DE102018110402A1 - METHOD FOR INCREASING THE TENSILE STRENGTH OF ALUMINUM CASTORS - Google Patents

Abstract

A multi-step process increases the net tensile strength of aluminum die castings (HPDC) through an alloy and process dependent heat treatment. The highest achievable temperature for solution treatment of an HPDC casting is determined by computational thermodynamics, kinetics, and gas laws based on alloy composition and gas pressure in the finally solidified parts. Setting the maximum solution temperature involves assigning the pressure in the bubbles of the solidified material to avoid the formation of bubbles by near-surface bubbles in the casting. In order to reduce the residual tensile strength, the HPDC components are air-cooled after the solution treatment. Finally, a specific, repeated temperature aging cycle is used to improve the aging behavior of HPDC air-cooled components and increase net tensile strength.

Description

TECHNICAL AREA

The present disclosure relates to aluminum die casting, and more particularly to a method of increasing the tensile strength of aluminum alloy die cast parts.

BACKGROUND

The explanations in this section merely provide background information that may or may not be related to the present disclosure and may be consistent with the prior art.

The high pressure casting (HPDC) process is used for mass production of metal components due to low cost, tight dimensional tolerances, i. H. the near-net shape and smooth surface textures used. Manufacturers in the automotive industry are increasingly challenged to produce final contoured aluminum components with a combination of high tensile strength and ductility by the die casting process, as this process is the most economical method of mass production. A disadvantage of the conventional HPDC process is that the parts thus produced are not solvent-suitable (T4) at high temperatures (eg 500 ° C.), which significantly reduces the precipitation hardening potential of a complete T6 or T7 heat treatment ,

This is due to the high porosity and voids in the finished HPDC components due to shrinkage during solidification, and particularly the trapped gases during mold filling, such as air, hydrogen or vapors formed from the decomposition of the matrix wall lubricants. It is virtually impossible to find a conventional HPDC component without large entrained gas bubbles. The internal pores containing gases or gas-forming components in the die cast parts expand during conventional solution treatment at elevated temperatures, resulting in the formation of surface bubbles. The presence of these bubbles affects not only the appearance of the castings, but also the dimensional stability and especially the mechanical properties of the components.

Because of the potential blistering problem, conventional HPDC aluminum components are mostly used in the as-cast state, or to a lesser extent, in aged conditions such as T5. Even in T5 cure, the yield strength increase is very limited, and sometimes there is no improvement, since the levels of hardening solutes for artificial cure (T5) in typical HPDC parts are very strong. As a result, the mechanical properties of the HPDC aluminum parts are typically low for a given aluminum alloy compared to other casting methods because the aluminum parts made by other casting methods can be heat treated in full T6 or T7 conditions.

In T5 curing, there are generally three types of aging conditions, commonly referred to as under age, peak aging and overaging. In the initial stage of aging, GP zones and fine shearable precipitates form, the structure being considered as underdeveloped. At this stage, the material hardness and yield strength are generally low. A longer time at a given temperature or curing at a higher temperature, the precipitate structure further develops and the hardness and yield strength increases to a maximum to reach the peak aging / curing state. Upon further aging, the hardness / yield strength decreases and the structure becomes more than crystallographic incoherence due to coarsening of the precipitate and the associated transformation.

Considering that typical HPDC aluminum components inevitably contain trapped air, each solution treatment must be specific to the quality of the casting, ie. H. the amount of trapped air and alloy used. Subsequent artificial aging (T5) is also a critical step to achieve the desired tensile properties without blistering problems. The increase in strength due to the curing occurs because the substances present in the supersaturated solid solution form precipitates which are finely distributed in the grains and which increase the ability of the casting to resist deformation by slippage and plastic flow. Maximum cure or strength enhancement may occur when the cure treatment results in the formation of a critical dispersion of at least one kind of these fine precipitates.

Last but not least, there is considerable residual stress on cast HPDC components, especially if the parts are quenched in water after being ejected from the mold at room temperature. A high internal stress reduces the material web strength during structural loading.

SUMMARY

The present invention provides a multi-step process for increasing the net tensile strength of HPDC aluminum components through an alloy and process dependent heat treatment. The highest achievable temperature for solution treatment of an HPDC casting is determined by computational thermodynamics, kinetics, and gas laws based on alloy composition and gas pressure in the finally solidified parts. Setting the maximum solution temperature involves assigning the pressure in the bubbles of the solidified material to avoid the formation of bubbles by near-surface bubbles in the casting. In order to reduce the residual tensile strength, the HPDC components are air-cooled after the solution treatment. A specific, repeated temperature aging cycle is used to improve the aging behavior of HPDC air-cooled components and to increase net tensile strength.

As such, one aspect of the present disclosure is a method of increasing the net tensile strength of HPDC aluminum components.

As such, another aspect of the present disclosure is a multi-stage process for increasing the net tensile strength of HPDC aluminum components.

As such, yet another aspect of the present disclosure is a multi-stage process for increasing the net tensile strength of HPDC aluminum components through an alloy and process dependent heat treatment.

