PSI - Issue 61

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Aptullah Karakaş et al. / Procedia Structural Integrity 61 (2024) 42 – 46 Aptullah Karakas / Structural Integrity Procedia 00 (2019) 000 – 000

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metallographically prepared according to ASTM E3, and examined under an optical microscope (Olympus BX53M) to measure coating thickness. Substrate-coating interface was examined by a Scanning Electron Microscope (SEM, Thermo Scientific Quattro) in order to check if there is crack and delamination or not. Hardness was measured from the cross section of the coating by microindentation with 300 g load (HV 0,3) for 5 seconds (Emco Test Dura Scan20) according to ASTM E384. Preform and flow-formed parts and hot dip aluminized specimens are indicated in Fig. 1. 3. Results and Discussions Fig. 2 shows the cross section of hot dip aluminized specimen without diffusion annealing. There was no crack, void or delamination at the interface, which is preferred for diffusion of atoms because crack, void or delamination play a barrier role for diffusion of atoms. In order to achieve a successful coating after hot dip aluminizing, molten aluminum must cover all surface with good wetting, without delamination and cracks. Otherwise, it is very difficult to apply a successful diffusion annealing. One of the main concerns in hot dip aluminizing process for good wetting is surface roughness. There should be sufficient capillary force during hot dip aluminizing process to provide good wetting, therefore, surface preparation becomes very critical. The wetting is also related with bath viscosity which can be controlled by bath temperature and composition (Springer et al. 2011). In this regards, it was seen that surface preparation and hot dip aluminizing process parameters were adequate.

aluminide

substrate

Figure 2: Cross sectional SEM micrographs of the coating on 4140 steel after hot dip aluminizing.

The coating characteristics such as coating thickness and hardness were measured and compared. The numerical results are indicated in Table 1. Coating thicknesses were increased dramatically after diffusion annealing because of top aluminum layer diffuses into the substrate and creates aluminide phases. After diffusion annealing, hardness level decreases about 100 HV, which is attributed to the transformation of Fe 2 Al 5 to FeAl because of FeAl is more ductile than Fe 2 Al 5 . Fe 2 Al 5 phase is one of the iron aluminide phases which has lower Gibbs free energy, therefore, after hot dip aluminizing process Fe 2 Al 5 phase is easily occurred. During the diffusion annealing process, Fe 2 Al 5 phase decomposes to phases such as FeAl 2 and FeAl, therefore, hardness values decreases. Cold plastic deformation by flow forming played a significant role on the coating thickness because of lattice defects and dislocations. The cross sectional optical micrographs are indicated in Fig. 3.