured using neutron diffraction. It reveals the difficulty in measuring the residual stress in HCWI due to its high volume fraction (VF) of carbide.

In this thesis, the microstructure is characterised by various techniques, including X-ray Diffraction (XRD) and scanning electron microscopy (SEM). Vickers hardness testing was used to evaluate the hardness in the alloys. Two wear testing systems, namely, a high-stress abrasion wear test using silica sands and a pin-on-disc wear test with a ruby pin, were used to study the microstructure-wear relationship in different wear conditions. 

The average hardness of the deposit is lower than the base alloy of the same composition. The base alloy is cast HCWI subjected to a four hours of post-heat treatment at temperature of 960°C. Further study shows the L-DEDed HCWI is less wear-resistant than the base alloy. New alloys are designed for L-DED to improve the wear resistance of the repairs. 

Titanium was selected as the additional element to improve the wear resistance of the deposit due to its ability to form hard TiC carbide and promote martensite in ferrous alloys. Two types of powders, i.e. Ti powder (spherical share) and TiC powder (non-spherical shape), were mixed with HCWI powder before the deposition. Composition analysis shows that the used of TiC powder results in lower Ti concentration in the deposit than designed, indicating that the non-spherical powder has lower flow resistance than the spherical powder. The Ti powder is melted completely in the deposition. It forms in-situ TiC carbides and destabilises austenite in the as-deposited microstructure. The TiC powder is partially melted in the deposition, resulting in fine in-situ TiC carbides and coarse ex-situ TiC carbides. The austenite is also destabilised by the TiC powder addition. The VF of carbide increases with the increasing TiC powder addition, improving the wear resistance of L-DEDed HCWI in the high-stress abrasion wear test. 

HCWI with 6.6wt.% Ti powder addition has a better wear resistance than the alloys with TiC powder addition, although it has lower harness and VF of carbide. It is attributed to the strain induced martensite (SIM) formation, which absorbs wear energy and hardens the alloy's surface during the wear test. The SIM formation is also found in the L-DEDed HCWI, but on a different scale. The SIM formation in the HCWI with 6.6 wt.% Ti addition is more extensive due to the destabilisation of austenitic caused by the Ti addition. Therefore, the HCWI with 6.6 wt.% Ti is more wear-resistant than L-DEDed HCWI. In the pin-on-disc wear test with a harder pin (ruby), the austenitic matrix is also advantageous and experiences lower wear loss. 

The wear resistance of L-DEDed HCWI is improved by the Ti addition, but still lower than that in the base alloy. Boron, a carbon substitute element in the ferrous alloy, is used to design the alloy with better wear resistance than the base alloy. The goal of the alloy design was achieved: the wear resistance of the HCWI with 1.1 wt.% B was better than the post-heat treated cast alloy. 

The addition of B turns the hypoeutectic alloy into a hypereutectic alloy containing primary M7(C,B)3. B also promotes M3(C,B) formation, resulting in a significant increase in VF of carboboride. The VF of carboboride reaches 60.3vol.% in the alloy with 1.1 wt.% B. In addition, the primary M7(C,B)3 is coarsened with the increasing B concentration. In terms of the matrix, the increase in the fraction of primary carbide and C-depletion region leads to the destabilisation of austenite. The improvement of the wear resistance is due to the high-fraction, coarser carboborides in the as-deposited microstructure. 

The addition of Ti and B in L-DED of HCWI illustrates that the hardness is still an important indicator of wear resistance. The carbide and the matrix in the refined microstructure are worn simultaneously. A harder matrix improves the bulk hardness. The matrix structure can be designed by adding strong carbide-forming element Ti or carbon substitute element B in this research. The added elements cause destabilisation of the austenite. 

The low wear resistance in L-DEDed HCWI is because the refined carbides at sub-micron size provide limited protection to the wear surface. This thesis demonstrates two methods to improve the wear resistance by modifying the carbide morphology. First is the use of TiC powder. The ex-situ TiC carbide in the L-DEDed HCWI shows a feasible way to use laser parameters to optimise the carbide size that cannot form during rapid cooling. The second method is the use of B addition, which not only increases the C equivalent of the alloy that results in higher fraction of carboboride, but also creates greater C-depletion zone that may promote martensite formation in the as-deposited microstructure. The wear losses in the two wear systems decrease monotonically with the increasing B concentration. Such a trend shows that the wear resistance of the repair can be designed to match the wear resistance of the base alloy by varying the B concentrations. 

The results in this thesis show that L-DED of crack-free HCWIs is possible by optimising the processing conditions, and the wear resistance can be improved by the alloy design. This makes L-DED more appealing for repairing components in the mining industry.