It is still another aspect of the present disclosure to provide a method of increasing the net tensile strength of HPDC aluminum components, including the step of setting the highest possible temperature for solution treatment of an HPDC casting.

It is still another aspect of the present disclosure to provide a method of increasing the net tensile strength of HPDC aluminum components, including the step of air cooling the component after solution treatment.

It is still another aspect of the present disclosure to provide a method of increasing the net tensile strength of HPDC aluminum components, including utilizing a specific aging cycle, to improve aging behavior and to maximize net tensile strength.

Other objects, advantages and applications will become apparent from the description presented herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

list of figures

• 1 ist ein Phasendiagramm einer A380-Aluminiumlegierung entsprechend dem Cu-Gehalt;
• 2 ist eine Reihenfolge von drei Ansichten, in denen die Entwicklung eines Teils eines HPDC-Gusses von der Verfestigung über die Raumtemperatur bis zu einer erhöhten Temperatur dargestellt wird;
• 3 ist ein Diagramm mit drei Kurven, welche die Partikelgröße auf der Ordinatenachse (Y) und die Lösungsbehandlungszeit auf der Abszissenachse (X) darstellen;
• 4 ist ein Diagramm mit mehreren Kurven, welche die Eigenspannung auf der Ordinatenachse (Y) und mehrere luft- und wassergekühlte Aluminiumkomponenten auf der Abszissenachse (X) darstellen;
• 5 ist ein Diagramm mit mehreren Kurven, welche die Streckgrenze auf der Ordinatenachse (Y) und mehrere Alterungsbehandlungen auf der Abszissenachse (X) darstellen;
• 6 ist ein qualitatives Diagramm des nicht-isothermen Alterungszyklus gemäß der vorliegenden Offenbarung zum Erhöhen der Festigkeit eines luftgekühlten HPDC-Aluminiumgusses mit der Temperatur auf der Ordinatenachse (Y) und der Zeit auf der Abszissenachse (X); und
• 7 ist der mehrstufige Alterungszyklus gemäß der vorliegenden Offenbarung, der die Eigenspannungen beim Aluminiumguss von unterschiedlichen Werkstoffen mit der Temperatur auf der Ordinatenachse (Y) und der Zeit auf der Abszissenachse (X) reduziert.

The drawings described herein are for illustration only and are not intended to limit the scope of the present disclosure in any way.

• 1 is a phase diagram of an A380 aluminum alloy according to the Cu content;
• 2 Figure 3 is a sequence of three views illustrating the evolution of a portion of an HPDC casting from solidification to room temperature to elevated temperature;
• 3 Fig. 13 is a graph with three curves representing the particle size on the ordinate axis (Y) and the solution treatment time on the abscissa axis (X);
• 4 Fig. 12 is a graph showing a plurality of curves showing the residual stress on the ordinate axis (Y) and a plurality of air and water cooled aluminum components on the abscissa axis (X);
• 5 Fig. 12 is a graph with several curves showing the yield strength on the ordinate axis (Y) and several aging treatments on the abscissa axis (X);
• 6 FIG. 12 is a qualitative diagram of the non-isothermal aging cycle according to the present disclosure for increasing the strength of an air-cooled HPDC cast aluminum having the temperature on the ordinate axis (Y) and the time on the abscissa axis (X); FIG. and
• 7 is the multi-stage aging cycle according to the present disclosure which reduces the residual stresses in aluminum casting of different materials with the temperature on the ordinate axis (Y) and the time on the abscissa axis (X).

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

Regarding 1 Now, a first part and step of the method or process related to determining the solution treatment (dissolution) temperature (T _ST ) will be discussed. In solution treatment, the dissolution rate increases with temperature. The higher the solution treatment temperature, the faster the element dissolution and spheroidization of the undissolved constituents as well as the homogenization of the dissolved concentrations in the metal or in the alloy. In practice, however, the temperature of the solution treatment may not be higher than a critical temperature above which blistering and melting occur.

The melting temperature of the metal or alloy is determined by the composition of the alloy and in particular by its thermodynamic properties such as solidus. As used herein, the term solidus refers to a line or curve on a graph of temperature versus the composition of an alloy below which the alloy is completely solidified. In the solution treatment, the dissolution temperature must not exceed the solidus of all phases in the structure of the alloy. The heavy, solid line 10 in the phase diagram, as 1 appears, illustrates the variation of the solidus depending on the Cu content in an aluminum die-casting alloy, such as the A380 alloy. For example, the solution treatment temperature for an alloy containing between 3 and 4% Cu may not exceed about 490 ° C.

