Method for producing Al-base alloy substrates for magnetic recording media

A method for producing Al-base alloy substrates for magnetic discs, which comprises continuously casting into a strip of a thickness of 4 to 15 mm a molten alloy composition containing alloy components consisting of 2-6% of Mg, less than 1% of Mn, less than 0.3% of Fe, less than 0.25% of Zn and less than 0.35% of Cr and a grain refiner comprising less than 0.08% of Ti and/or less than 0.01% of B, followed by rolling.

BACKGROUND OF THE INVENTION 
This invention relates to a method for producing aluminum-base alloy 
substrates for magnetic recording media such as magnetic memory discs, and 
more particularly to a method for producing aluminum-base alloy substrates 
for magnetic discs, which have few surface defects and excellent surface 
uniformity after grinding or polishing. 
The magnetic discs which are most widely used as a memory medium for 
computers are generally produced by machining a substrate of an aluminum 
alloy to a predetermined thickness followed by precision grinding and 
coating of the surface of the substrate with a thin film of a material 
which is magnetizable for recording signals. The magnetic disc substrate 
of this sort is required to meet the following conditions. 
(1) The surface roughness after precision polishing should be small enough 
to maintain a constant flying height of the magnetic head for stable 
read/write characteristics. 
(2) On polished substrate the surface defects which adversely affect the 
uniform formation of the coated film of the magnetizable material should 
be few and of sufficiently small dimensions. 
(3) The substrate should have sufficient mechanical strength to withstand 
machining and grinding in the fabrication process as well as highspeed 
rotation while in the softened state. 
(4) The substrate should have good corrosion resistance and a certain 
degree of heat resistance. 
(5) It should be non-magnetic, light in weight and low in cost. 
The magnetic disc substrates which are currently most popular in the art 
are made of an aluminum alloy AA 5086. However, recently there has been an 
increasing demand for magnetic discs having a large capacity and high 
density. It is necessary to obtain such high density disc that magnetic 
area for each bit of signal, thickness of magnetic film and flying height 
of the head should be reduced. Consequently, the finished surface of 
substrate have excellent uniformity to provide such high density disc. In 
this regard, the abovementioned alloy AA5086 is unsatisfactory in surface 
uniformity. 
SUMMARY OF THE INVENTION 
Under these circumstances, the present inventors conducted a study to find 
out the cause of the unsatisfactory surface uniformity and as a result 
discovered the following facts. 
(a) In the metallic matrix of the substrate of the above-mentioned alloy, 
there exist various intermetallic compounds (e.g., of the Al-Fe-Mn and 
Mg-Si phases) having a size of about 10 .mu.m in diameter of length. 
Because the interface of their intermetallic compounds is incoherent with 
the matrix and the hardness of the compound is higher than that of the 
matrix, their compounds tend to remain as projections or come off to form 
voids on the surface of the substrate when it is machine or polished. 
Therefore, it is difficult to reduce the size of surface defects even 
after an elaborate polishing operation. 
(b) When a grain refiner is added in little amount at the time of direct 
chill casting, the ingot portion which will form a surface of the grinded 
substrate after rolling bears feather-structure of about 1 mm in which 
with micro-segregation of intermetallic compounds at their boundaries. The 
micro-segregation imposes an adverse effect on the recrystallizing 
behavior in the subsequent hot-rolling stage and the result is a 
macrostructure having a period as large as about 1 mm in width when the 
surface of the grinded substrate is etched with aqua regia. The material 
therefore contains variations in hardness according to the period of the 
macro-structure resulting from the micro-segregation in the ingot. Due to 
such variations in hardness, fine undulations with a period of about 1 mm 
appear on the surface of the disc substrate in the cutting or grinding 
stage (generally referred to as "trough phenomenon:, which ordinarily have 
a period of 0.5-2 mm and a height of about 0.1 .mu.m), making it difficult 
to maintain a constant flying height of the head. 
In view of these results, it is an object of this invention to improve the 
surface uniformity of the aluminum substrate by reducing as much as 
possible the size of intermetallic compound in the ingot by preventing the 
above-mentioned micro-segregation, with addition of a grain refiner. 
A further object is to provide an alloy composition and casting conditions 
which permit the manufacture of aluminum discs having very uniform 
surfaces. It has now been found that the above-mentioned objects can be 
achieved by restricting the kinds and contents of additive alloy 
components and especially by reducing the thickness of strips produced by 
continuous casting. By so doing, the intermetallic compounds are refined 
to a considerable degree due to a rapid-cooling effect and by addition of 
grain refiner, the micro-segregations of the intermetallic compounds are 
reduced. 
