Patent Application
20200017945
The present disclosure relates to the joining of high pressure die cast (HPDC) materials, including various aluminum alloys, with highly localized forming operations such as self-piercing riveting (SPR).
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
For aluminum alloys, a T5-temper is an artificial age-hardening (strengthening) treatment that is applied to an as-cast (F-temper) Aluminum alloy. The T5 treatment often results in sufficient yield strength (YS) and elongation (% EL or EL) values without having to fully solution heat treat the aluminum alloy. The T5-temper is an advantageous heat treatment for High Pressure Die Cast (HPDC) aluminum alloys, especially Al—Si—Mg—Mn alloys in vehicle body structural applications.
Vehicle parts are often joined together to form an assembly, including HPDC parts to other HPDC or non-HPDC parts, which may be aluminum. A relatively inexpensive and reliable method of joining HPDC parts to other parts is self-piercing riveting (SPR). SPR is a mechanical fastening process for point joining sheet material, typically steels and aluminum alloys. The SPR process demands a reasonable amount of through-thickness fracture toughness to produce a robust joint. Normally, the reasonable amount of through-thickness fracture toughness is enabled by heat-treating the casting prior to riveting.
Unfortunately, when HPDC materials are subjected to a highly localized forming operation such as SPR, the HPDC material is susceptible to cracking. This cracking renders HPDC materials, generally, unfit for SPR processes, which significantly plastically deforms the substrates/parts being joined.
A fully solution treated and overaged HPDC aluminum alloy (e.g. T7-temper) does not readily form cracks during localized forming operations. Thus, the T7-temper has become the default temper for delivering material rivetability because the T7-temper has adequate through-thickness toughness. The component also satisfies designer requirements, offering high elongation with sufficient strength.
Unfortunately, the T7-heat treatment usually consists of a solution step followed by an air-blast quench then artificial aging. In addition to the cost of the heat treatment, additional costs are associated with increased dimensional distortion and blister scrap.
Now referring to
To further complicate HPDC aluminum alloy processing, HPDC aluminum alloys may be used both in the “as-cast” and T4-temper conditions where the T4-temper is defined as material that has been solution treated and quenched after fabrication. It is often an unstable condition as natural aging (e.g. room temperature aging) may occur resulting in an increase in yield stress but a decrease in ductility. Additionally, “as-cast” HPDC aluminum alloys age during storage, thus altering (degrading) their rivetability.
The present disclosure addresses the issues of cracking when joining HPDC materials, such as aluminum alloys, particularly using SPR processes, among other issues related to joining HPDC materials.
In one form of the present disclosure, a method of treating a high pressure die cast (HPDC) material to improve rivetability is provided. The method comprises exposing the HPDC material to a temperature between 300° C. and 450° C. for a time period between 10 minutes and 5 hours in a heat treatment step and quenching the HPDC material. In this method of the present disclosure, the heat treatment results in eutectic silicon spheroidization of the HPDC material such that the HPDC material does not crack after rivet installation, and more specifically self-piercing rivet installation in one form. In order to inhibit cracking, in one form, the eutectic silicon spheroidization comprises fewer than a minimum of about 3.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in any of the Al—Si eutectic microstructural constituents of the HPDC material. The primary aluminum phase, which is a dendritic structure, has no Si particles and therefore cannot be in the field of view for this measurement as it would bias the particle density values.
In a variation of the method of the present disclosure, the HPDC material is exposed to a subsequent heat treatment step, e.g. a paint bake step, at about 180° C. for about 30 minutes after quenching. In this variation, the subsequent heat treatment step results in a yield strength of at least about 80 MPa for an aluminum alloy.
In another variation of the method of the present disclosure, the temperature of the heat treatment step is 450° C. and the time period is between 10-30 minutes to result in full spheroidization of silicon eutectic, wherein there are fewer than about 1.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in an HPDC material.
In still another variation, the temperature of the heat treatment step is between 300° C. to 450° C. and the time period is between 30 minutes to 2 hours to result in intermediate spheroidization of silicon eutectic, wherein there are between about 3.50×10{circumflex over ( )}5 and about 8.10×10{circumflex over ( )}4 silicon particles per mm{circumflex over ( )}2 in an HPDC material.
In yet another variation of the method of the present disclosure, the temperature of the heat treatment step is 300° C. and the time period is between 2 to 5 hours to result in partial spheroidization of silicon eutectic, wherein there are fewer than about 3.5×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in an HPDC material.
In one form, the HPDC material is an aluminum alloy, although other materials are contemplated according to the teachings of the present disclosure. Rather than heat treating and quenching an entire part, in one form of the present disclosure, at least one heat treatment and at least one quenching are localized over predetermined portions of the HPDC material.