The appearance of the bubbles is determined by the balance between pressure in the air bubbles and the strength of the alloy at a given temperature. In general, the severity of blistering is significantly increased with increasing temperature as both the pressure in the air bubbles increases and the alloy strength decreases drastically with temperature.

wobei P_solidus der Druck innerhalb der eingeschlossenen Luftblase bei Solidus (MPa) ist, V_solidus das Volumen der eingeschlossenen Luftblase bei der Solidustemperatur (m^3) ist, T_solidus ist die Solidustemperatur (°K), P_RT ist der Druck innerhalb der eingeschlossenen Luftblase bei der Raumtemperatur (MPa), V_RT ist das Volumen der eingeschlossenen Luftblase bei der Raumtemperatur (m^3), T_RT ist die Raumtemperatur (°K), σ_ys@ST ist die Streckgrenze der Legierung bei der Lösungsbehandlungstemperatur (MPa), V_ST ist das Volumen der eingeschlossenen Luftblase 20C bei der Lösungsbehandlungstemperatur (m^3) und T_ST ist die Lösungsbehandlungstemperatur (°K).With reference to 2 is the development of trapped air bubbles, one of which is in 2 shown from the solidification under pressure during the die casting process, designated as 20A, via the cooling at room temperature (RT), designated as 20B, to the bubbling at an elevated temperature (ET), designated as 20C. Basically, the gas pressure in the bubbles 20A, 20B, and 20C and the volume of bubbles correspond to the combined gas laws at the different temperatures. The actual volume of bubbles 20A, 20B and 20C is balanced by either the yield strength of the alloy, the externally applied pressure or both. The following equation describes this activity: $$\frac{{\text{P}}_{\text{solidus}}{\text{V}}_{\text{solidus}}}{{\text{T}}_{\text{solidus}}}=\frac{{\text{P}}_{\text{RT}}{\text{V}}_{\text{RT}}}{{\text{T}}_{\text{RT}}}=\frac{{\mathrm{\sigma }}_{\text{ys @ ST}}{\text{V}}_{\text{ST}}}{{\text{T}}_{\text{ST}}}$$

where P _{solidus is} the pressure inside the enclosed bubble at Solidus (MPa), V _{solidus is} the volume of the trapped bubble at the solidus temperature (m ^ 3), T _solidus is the solidus temperature (° K), P _RT is the pressure inside the enclosed air bubble at room temperature (MPa), V _RT is the volume of trapped air bubble at room temperature (m ^ 3), T _RT is the room temperature (° K), σ _{ys @ ST} is the yield strength of the alloy at the solution _treatment temperature (MPa), V _ST is the volume of the trapped air bubble 20C at the solution treatment temperature (m ^ 3) and T _ST is the solution treatment temperature (° K ).

When solidifying, the aluminum surrounding the entrained air bubble is naturally solid so that the volume of the air bubble 20B is solid and can be considered constant at room temperature.

wenn T<=T_min $${\mathrm{\sigma }}_{\text{ys}}\left(\text{T}\right)={\mathrm{\sigma }}_{\text{ys}}\left({\text{T}}_{\text{min}}\right)\left[1-{\text{C}}_{\text{ys}}\left(\frac{{\text{E}}_{\text{LT}}-\text{E}\left(\text{T}\right)}{{\text{E}}_{\text{LT}}}\right)\right]$$

wenn T>T_min wobei C_ys eine empirisch ermittelte Proportionalitätskonstante (1,5 ~ 2,0) ist. σ_ys (T_min) = (0.85∼0.9) σ_ys (298). Die Niedrigtemperatur (T_min) liegt zwischen 423 ∼ 453°K; Die maximale Temperatur (T_max) liegt zwischen 673° und 723° K; T ist die Temperatur (°K); $$\text{E}\left(\text{T}\right)={\text{E}}_{\text{0}}-\frac{{\text{E}}_{\text{LT}}-{\text{E}}_{\text{HT}}}{1+\text{exp}\square \left(\frac{\text{T}*-\text{T}}{\mathrm{\phi }}\right)}$$

$$\text{T}*=\left({\text{T}}_{\text{min}}+{\text{T}}_{\text{max}}\right)/2$$

$$\mathrm{\phi =}\left({\text{T}}_{\text{max}}-{\text{T}}_{\text{min}}\right)/4$$

wobei E(T) das Elastizitätsmodul bei Temperatur T (MPa) ist; E_LT ist das Elastizitätsmodul bei einer niedrigen Temperatur (T_min); E_HT ist das Elastizitätsmodul bei einer hohen Temperatur (T_max); Eo ist der Elastizitätsmodul bei einer Raumtemperatur (73000 ∼ 77000 MPa); und σ_ys (298) ist die Materialstreckgrenze bei einer Raumtemperatur. $${\mathrm{\sigma }}_{\text{ys}}\left(298\right)={\mathrm{\sigma }}_{\text{i}}+\mathrm{\Delta }{\mathrm{\sigma }}_{\text{ss}}\left(\text{t},\text{T}\right)+\mathrm{\Delta }{\mathrm{\sigma }}_{\text{ppt}}\left(\text{t},\text{T}\right)$$