In summary, the present invention resides in a method for producing an 
aluminum-base alloy substrate for magnetic discs, comprising continuously 
casting into a strip of a thickness of 4 to 15 mm a molten alloy 
composition containing alloy components consisting essentially of 2-6% of 
Mg, less than 1% of Mn, less than 0.3% of Fe, less than 0.25% of Zn and 
less than 0.35% of Cr and a grain refiner consisting essentially of less 
than 0.08% of Ti and/or less than 0.01% of B, balance Al and inevitable 
impurities, followed by rolling of the resulting cast strip. All 
proportions of the ingredients are by weight. 
The reasons underlying the above-defined ranges and amounts of the additive 
alloy elements and the conditions of the continuous casting operation are 
discussed more particularly in the following description along with the 
resulting effects. However, it is to be understood that the present 
invention is not limited to the particulars shown herein and includes all 
alterations and modifications are encompassed by the appended claims. 
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
Of the additive alloy elements employed in the present invention, Mg is a 
component which is essential for imparting a predetermined mechanical 
strength to the substrate and therefore should be included in a proportion 
of at least 2% or more. However, the additive amount of Mg should be less 
than 6% since excessive addition of Mg will invite increased formation of 
an intermetallic compound of Mg-Si and the increased formation of MgO 
(non-metallic inclusion) which is produced by high-temperature oxidation 
in the melting and continuous casting stages. The surface uniformity will 
be lowered by thus formed: Mg-Si and MgO. 
The alloy component Mn serves to enhance the corrosion resistance of the 
alloy plate while Fe contributes to improve the strength of the plate. 
Excessive addition of these elements leads to production of large 
inter-metallic compounds of Al-Fe-Mn phase so that Mn and Fe should be 
added in an amount of less than 1% and less than 0.3%, respectively. 
The Cr-component, which acts to enhance the corrosion resistance of the 
alloy plate similarly to Mn, should be added in an amount less than 0.35% 
Since otherwise it will lower the surface uniformity by enlarging the 
inter-metallic compounds of Al-Fe-Mn phase. 
The Zn-component has a tendency to lower the corrosion resistance of the 
ultimate product but this defect is almost unnoticeable when its content 
is less than 0.25%. 
The Ti and B components are effective for making the ingot structure finer 
and for preventing the micro-segregations, and, in order to achieve these 
effects to a significant degree, it is preferred to add more than 0.001% 
of Ti and 0.0005% of B singly or jointly. However, an additive amount of 
Ti in excess of 0.08% and an additive amount of B in excess of 0.01% are 
wasteful since the excess portions are mostly filtered out in the molten 
metal filtering stage using an electrochemical adsorption filter. 
The Al-component used in the present invention has a purity of 99% or 
higher, so that Cu and Si which are unabidably contained in the 
Al-component are only of the order of impurities and thus have no 
possibility of enlarging the intermetallic compounds in any practically 
objectionable degree. 
According to the present invention, the Al-base alloy of the 
above-described composition is melted and the molten metal is passed 
through an electrochemical adsorption filter to remove non-metallic 
inclusions therefrom prior to the continuous casting stage in which the 
molten metal has to be formed into a 4 to 15 mm thick strip in order to 
achieve the object mentioned hereinbefore. In the ordinary direct chill 
casting process, it is the usual practice to form a slab of 300 to 600 mm 
in thickness. However, it has been found that it is difficult to solidify 
a slab of the usual large thickness in such a manner that the 
intermetallic compounds are reduced to a size smaller than 3 .mu.m, no 
matter, what the alloy composition. However, if the thickness of the cast 
strip formed in the continuous casting stage is reduced to 15 mm or 
smaller, the cast strip is solidified very quickly, permitting formation 
of fine intermetallic compounds. For example, the intermetallic compound 
size in an ordinary 300-600 mm thick slab obtained by the conventional 
direct chill casting process is about 12 .mu.m. On the other hand, a cast 
strip having a thickness of smaller than 15 mm can be solidified at a rate 
more than ten times greater, with the simultaneous reduction of the 
intermetallic compound to a size smaller than 3 .mu.m. In addition, there 
occurs almost no micro-segregation owing to the formation of the fine 
crystal grains with the addition of grain refiner. 
Therefore, the Al-base alloy plate which is obtained after hot-, warm- or 
cold-rolling contains very fine and uniform macro-structure, in the 
subsequent machining or grinding operation, it is free of surface 
undulations caused by enlarged macro-structure or caused by the 
micro-segregations of intermetallic compounds. 
