Investigation of Precision-Determing Factors for the Minimization of Warpage in Moulding and Die-Casting

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Andreas Bührig-Polaczek

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Distortion and hot tearing are prominent quality-reducing component features in foundry technology. The reason for this is that the solidification and cooling process of most engineering alloys is associated with volume contraction. This, in combination with the geometric constraint applied to the casting by the mold, induces stresses in the casting. These stresses can already lead to component damage during the solidification process due to hot tearing. However, they cause distortion if they are asymmetrical or inhomogeneous. This distortion problem is relevant for practically all molded parts. This problem is particularly pronounced in high pressure die casting, which is highly relevant from an economic point of view, since here the rapid heat extraction makes intrinsic stress relief more difficult. Moreover, additional residual stresses can occur in high pressure die casting as a result of the holding pressure applied to compensate for shrinkage. Neither the necessary subsequent straightening of the components nor the application of allowances for the purpose of subsequent machining are economically and technically sensible corrective measures.

The formation of undesirable hot tears and distortion can in principle be taken into account on the component side during casting design - e.g. via geometry - as well as in the casting process via the boundary conditions - e.g. design of the molds - or the setting of suitable process variables - such as temperature control. However, there are currently only insufficient models, guidelines or measures for these approaches, as the basic knowledge in this area still needs to be expanded.

The primary objective of the subproject is therefore to increase manufacturing precision in the permanent mold casting of metallic cast materials. In particular, the focus is on minimizing distortion and hot tearing in aluminum permanent mold and die casting, as well as on creating the necessary improved understanding of the fundamentals of materials science. Therefore, the systematic determination of the relationships between heat balance, microstructure formation and stresses during and after solidification, combined with the control of distortion and hot tearing tendency, is the final objective. For this purpose, process-relevant phenomena as well as their coupled mechanisms of action are to be analyzed in a suitable manner and, as a result, previously unavailable basic knowledge is to be acquired and transferred into new effective models. Due to the approach, new or previously unused concepts for the prevention of hot tears and distortion are also expected. To achieve these goals, a three-stage approach (analyze, understand, master) is followed in SFB1120. From this, in turn, phase-related sub-goals are derived.

In the first phase, the focus was on analyzing the influences of the input variables on the measurable process variables and, finally, on the component distortion as a target variable. The overall objective was to investigate the effective heat transfer between the component and the mold. This was broken down into its individual influencing variables. The influence of various contact conditions between mold and component was also taken into account. At the end of the first phase, findings were obtained on the key variables of heat transfer and their influence on the formation of the measured variables. Initial findings on the development of distortion were also obtained.

In the second phase, these analyses were further refined and initial approaches to influencing component distortion were evaluated. For aluminum gravity die casting with thermally well-conducting metal molds of high stiffness, it was possible to identify the control of solidification by spatially resolved influence on the heat balance as a measure for influencing the above-mentioned damage mechanisms. Furthermore, influencing the mold constraint, by predictive geometric compensation or process-immanent by the demolding time, was identified as a target-oriented way. These findings were obtained from laboratory-scale tests developed specifically for these issues. In addition, the investigations based on the knowledge gained to date were expanded in the second project phase to include the research focus of hot tearing. Thus, the tear formation could be observed in-situ and correlated with subsequent metallographic analyses. With the results obtained, the knowledge base on the mechanisms of hot tear initiation in aluminum casting could be considerably expanded.

In the third application phase, the focus is on controlling distortion and hot tearing tendencies and transferring the compensation strategies developed for this - for example thermally - to practical complex castings and processes. This transfer leads to further boundary conditions, such as the interaction of thermal and geometric influence, linked by varying contact conditions between component and mold. In the further course of the project, the knowledge gained in the first phases on the control of the thermal budget and the geometric influence will now be used in combination. Likewise, the test and measurement concepts developed in the first phases for investigating component distortion and hot tear formation will be further applied and adapted. The next step will be the transfer to practical components and processes. The influence of the above-mentioned quality-reducing phenomena will be combined into a compensation methodology. With the methods then developed, it will be possible at the end of the third phase to quantifiably control and thus compensate for component distortion and the tendency to hot tearing in the permanent mold and high pressure die casting process.

The experimental work of this subproject is carried out throughout the entire duration of the project in close cooperation with methodologically related or thematically complementary subprojects. For example, in the area of thermal influence in the casting process, there is close cooperation with subprojects B01 and B03, which are based in the area of injection molding, and in the area of analysis with subprojects A02 and A06, in order to be able to exploit synergies in expertise. Furthermore, the approach and experimental design are coordinated with the simulation subprojects A13 (N), B07, B09 and B05 so that the models develop

  Exemplary input, measurement and target variables of the subproject Copyright: © SFB 1120 Exemplary input, measurement and target variables of the subproject
 
  Video of a casting to investigate the heat balance (with subsequent filling and solidification simulation).
 
  Example for the first project phase: On the left side cooling curves and on the right side heat transfer coefficients (HTC) for different mold temperature control settings Copyright: © SFB 1120 Example for the first project phase: On the left side cooling curves and on the right side heat transfer coefficients (HTC) for different mold temperature control settings
 
  Video of hot tear initiation in AlSi1.75
 
  Photographs of the structure in the crack surface of the hot crack in Al99.8 at different scales, from a cooperation with TP A02. Copyright: © SFB 1120 Photographs of the structure in the crack surface of the hot crack in Al99.8 at different scales, from a cooperation with TP A02.
 
  Above the simulation of a distorted casting (distortion increased by a factor of 25 shown) and below the measured distortion and standard deviations for different component series, once with core with nominal geometry and once with compensated geometry, Copyright: © SFB 1120
 
 

Above the simulation of a distorted casting (distortion increased by a factor of 25 shown) and below the measured dis tortion and standard deviations for different component series, once with core with nominal geometry and once with compensated geometry, each for the initial component - left - and once - right - for one with reduced wall thickness (from project phase two)