wobei t und T Alterungszeiten (Sekunden) und die Temperatur (°K) sind. The temperature dependence of the material _yield strength σ _ys (T) can be determined by linearly reducing the room temperature _yield strength at temperatures below T _min and by setting the low temperature _yield strength σ _ys (T _min ) for the loss of stiffness associated with the temperature rise. $${\mathrm{\sigma }}_{\text{ys}}\left(\text{T}\right)={\mathrm{\sigma }}_{\text{ys}}\left({\text{T}}_{\text{min}}\right)+\left({\mathrm{\sigma }}_{\text{ys}}\left(298\right)-{\mathrm{\sigma }}_{\text{ys}}\left({\text{T}}_{\text{min}}\right)\right)\left[\frac{{\text{T}}_{\text{min}}-\text{T}}{{\text{T}}_{\text{min}}-298}\right]$$

if T <= T _min $${\mathrm{\sigma }}_{\text{ys}}\left(\text{T}\right)={\mathrm{\sigma }}_{\text{ys}}\left({\text{T}}_{\text{min}}\right)\left[1-{\text{C}}_{\text{ys}}\left(\frac{{\text{e}}_{\text{LT}}-\text{e}\left(\text{T}\right)}{{\text{e}}_{\text{LT}}}\right)\right]$$

if T> T _min where C _{ys is} an empirically determined proportionality constant (1.5 ~ 2.0). σ _ys (T _min ) = (0.85~0.9) σ _ys (298). The low temperature (T _min ) is between 423 ~ 453 ° K; The maximum temperature (T _max ) is between 673 ° and 723 ° K; T is the temperature (° K); $$\text{e}\left(\text{T}\right)={\text{e}}_{\text{0}}-\frac{{\text{e}}_{\text{LT}}-{\text{<figure-callout id="1" label="e HT" filenames="DE102018110402A1\_0030.png" state="{{state}}">e </figure-callout>}}_{\text{HT}}}{1+\text{exp}\square \left(\frac{\text{T}*-\text{T}}{\mathrm{\phi }}\right)}$$

$$\text{T}*=\left({\text{T}}_{\text{min}}+{\text{T}}_{\text{Max}}\right)/2$$

$$\mathrm{\phi \; =}\left({\text{T}}_{\text{Max}}-{\text{T}}_{\text{min}}\right)/4$$

where E (T) is the modulus of elasticity at temperature T (MPa); E _LT is the elastic modulus at a low temperature (T _min ); E _HT is the modulus of elasticity at a high temperature (T _max ); Eo is the elastic modulus at room temperature (73,000 ~ 77,000 MPa); and σ _ys (298) is the material _yield strength at room temperature. $${\mathrm{\sigma }}_{\text{ys}}\left(298\right)={\mathrm{\sigma }}_{\text{i}}+\mathrm{\Delta }{\mathrm{\sigma }}_{\text{ss}}\left(\text{t}.\text{T}\right)+\mathrm{\Delta }{\mathrm{\sigma }}_{\text{ppt}}\left(\text{t}.\text{T}\right)$$

where t and T are aging times (seconds) and temperature (° K).

wobei $$\Delta {\sigma }_{\text{ssi}}={\mathrm{\sigma }}_{\text{q}}-{\mathrm{\sigma }}_{\text{i}}$$

$$\Delta {\sigma }_{\text{ss}\mathrm{\theta }}\left(\text{T}\right)={\mathrm{\sigma }}_{\text{oa}}\left(\text{T}\right)-{\mathrm{\sigma }}_{\text{i}}$$

$${\mathrm{\sigma }}_{\text{oa}}\left(\text{T}\right)={\mathrm{\sigma }}_{\text{i}}+\left({\mathrm{\sigma }}_{\text{q}}-{\mathrm{\sigma }}_{\text{i}}\right)\text{exp}\frac{-2{\text{Q}}_{\text{s}}}{2\text{R}}\left(\frac{1}{\text{T}}-\frac{\text{1}}{{\text{T}}_{\text{s}}}\right)$$

$${\mathrm{\tau }}_1\left(\text{T}\right)={\text{K}}_1{\text{P}}_{\text{p}}\text{Texp}\left(\frac{\text{Qa}}{\text{RT}}\right)$$

The contribution of solid solution hardening can be described by structural variables in the form of an equilibrium concentration at the aging temperature, as shown below: $$\Delta {\sigma }_{\text{ss}}\left(\text{t}.\text{T}\right)={\left\{\Delta {\mathrm{<figure-callout\; id="3"\; label="\sigma \; ss\; \theta "\; filenames="DE102018110402A1\_0030.png"\; state="\{\{state\}\}">\sigma \; </figure-callout>}}_{\text{ss}\mathrm{\theta }}{}^{\frac{3}{2}}\left(\text{T}\right)+exp\left(\frac{-\text{t}}{{\mathrm{\tau }}_1\left(\text{t}\right)}\right)\left[\mathrm{<figure-callout\; id="3"\; label="\Delta \; \sigma \; ssi"\; filenames="DE102018110402A1\_0030.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{\sigma }_{\text{ssi}}{}_{}{}^{\frac{3}{2}}-\Delta {\mathrm{<figure-callout\; id="3"\; label="\sigma \; ss\; \theta "\; filenames="DE102018110402A1\_0030.png"\; state="\{\{state\}\}">\sigma \; </figure-callout>}}_{\text{ss}\mathrm{\theta }}{}^{\frac{3}{2}}\left(\text{T}\right)\right]\right\}}^{\frac{2}{3}}$$

