About Residual Stress State of Castings: The Case of HPDC Parts and Possible Advantages through Semi-Solid Processes

Nowadays, one of the most crucial focus in the aluminium-foundry sector is the production of high-quality castings. Mainly, High-Pressure Die Casting (HPDC) is broadly adopted, since by this process is possible to realize aluminium castings with thin walls and high specific mechanical properties. On the other hand, this casting process may cause tensile states into the castings, namely residual stresses. Residual stresses may strongly affect the life of the product causing premature failure of the casting. Various methods can assess these tensile states, but the non-destructive X-Ray method is the most commonly adopted. Namely, in this work, the residual stress analysis has been performed through Sinto-Pulstec μ-X360s. Detailed measurements have been done on powertrain components realized in aluminium alloy EN AC 46000 through HPDC processes to understand and prevent dangerous residual stress state into the aluminium castings. Furthermore, a comparison with stresses induced by Rheocasting processes is underway. In fact, it is well known that Semi-Solid metal forming combines the advantages of casting and forging, solving safety and environmental problems and possibly even the residual stress state can be positively affected.

Implementation of a viscoplastic substrate creep model in the thermomechanical simulation of the WAAM process

This research work focusses on the implementation of a viscoplastic creep model in the thermomechanical simulation of the wire arc additive manufacturing (WAAM) process for Ti-6Al-4 V structures. Due to the characteristic layer by layer manufacturing within the WAAM process, viscoplastic material effects occur, which can be covered by implementing a creep model in the thermomechanical simulation. Experimental creep tests with a wide temperature, load and time range were carried out to examine short-term creep behaviour in particular. A Norton-Bailey creep law is used to accurately fit the experimental data and describe the base material’s creep behaviour. Subsequently, the fitted Norton-Bailey creep law was implemented in the thermomechanical simulation of the WAAM process. Finally, to determine the effect of creep on global distortion and local residual stress state in the substrate, simulations of a simplified linear, three-layer WAAM structure, with and without applying the implemented creep law, were carried out and compared to experimental data. The thermomechanical simulation with implemented creep model reveals a significant improvement in the numerical estimation of distortion and residual stress state in the substrate. The maximum distortion is reduced by about 13% and respectively the mean absolute percentage error between simulation and experiment decreases by about 34%. Additionally, the estimation accuracy with respect to the local residual stress state in the substrate improved by about 10%.

Analysis of the Influence of Surface Modifications on the Fatigue Behavior of Hot Work Tool Steel Components

Hot work tool steels (HWS) are widely used for high performance components as dies and molds in hot forging processes, where extreme process-related mechanical and thermal loads limit tool life. With the functionalizing and modification of tool surfaces with tailored surfaces, a promising approach is given to provide material flow control resulting in the efficient die filling of cavities while reducing the process forces. In terms of fatigue properties, the influence of surface modifications on surface integrity is insufficiently studied. Therefore, the potential of the machining processes of high-feed milling, micromilling and grinding with regard to the implications on the fatigue strength of components made of HWS (AISI H11) hardened to 50 ± 1 HRC was investigated. For this purpose, the machined surfaces were characterized in terms of surface topography and residual stress state to determine the surface integrity. In order to analyze the resulting fatigue behavior as a result of the machining processes, a rotating bending test was performed. The fracture surfaces were investigated using fractographic analysis to define the initiation area and to identify the source of failure. The investigations showed a significant influence of the machining-induced surface integrity and, in particular, the induced residual stress state on the fatigue properties of components made of HWS.

Influence of the Deep Cryogenic Treatment on AISI 52100 and AISI D3 Steel’s Corrosion Resistance

The effect of deep cryogenic treatment (DCT) on corrosion resistance of steels AISI 52100 and AISI D3 is investigated and compared with conventional heat-treated counterparts. DCT’s influence on microstructural changes is subsequently correlated to the corrosion resistance. DCT is confirmed to reduce the formation of corrosion products on steels’ surface, retard the corrosion products development and propagation. DCT reduces surface cracking, which is considered to be related to modified residual stress state of the material. DCT’s influence on each steel results from the altered microstructure and alloying element concentration that depends on steel matrix and type. This study presents DCT as an effective method for corrosion resistance alteration of steels.