As will be gathered from the foregoing description, it is desirable that 
the cast strip formed in the continuous casting stage have as small a 
thickness as possible, but is preferred to have at least a thickness of 4 
mm in consideration of the 50% reduction by cold-rolling which is required 
subsequently to ensure machining accuracy in the punching, cutting and 
grinding operations of the disc plate. On the other hand, if the thickness 
of the strip exceeds 15 mm, it becomes difficult to solidify it at a 
sufficiently high rate, with consequent failure to form fine intermetallic 
compounds. 
It is to be understood that the term "continuous casting" as herein used 
includes semi-continuous casting. 
The effects resulting from the above-described method of the present 
invention can be summarized as follows. 
(1) The constituent elements and their proportions in the alloy are 
restricted, and the alloy strip is formed into as small a thickness as 
possible in the continuous casting operation to allow rapid solidifying of 
the strip, so that the size of the intermetallic compounds minimized and 
the intermetallic compounds are uniformly dispersed in the matrix. As a 
result, there can be obtained an Al-base alloy plate for magnetic discs, 
which has high surface uniformity and which is free of surface undulation 
due to the micro-segregations of the intermetallic compounds and due to 
the enlarged macro-stracture caused by the above-mentioned 
micro-regregations. 
(2) Consequently, it becomes possible to reduce the thickness of the 
magnetic coating film and the flying height of the head in order to cope 
with the requirements of larger capacity and higher in formation density 
on magnetic discs. 
(3) An alloy substrate which is comparable to or higher than the 
conventional counterparts in mechanical strength, heat resistance and 
corrosion resistances can be obtained by a simple method, so that it 
becomes possible to provide magnetic disc substrates of higher quality at 
a similar cost.

The invention is illustrated more particularly by the following Examples. 
EXAMPLE 1 
Molten Al-base alloys of the compositions Nos. 1 to 4 of Table 1 were 
passed through an ordinary electrochemical adsorption filter for the 
removal of non-metallic inclusions and then formed into a 5 mm thick strip 
by continuous casting. The grain size on the surface of the cast strip was 
adjusted to 0.055 mm by addition of a grain refiner (Ti and/or B). The 
resulting cast strip was cold-rolled into a thickness of 2 mm, and was 
punched to substrates followed by heat treatment (250.degree. C. for 2 
hours) to get flatness of the material. 
For the purpose of comparison, molten Al-base alloy of the composition No. 
5 of Table 1 was, after removing non-metallic inclusions in a similar 
manner, formed into a 360 mm thick slab by the ordinary direct chill 
casting and scalping. The slab had a feather structure. After a 
homogenizing heating treatment (520.degree. C. for 6 hours), the slab was 
hot-rolled to a thickness of 5 mm and, after reduction to 2 mm by 
cold-rolling, subjected to a heat treatment (250.degree. C. for 2 hours) 
to get flatness. 
After cutting 0.1 mm from both sides of each sample strip according to the 
procedure of the ordinary disc manufacturing process, the sample was 
softened by re-annealing to get flatness and then finished by buffing to 
measure the size and number of surface defects (surface defects 
attributable to the intermetallic compound), surface roughness (trough 
phenomenon) and mechanical properties. The results are shown in Table 2. 
The surface roughness was determined by buffing the surface of each sample 
with powder of aluminum oxide and measuring the maximum surface roughness 
by means of a surface roughness meter. The size and number of 
intermetallic compound was assessed by observing the polished surface with 
a scanning electron microscope at a magnification of 500, spotting the 
microscope on ten randomly selected rectangular areas of 1 mm.sup.2 having 
a dimension of 0.2 mm in the rolled direction and 5 mm in the transverse 
direction and counting the number of intermetallic compounds in the view 
field with respect to four ranks, namely an intermetallic compound size, 
greater than 2.5 .mu.m, an intermetallic compound size greater than 3.75 
.mu.m, an intermetallic compound size greater than 5 .mu.m and an 
intermetallic compound size greater than 10 .mu.m. 