in which $$\Delta {\sigma }_{\text{ssi}}={\mathrm{\sigma }}_{\text{q}}-{\mathrm{\sigma }}_{\text{i}}$$

$$\Delta {\sigma }_{\text{ss}\mathrm{\theta }}\left(\text{T}\right)={\mathrm{\sigma }}_{\text{oa}}\left(\text{T}\right)-{\mathrm{\sigma }}_{\text{i}}$$

$${\mathrm{\sigma }}_{\text{oa}}\left(\text{T}\right)={\mathrm{\sigma }}_{\text{i}}+\left({\mathrm{\sigma }}_{\text{q}}-{\mathrm{\sigma }}_{\text{i}}\right)\text{exp}\frac{-2{\text{Q}}_{\text{s}}}{2\text{R}}\left(\frac{1}{\text{T}}-\frac{\text{1}}{{\text{T}}_{\text{s}}}\right)$$

$${\mathrm{\tau }}_1\left(\text{T}\right)={\text{K}}_1{\text{P}}_{\text{p}}\text{texp}\left(\frac{\text{Qa}}{\text{RT}}\right)$$

$$\text{P}*\left(\text{t},\text{T}\right)=\frac{\text{T}\left(\text{t},\text{T}\right)}{{\text{P}}_{\text{p}}}$$

$$\text{P}\left(\text{t},\text{T}\right)=\frac{\text{t}}{\text{T}}\text{exp}\square \left(\frac{-\text{Qa}}{\text{RT}}\right)$$

$$\text{S}{\left(\text{t}\text{,T}\right)}^2={\left({\text{S}}_{\mathrm{\theta }}\right)}_{{\text{max}}^2}\left[1-\text{exp}\frac{-\text{Qs}}{\text{R}}\left(\frac{1}{\text{T}}-\frac{1}{{\text{T}}_{\text{s}}}\right)\right]\left[1-\text{exp}\left(\frac{-\text{t}}{{\mathrm{\tau }}_1\left(\text{t}\right)}\right)\right]$$

Tabelle 1 zeigt mehrere Materialkonstanten für die Streckgrenzmodelle. Parameter des Streckgrenzmodells Aluminiumgusslegierungen Intrinsische Streckgrenze (MPa), □_i 40∼60 Streckgrenze wie abgeschreckt (MPa), □_q 70∼90 Aktivierungsenergie für die Alterung (kJ/mol), Qa 180∼230 Universelle Gaskonstante (J/mol. °K), R 8,314 Spitzentemperatur-Korrekturzeit (s/°K), Pp 3,5E-22 ∼3,5E-22 Metastabile Solvustemperatur (°K), Ts 500∼550 Max. Festigkeitsparameter bei 0 K (MPa), (S₀)_max 240-280 Solvus-Enthalpie (kJ/mol), Qs 40∼50 Konstante bezogen auf □1 bis tp, K1 0,5∼0,6 The contribution to hardening by precipitations of the second phase is given by: $$\Delta {\sigma }_{\text{ppt}}\left(\text{t}.\text{T}\right)=\frac{2\text{S}\left(\text{t}.\text{T}\right){\left[\text{P}*\left(\text{t}.\text{T}\right)\right]}^{\frac{1}{6}}}{1+{\left[\text{P}*\left(\text{t}.\text{T}\right)\right]}^{\frac{1}{2}}}$$

$$\text{P}*\left(\text{t}.\text{T}\right)=\frac{\text{T}\left(\text{t}.\text{T}\right)}{{\text{P}}_{\text{p}}}$$

$$\text{P}\left(\text{t}.\text{T}\right)=\frac{\text{t}}{\text{T}}\text{exp}\square \left(\frac{-\text{Qa}}{\text{RT}}\right)$$

$$\text{S}{\left(\text{t}\text{T}\right)}^2={\left({\text{S}}_{\mathrm{\theta }}\right)}_{{\text{Max}}^2}\left[1-\text{exp}\frac{-\text{qs}}{\text{R}}\left(\frac{1}{\text{T}}-\frac{1}{{\text{T}}_{\text{s}}}\right)\right]\left[1-\text{exp}\left(\frac{-\text{t}}{{\mathrm{\tau }}_1\left(\text{t}\right)}\right)\right]$$