Influence of grinding zones on the tooth root bending strength of case carburized gears

Knowledge of the expected tooth root bending strength plays a decisive role in the design of gear sets. Due to dimensional and shape changes resulting from distortion due to the heat treatment, unintentional, partial grinding in the tooth root area may occur, particularly in the application range of large gears. The influences of an unintentional grinding zone on the tooth root bending strength have not yet been clarified with sufficient accuracy. As a result, grinding zones lead to uncertainties when evaluating the tooth root bending strength and thus to a loss of time and cost in the field of industrial practice.This paper presents experimental investigations on the influence of grinding zones on the tooth root bending strength of case carburized gears. For the experimental investigations, there are three unground reference variants with different blasting treatments: non-blasted, mechanical cleaned by shot blasting and shot peened. The unground reference variants are examined regarding their tooth root bending strength. For the other test gear variants, different grindings zones are applied resulting in light and strong material removal by grinding. The variants with the different grinding zones are examined analogously regarding their tooth root bending strength and are subsequently compared to the reference variants.The results of the experimental investigations show that grinding zones can have diverse influences on the tooth root bending strength of case carburized gears– Non-blasted gears do not show changes regarding the tooth root bending strength with regard to light or strong grinding zones applied within this investigation.– Shot blasted (mechanical cleaned) gears show no change in the tooth root bending strength for light grinding zones (grinding application does not significantly alter the original residual stress state in the tooth root area).– Shot blasted (mechanical cleaned) gears show a reduction of the tooth bending strength of up to 20 % with regard to strong grinding zones (grinding application does significantly alter the original residual stress state in the tooth root area).– Shot peened gears show a behavior similar to that of shot blasted gears with reductions of the tooth root bending strength of up to 30 %.– Shot peening the strong grinding zones as a repair measure can increase the reduced tooth root bending strength again. However, for the investigated test gears, the resulting tooth root bending strength was below the shot blasted reference variant.The results of this paper help to evaluate the influence of grinding zones on the tooth root bending strength of case carburized gears more precisely compared to the generalized reductions of current standards and classifications. The results can be incorporated in standards such as DIN 3390 as well as ISO 6336 and can be applied in the field of industrial practice. Eventually, the findings help to reduce the current loss of time and cost caused by uncertainties regarding grinding zones.

Changes in Residual Stress State During Quench and Temper of Vacuum Carburized Gear Steel

Vacuum carburizing of 9310 gear steel followed by austenitizing, oil quench, cryogenic treatment, and tempering is known to impact residual stress state of the steel. Residual stress magnitude and depth distribution can have adverse effects on part distortion during intermediary and finish machining steps. The present research provides residual stress measurement, microstructural, and mechanical property data for samples taken along a specific heat treat sequence. Test rings of AISI 9310 steel are subjected to a representative gear manufacturing sequence that includes normalizing, rough machining, vacuum carburizing to 0.03”, austenitizing, quench, cryo-treatment, temper, and finish machining. Characterization of a test ring and a metallurgical sample after each manufacturing step allows tracking of residual stress and microstructural changes along the sequence. The results presented here are particularly interesting because the highest compressive residual stresses appear after removal of copper masking, not after quench as expected. Data can be used for future ICME models of the heat treat and subsequent machining steps. Analytical methods include X-Ray Diffraction, optical and electron microscopies, mechanical testing, and hardness testing.

Relationship Between Deep Case Carburizing and Residual Stress in Rolling Contact Service

The proposition that compressive residual stresses are beneficial in improving the service life of components subject to rolling contact fatigue is well documented. However, the exact nature of the relationship between effective case depth (ECD) and the residual stress state is not well understood for components with deep case depth (>0.050inches, 1.27mm). It is expected that compressive residual stresses will gradually transition to tensile stresses as the case depth increases beyond a threshold value. In addition, the strain-induced transformation of retained austenite and its influence on the residual stress state of components resulting from service will be explored. This study will measure the residual stress state of components prepared with various ECD before and after simulated service with the goal of determining where the compressive to tensile transition occurs. Residual stress and retained austenite measurements will be conducted using X-ray diffraction.

Thermally Induced Reduction of Hot Cracking Susceptibility in Electron Beam Welding of CuCr1Zr: A Numerical Approach

Electron Beam Welding (EBW) is a highly effective and accurate welding process that is being increasingly used in industrial work and is of growing importance in manufacturing. In the current study, solidification cracking in EBW of a CuCr1Zr cylindrical geometry was explored. To investigate and prevent occurrence of hot cracking, a thermomechanically coupled numerical model was developed using Finite Element Method (FEM). An additional heat source was considered, in order to influence the resulting residual stress state, namely to minimize tensile stresses in the fusion zone during solidification. Hence, a methodical assessment of relevant parameters, such as the power, the diameter of the additional heat source and the distances between both heat sources was employed using Design of Experiments (DoE). It was found that for a particular parameter configuration, solidification cracking most likely could be averted.