In yet another method of the present disclosure, the quenching is a forced air quench for a period of 6° C./s.
The present disclosure also includes a part and an assembly formed/treated according to the various methods disclosed herein.
In another form of the present disclosure, a method of treating a high pressure die cast (HPDC) material to improve rivetability is provided. The method comprises exposing the HPDC material to a temperature between 300° C. and 450° C. for a time period between 10 minutes and 2 hours in an initial heat treatment step, quenching the HPDC material, and exposing the HPDC material to a subsequent heat treatment step at about 180° C. for about 30 minutes after quenching. In this method of the present disclosure, the heat treatment results in eutectic silicon spheroidization of the HPDC material such that the HPDC material does not crack due to rivet installation and the subsequent heat treatment step increases yield strength of the HPDC material. According to this method, the subsequent heat treatment step results in a yield strength of at least about 80 MPa for an aluminum alloy, and the eutectic silicon spheroidization comprises fewer than an average of about 3.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in the HPDC material. Similarly, the heat treatments and the quenching may be localized over predetermined portions of the HPDC material rather than over the entire part.
In yet another form of the present disclosure, a method of treating an aluminum high pressure die cast (HPDC) material to improve rivetability is provided. The method comprises exposing the aluminum HPDC material to a temperature between 300° C. and 450° C. for a time period between 10 minutes and 2 hours in an initial heat treatment step, quenching the aluminum HPDC material, and exposing the aluminum HPDC material to a subsequent heat treatment step at about 180° C. for about 30 minutes after quenching. In this method of the present disclosure, the heat treatment results in eutectic silicon spheroidization of the aluminum HPDC material such that the aluminum HPDC material does not crack after rivet installation and the subsequent heat treatment step increases yield strength of the aluminum HPDC material to at least 80 MPa.
According to this variation, the eutectic silicon spheroidization comprises fewer than an average of about 3.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in the HPDC material. Further, the heat treatment may be localized over predetermined portions of the HPDC material.
Further areas of applicability will become apparent from the description provided 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.
In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.
To address the issues of highly localized forming and joining operations on HPDC materials, the present disclosure rivets the parts in the “spheroidized” condition. The present disclosure also stabilizes the castings to address issues with the HPDC aluminum alloys in both the “as-cast” and T5-temper conditions aging at room temperature. The present disclosure further provides an inexpensive mechanical test to characterize and correlate heat treatments to rivetability.
The inventors have discovered that one method to characterize material rivetability is to assess energy absorption to failure. Factors that contribute to energy absorption include alloy chemistry, temper and thickness, and the energy absorption to failure, which can be determined by piercing a subject material with a cylindrical punch of similar diameter to that of the rivet.
A clinch test, as shown in
Referring now to
The innovative clinch test correlated new HPDC aluminum alloy heat treatments with their respective rivetabilities by determining energy absorption to failure for a series of time and temperature conditions. Surprisingly, energy absorption was linked to eutectic silicon spheroidization, where energy absorption to failure increases with increasing spheroidization of the silicon eutectic. Therefore, the inventors discovered that a material becomes more rivetable with increasing spheroidization of the silicon eutectic.
As described in greater detail below, the inventors discovered that temperatures ranging from 300° C. to 450° C. are sufficient to alter the silicon morphology to achieve a higher energy absorption to failure energy, which translates to improved rivetability. Generally, full spheroidization of the silicon eutectic is most rapid at 450° C., taking place within 10-30 mins at temperature. Intermediate spheroidization takes place at 350° C. at times from 30 mins to 2 hours. Partial spheroidization takes place at temperatures of 300° C. after 2-5 hours. Negligible beneficial silicon morphology changes were observed at temperatures less than or equal to 280° C. for the durations of interest to the inventors.
Now referring to
Referring now to
Therefore, the inventors have used an HPDC aluminum alloy having a T5 heat treatment, which is attractive due to its commercial availability, and the material does not experience high temperature (>450° C.) solution heat treatment, only a simple aging treatment (220° C.-280° C.) after casting. However, HPDC material in the T5-temper is prone to cracking when mechanically joined. To form a successful SPR button in HPDC aluminum alloys (i.e., no cracking) the material/part must be exposed to temperatures sufficient to spheroidize the silicon morphology within a reasonable amount of time after casting.
Therefore, the present disclosure provides an improved heat treatment designed to modify silicon morphology to obtain sufficient spheroidization for rivetability. This improved heat treatment includes a eutectic silicon spheroidization step comprising material exposure at a temperature ranging from 300° C. to 450° C. for a time ranging from 10 minutes to 2 hours, as described in greater detail below.