Table 1 shows several material constants for the yield point models. Parameters of the yield point model Cast aluminum alloys Intrinsic yield strength (MPa), □ _i 40~60 Yield strength as quenched (MPa), □ _q 70~90 Activation energy for aging (kJ / mol), Qa 180~230 Universal gas constant (J / mol. ° K), R 8.314 Peak temperature correction time (s / ° K), pp 3.5E-22 ~ 3.5E-22 Metastable solvus temperature (° K), Ts 500~550 Max. Strength parameters at 0 K (MPa), (S ₀ ) _max 240-280 Solvus enthalpy (kJ / mol), Qs 40~50 Constant with respect to □ 1 to tp, K1 0,5~0,6

wobei P_solidus der Druck in der endgültig verfestigten Blase ist, der durch eine Gusssimulation bestimmt werden kann. In einem Gussbauteil wie etwa einem Motorblock kann der Druck in jeder mitgeführten Gasblase in Bezug auf die Blasenposition zugeordnet werden. Basierend auf dem zugeordneten Blasendruck und der Materialstreckgrenze bei einer erhöhten Temperatur kann die erreichbare Lösungsbehandlungstemperatur optimiert werden.If there is no bubbling, the bubble volume should be constant at the specific solution temperature (V _ST ), ie the V _ST should equal V _solidus . Consequently, $${\text{T}}_{\text{ST}}=\frac{{\mathrm{\sigma }}_{\text{ys @ ST}}}{{\text{P}}_{\text{solidus}}}{\text{T}}_{\text{solidus}}<{\text{T}}_{\text{solidus}}$$

where P _{solidus is} the pressure in the finally solidified bubble that can be determined by casting simulation. In a cast component such as an engine block, the pressure in each entrained gas bladder may be associated with respect to the bladder position. Based on the associated bubble pressure and the material yield point at an elevated temperature, the achievable solution treatment temperature can be optimized.

A second part and step of the method or process relates to determining the solution treatment time (t _ST ). Solution time, also called dissolution time (t _ST ), is based on the times at which particles dissolve in the material or alloy and the critical time at which the bubble begins to grow. The maximum solution treatment time (maximum dissolution time) should be less than the time required to dissolve the particles and less than the critical time over which the entrained gas bubbles grow. $$t_{ST}=\text{min}\left\{{\text{Dissolution time of <figure-callout id="2" label="Mg" filenames="DE102018110402A1\_0030.png,DE102018110402A1\_0032.png" state="{{state}}">Mg</figure-callout>}}_2\text{Si}.{\text{<figure-callout id="2" label="al" filenames="DE102018110402A1\_0030.png,DE102018110402A1\_0032.png" state="{{state}}">al</figure-callout>}}_2\text{Cu}.\text{critical growth phase}.\text{ups},\right\}$$

$$\text{D}={\text{D}}_{\mathrm{\theta }}\text{exp}\left(-\frac{{\text{Q}}_{\text{d}}}{\text{RT}}\right)$$

wobei C der Legierungselementgehalt (at% oder Gew.-%), t die Zeit (Sekunden), x die Entfernung (Meter) ist; D ist der Diffusionskoeffizient (m^2s^-1), Do ist die Diffusionskonstante (m^2s^-1), R ist die universelle Gaskonstante (J/(mol. °K)); T ist die Temperatur (°K); und Q_d ist die Aktivierungsenergie (J/mol).In the solution treatment, the dissolution time of the particles follows the second Fick's law. $$\frac{\partial \text{C}}{\partial \text{t}}=\text{D}\frac{{\partial }^2\text{C}}{\partial {\text{x}}^2}$$

$$\text{D}={\text{D}}_{\mathrm{\theta }}\text{exp}\left(-\frac{{\text{Q}}_{\text{d}}}{\text{RT}}\right)$$

where C is the alloying element content (at% or wt%), t is the time (seconds), x is the distance (meters); D is the diffusion coefficient (m ^ 2s ^ -1), Do is the diffusion constant (m ^ 2s ^ -1), R is the universal gas constant (J / (mol. ° K)); T is the temperature (° K); and Q _d is the activation energy (J / mol).

3 FIG. 12 shows an example of the calculated dissolution times for various sizes of Mg ₂ Si particles in an aluminum casting alloy during the solution treatment at 400 ° C. with a particle size on the Y-axis and a solution treatment time on the X-axis, starting at 10 seconds. A first, lower curve 30A tracks the resolution of nominal 2.5 μm particle sizes; a second, middle curve 30B tracks the resolution of the nominal 5 μm particle size; and the third, upper curve 30C tracks the resolution of the nominal 10 μm particle size. Out 3 shows that a much longer dissolution time is required for complete dissolution of the larger particles.

wobei ε die Dehnung (dimensionslos) ist und A, n und m die Materialparameter sind. A variiert zwischen 1×10^-10 und 1×10^-18, n liegt im Bereich von 1 bis 10, m variiert von -0.1 bis 1.0 und σ ist die angelegte Spannung (MPa). Im Blasenbildungsprozess ist σ=P_ST-σ_ys(ST), wobei P_ST der Gasdruck innerhalb der Blase bei der Lösungsbehandlungstemperatur, σ_ys(ST) die Materialstreckgrenze bei der Lösungsbehandlungstemperatur und t ist die kritische Blasenbildungszeit (Sekunden). Wie vorstehend beschrieben, sollte die maximale Lösungsbehandlungszeit (t_ST) unter der Zeit liegen, die zum Auflösen der Partikel benötigt wird, und unter der kritischen Zeit, über der die mitgeführten Gasblasen wachsen.As mentioned above, the solution treatment time (t _ST ) is limited by the critical time when the bubble begins to grow. The critical blistering time depends on how fast the material creeps. The creep strain rate is given by: $\stackrel{,}{\mathrm{\epsilon }}\mathrm{=}\frac{\text{d}\mathrm{\epsilon }}{\text{dt}}=\text{A}{\mathrm{\sigma }}^{\text{n}}{\text{t}}^{\text{m}}$

where ε is the strain (dimensionless) and A, n and m are the material parameters. A varies between 1 × 10 ^ -10 and 1 × 10 ^ -18, n ranges from 1 to 10, m varies from -0.1 to 1.0, and σ is the applied stress (MPa). In the bubbling process, σ = P _ST -y _ys (ST), where P _{ST is} the gas pressure within the bubble at the solution treatment temperature, σ _ys (ST) is the material _yield strength at the solution treatment temperature, and t is the critical bubble formation time (seconds). As described above, the maximum solution treatment time (t _ST ) should be less than the time required to dissolve the particles and less than the critical time over which the entrained gas bubbles grow.

With reference to 4 Now, a comparison of residual stresses in air- and water-quenched typical die castings, such as cylinder heads, presented. The y-axis (ordinate) stands for increasing residual stresses, ie maximum principal stress or MPa. Along the X-axis (abscissa) are on the left three groups 40A, 40B and 40C of plots, representing three different castings, which were quenched with air. Within each group 40A, 40B and 40C, each of the three diagrams represents the stress at three different locations of each casting 4 There are six groups 42A, 42B, 42C, 42D, 42E and 42F of plots representing six different castings quenched with water. Within each group 42A, 42B, 42C, 42D, 42E and 42F, each of the three diagrams represents the stress at three different locations within each casting. By reference and examination of 4 It should be noted that, compared to a water quench, an air quench can significantly reduce the residual stress by an average of at least 50 MPa. Accordingly, a third part and step of the method or process refers to determining and applying an artificial aging step.

With reference to 5 and, nevertheless, it is understood that air quenching generally provides both lower yield strength and higher tensile strength compared to water quenching at the same aging condition. To compensate for the loss of tensile strength by air quenching, various stage aging cycles are known for improving yield strength. 5 illustrates the improvement in the yield strength of air-cured HPDC A380 alloy casting through various aging cycles and, for better comparison, a water-hardened HPDC A380 alloy casting. The y-axis (ordinate) of 5 represents the increasing yield strength (MPa). Along the X axis (abscissa) are five graphs, with the first graph 50A representing air cooling at room temperature; the second curve 50B represents air cooling and is then held at 200 ° C for 2 hours; the third curve 50C is air-cooled, then held at 95 ° C for 2.5 hours and then held at 200 ° C for 2 hours; the fourth curve 50D is air-cooled, then held at 95 ° C for 2.5 hours, 180 ° C for 4 hours and then 200 ° C for 1 hour; and finally, the fifth curve 50E, which is a water quench and then held at 200 ° C for 2 hours.

$$\mathrm{\Omega }=\left\{0<T<T_c;0<t<\infty ;0<C<C_0\right\}$$

$$\Delta {\sigma }_{ppt}\left(T,t,C\right)\ge \Delta {\sigma }_{t\text{arg}et}$$

wobei E(T,t) der Energieeintrag ist, der eine Funktion der Temperatur (T) und der Zeit (t) ist, Δσ_ppt(T,t,C) die Erhöhung der Materialfestigkeit durch Ausscheidungshärtung ist, Δσ_{t arg et} die gewünschte Festigkeitssteigerung ist, die für den luftabgeschreckten Aluminiumguss erforderlich ist, Co und C sind der anfängliche und aktuelle Gehalt (in % oder Gew.-%) eines härtenden Legierungselements in einer Aluminiummatrix während des Alterungszyklus und Tc ist die kritische Obergrenze der Alterungstemperatur (°K).As in 6 illustrates, the non-isothermal scheme T (t) for an aluminum alloy can be optimized to achieve the desired yield strength and tensile strength with minimum energy input, E (T, t) and aging time t. That is, and like the qualitative curve 60 of FIG 6 illustrated, the temperature is first increased relatively quickly to a maximum, then reduced and maintained at a lower temperature and finally reduced slowly. This cross-issue issue with limitations can be defined as: $$\{\begin{array}{l}\underset{\left(T.t\right)\in \mathrm{\Omega }}{{\displaystyle Min}}e\left(T.t\right)=\underset{\left(T.t\right)\in \mathrm{\Omega }}{{\displaystyle Min}}{\displaystyle \oint T\left(t\right)dt}\hfill \\ \underset{\left(T.t\right)\in \mathrm{\Omega }}{{\displaystyle Max}}\Delta {\sigma }_{ppt}\left(T.t.C\right)\hfill \end{array}$$

$$\mathrm{\Omega }=\left\{0<T<T_c;0<t<\infty ;0<C<C_0\right\}$$

$$\Delta {\sigma }_{ppt}\left(T.t.C\right)\ge \Delta {\sigma }_{t\text{bad}et}$$

where E (T, t) is the energy input that is a function of temperature (T) and time (t), Δσ _ppt (T, t, C) is the increase in material strength by precipitation hardening, Δσ _{t arg et} the desired one Co and C are the initial and current content (in% or wt%) of a hardening alloying element in an aluminum matrix during the aging cycle and Tc is the critical upper limit of the aging temperature (° K) ,

Related to 7 The method also includes the use of a multi-stage aging cycle, represented by the curve 70, at a higher temperature 72 during the early part of the aging cycle, followed by a reduced temperature 74 during a final part of the aging cycle that reduces the residual stress in aluminum die castings, in particular in components in which dissimilar materials, such as aluminum, are present which enclose the cast iron bushes of an engine block.