Furthermore, to mitigate room temperature aging of “as-cast” HPDC materials during storage, the present disclosure exposes the “as-cast” castings to a slightly elevated temperature for a controlled time, thus stabilizing its mechanical properties. This stabilization enables months-long storage of the castings and reduces or negates the room temperature aging of the castings. Castings can thus be stored and eliminate the “expiration date” problem that also is a factor with rolled sheet in the naturally aged condition.
In one form, the improved heat treatment according to the teachings of the present disclosure also includes a rapid forced air quench. Subsequently, the improved heat treatment includes a paint curing (i.e. paint bake) process at 180° C. for 30 minutes, imparting an additional stabilization heat treatment resulting in a minimum alloy strength of 80 MPa, which is an acceptable strength for most stiffness driven designs.
In other words, the improved heat treatment of the present disclosure includes a partial solution heat treatment to modify eutectic silicon morphology to achieve high energy absorption to failure energy (i.e. rivetability), while the paint curing stabilizes and modestly improves the strength of the HPDC aluminum alloy. A process for manufacturing an HPDC aluminum alloy part with a temper approximating the aluminum T5-temper according to the teachings of the present disclosure comprises high pressure die casting an aluminum alloy part to form an “as-cast” aluminum alloy part, a partial solution heat-treatment of the “as-cast” aluminum alloy part at about 300° C.-450° C. for about 1-5 hours, and forced air quenching the partially solution heat treated aluminum alloy part. The inventors have found desirable results with forced air quench rates in the range of about 2° C./sec to about 15° C./sec, with an alternate range of about 5° C./sec to about 8° C./sec, and at about 6° C./sec. The process further comprises paint curing the air quenched aluminum alloy part at about 180° C. for about 30 mins. to manufacture an HPDC aluminum alloy part with a temper approximating the aluminum T5-temper, but with improved rivetability.
As the rivetability of the HPDC aluminum alloy improves, the manufacturability of the HPDC aluminum alloy improves. Improved manufacturability reduces the design margins, manufacturing costs, and part weight. Improved manufacturability also improves fuel efficiency, logistics costs, and lifecycle costs.
The present disclosure provides a robust production riveting process, followed by an aging process during a coating or sealing process that concurrently ages the HPDC aluminum alloy parts and assemblies analogous to at least a 6XXX series aluminum sheet. The concurrent aging process increases yield strength of the HPDC Aluminum alloy parts, while slightly decreasing ductility, leaving the parts in an approximation of a T5-temper. The loss in ductility negligibly or nominally affects either the assembly process or service characteristics.
The HPDC aluminum alloy material of the present disclosure comprises the compositional ranges of TABLE 2. The inventors have found desirable results throughout Range A, as well as the alternate Ranges B and C.
Referring to
In one method of the present disclosure, the method further comprises the step of exposing the HPDC material to a subsequent heat treatment step at about 350° C. for about 30 minutes after quenching. In another method of the present disclosure, the subsequent heat treatment step results in a yield strength of at least about 80 MPa for an aluminum alloy.
In one form, the eutectic silicon spheroidization comprises fewer than about 3.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in any of the Al—Si eutectic microstructural constituents of an HPDC material.
In another form, the temperature of the heat treatment step is about 450° C. and the time period is between about 10-30 minutes to result in full spheroidization of silicon eutectic, wherein there are fewer than an average of about 1.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in any of the Al—Si eutectic microstructural constituents of an HPDC material.
In yet another form, the temperature of the heat treatment step is between about 300° C. to about 450° C. and the time period is between about 30 minutes to about 2 hours to result in intermediate spheroidization of silicon eutectic, wherein there are between about 3.50×10{circumflex over ( )}5 and about 1.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in any of the Al—Si eutectic microstructural constituents of an HPDC material.
In still another form, the temperature of the heat treatment step is about 300° C. and the time period is between about 2 to about 5 hours to result in partial spheroidization of silicon eutectic, wherein there are fewer than an average of about 3.50×10{circumflex over ( )}5 silicon particles per mm{circumflex over ( )}2 in any of the Al—Si eutectic microstructural constituents of an HPDC material.
Now referring to
Referring to
Characterization of Spheroidization of Eutectic Silicon
With reference to
Unless otherwise expressly indicated herein, all numerical values indicating mechanical/thermal properties, compositional percentages, dimensions and/or tolerances, or other characteristics are to be understood as modified by the word “about” or “approximately” in describing the scope of the present disclosure. This modification is desired for various reasons including industrial practice, manufacturing technology, and testing capability.
The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.