Accordingly, it will be seen from the foregoing that a multi-stage process or process for treating aluminum die cast parts for maximum net tensile strength and residual stress reduction includes the steps of determining the highest temperature and the shortest possible solution treatment time of the casting (s). e) by computer-aided thermodynamics, kinetics and gas laws based on the composition of the aluminum alloy and the gas pressure in the solidified parts. This aspect in determining the maximum solution temperature is achieved by assigning the pressure in the bubbles of the solidified material, since it is essential to avoid blistering by near-surface bubbles in the casting. After the solution treatment, the parts are air-cooled to reduce the residual tensile strength. Finally, the parts undergo a non-isothermal or multi-stage aging process that maximizes net tensile strength.

The above multi-step process or process improves net tensile strength by at least 50%, relieves the residual casting stress by 40% to 80%, and eliminates cracking in parts with various materials, such as cast iron cylinder liner aluminum engine blocks.

The description is merely exemplary and variations that do not depart from the general spirit of this disclosure are deemed to be within the scope of the disclosure. Such variants are not to be regarded as a departure from the spirit and scope of the invention.

Claims (7)

A method of improving the net tensile strength of a high pressure aluminum die casting comprising the steps of: Determining a highest temperature and a shortest time for the solution treatment of an aluminum alloy die-cast member by applying pressure to bubbles of a solidified aluminum alloy using pressure and volume gas laws; subjecting the aluminum die-cast solution treatment to the set highest temperature and shortest time, air cooling the An aluminum die casting for reducing residual tensile strength, and subjecting the aluminum die casting to a non-isothermal multi-stage aging cycle to improve net tensile strength. Method according to Claim 1 further comprising the step of determining a solidus temperature for the aluminum alloy die casting. Method according to Claim 1 wherein the temperature and time for solution treatment avoid bubble formation. Method according to Claim 1 wherein the solution treatment time is below a time when the entrained gas bubbles grow. Method according to Claim 1 in which the assignment of pressure in bubbles uses the following equation: $\frac{{\text{P}}_{\text{solidus}}{\text{V}}_{\text{solidus}}}{{\text{T}}_{\text{solidus}}}=\frac{{\text{P}}_{\text{RT}}{\text{V}}_{\text{RT}}}{{\text{T}}_{\text{RT}}}=\frac{{\mathrm{\sigma }}_{\text{ys @ ST}}{\text{V}}_{\text{ST}}}{{\text{T}}_{\text{ST}}}$

Method according to Claim 1 wherein determining the solution treatment time applies the following equations: $$\frac{\partial \text{C}}{\partial \text{t}}=\text{D}\frac{{\partial }^2\text{C}}{\partial {\text{x}}^2}\text{D}={\text{D}}_{\mathrm{\theta }}\text{exp}\left(-\frac{{\text{Q}}_{\text{d}}}{\text{RT}}\right)$$

where C is the alloying element content (at% or wt%), t is the time (seconds), x is the distance (meters); D is the diffusion coefficient (m ^ 2s ^ -1), Do is the diffusion constant (m ^ 2s ^ -1), R is the universal gas constant (J / (mol. ° K)); T is the temperature (° K); and Q _d is the activation energy (J / mol). Method according to Claim 1 wherein determining the non-isothermal multi-stage aging process employs the following equations: $$\{\begin{array}{l}\underset{\left(T.t\right)\in \mathrm{\Omega }}{{\displaystyle Min}}e\left(T.t\right)=\underset{\left(T.t\right)\in \mathrm{\Omega }}{{\displaystyle Min}}{\displaystyle \oint T\left(t\right)dt}\hfill \\ \underset{\left(T.t\right)\in \mathrm{\Omega }}{{\displaystyle Max}}\Delta {\sigma }_{ppt}\left(T.t.C\right)\hfill \end{array}$$

$$\mathrm{\Omega }=\left\{0<T<T_c;0<t<\infty ;0<C<C_0\right\}$$

$$\mathrm{\Delta }{\sigma }_{ppt}\left(T.t.C\right)\ge \mathrm{\Delta }{\sigma }_{a\text{rg}et}$$

where E (T, t) is the energy input that is a function of temperature (T) and time (t), Δσ _ppt (T, t, C) is the increase in material strength by precipitation hardening, Δσ _{t arg et} the desired one Co and C are the initial and current content (in% or wt%) of a hardening alloying element in an aluminum matrix during the aging cycle and Tc is the critical upper limit of the aging temperature (° K) ,

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