TABLE 1 
__________________________________________________________________________ 
Alloy Compositions (wt %) 
No. Si Fe Cu Mn Mg Zn Cr Ti B 
__________________________________________________________________________ 
1 Invention 
0.095 
0.158 
0.005 
0.09 
2.89 
0.011 
0.175 
0.011 
-- 
2 " 0.070 
0.178 
0.010 
0.36 
3.78 
0.009 
0.085 
0.019 
-- 
3 " 0.080 
0.165 
0.003 
0.45 
3.89 
0.010 
0.070 
-- 0.005 
4 " 0.085 
0.172 
0.002 
0.10 
2.65 
0.012 
0.155 
0.011 
0.005 
5 Comparative 
0.066 
0.169 
0.009 
0.37 
3.98 
0.008 
0.084 
0.021 
-- 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Results of Measurements 
(Number of ups and 
downs due to inter- 
metallic compounds/ 
Mechanical properties 
Surface 1 mm.sup.2) Tensile 
Yield Elon- 
roughness 
Surface defects 
maximum 
strength 
strength 
gation 
No. .mu.Rmax 
2.5.mu.m 
3.75.mu.m 
5.mu.m 
10.mu.m 
length (.mu.m) 
(kg/mm.sup.2) 
(kg/mm.sup.2) 
(%) 
__________________________________________________________________________ 
1 Invention 
0.01 1.8 0 0 0 2.8 21.5 10.0 27.4 
2 " 0.01 2.0 0 0 0 3.0 25.9 12.0 25.5 
3 " 0.01 1.9 0 0 0 3.0 26.0 12.2 25.3 
4 " 0.01 1.8 0 0 0 2.8 21.5 10.0 27.6 
5 Comparative 
0.12 40 28 14 2 12.5 26.7 12.8 24.9 
__________________________________________________________________________ 
As is evident from Tables 1 and 2, the Al-base alloy substrates (Nos. 1 to 
4) are far superior in surface roughness and the size of surface defects 
to the comparative example prepared by the conventional method (No. 5). At 
present, in order to increase the capacity and density in data processing 
by computers, the surface defects of the substrate which are attributable 
to the intermetallic compound are required to be smaller than 3 .mu.m in 
length. This requirement cannot be met by the substrate of the 
conventional method. In contrast, an Al-base alloy substrate in which the 
maximum surface defect is smaller than 3.0 m in length can be easily 
obtained by the method of the present invention. In addition, the method 
of the present invention forms very fine grains in the continuous casting 
stage, preventing micro-segregations which would cause the through 
phenomenon in the subsequent polishing operation. 
The respective sample strips have almost no difference in mechanical 
properties, since these depend mainly on the Mg-content. 
EXAMPLE 2 
Molten Al-base alloy compositions Nos. 6 and 7 of Table 3 were cast into a 
12 mm thick strip after removal of non-metallic inclusions in the same 
manner as in Example 1. The grain size on the surface of the cast strip 
was adjusted to 0.05 mm by addition of grain refining agent (Ti). The 
resulting cast strips were hot-rolled to a thickness of 5 mm and with a 
final temperature of 330.degree. C. and then cold-rolled to a thickness of 
2 mm, were punched to the substrates, followed by a heat treatment 
(250.degree. C. for 2 hours) to get flatness. 
The compositions Nos. 8 and 9 were similarly formed into 20 mm thick strips 
by continuous casting (grain size: 0.055 mm), hot-rolled into a thickness 
of 5 mm with a final temperature of 300.degree. C., and then cold-rolled 
to a thickness of 2 mm. The resulting strips were punched to the 
substrates, and heat treated under the same conditions as mentioned above. 
The composition No. 10 was similarly continuously cast into a 12 mm thick 
strip, reduced to a thickness of 4 mm by cold-rolling, and, after 
annealing at 340.degree. C. for 3 hours, cold-rolled to a thickness of 2 
mm was punched to the substrates, followed by a heat treatment 
(250.degree. C. for 2 hours) to get flatness. 
The resulting samples were machined and finished by buffing in the same 
manner as in Example 1, and the size and number of intermetallic 
compounds, surface roughness and mechanical properties were measured. The 
results of the measurements are shown in Table 4. 
TABLE 3 
__________________________________________________________________________ 
Alloy Compositions (Wt %) 
No. Si Fe Cu Mn Mg Cr Zn Ti 
__________________________________________________________________________ 
6 Invention 
0.07 
0.13 
0.003 
0.08 
2.78 
0.19 
0.008 
0.014 
7 " 0.08 
0.15 
0.006 
0.35 
3.85 
0.08 
0.008 
0.020 
8 Comparative 
0.09 
0.15 
0.003 
0.08 
2.83 
0.18 
0.007 
0.012 
9 " 0.09 
0.16 
0.003 
0.37 
4.07 
0.09 
0.010 
0.020 
10 Invention 
0.07 
0.14 
0.010 
0.36 
3.95 
0.07 
0.009 
0.021 
__________________________________________________________________________ 
TABLE 4 
__________________________________________________________________________ 
Results of Measurements 
(Number of ups and 
downs due to inter- 
metallic compounds/ 
Mechanical properties 
Surface 1 mm.sup.2) Tensile 
Yield Elon- 
roughness 
Surface defects Maximum 
strength 
strength 
gation 
No. .mu.Rmax 
&gt;2.5.mu.m 
&gt;3.75.mu.m 
&gt;5.mu.m 
&gt;10.mu.m 
length (.mu.m) 
(kg/mm.sup.2) 
(kg/mm.sup.2) 
(%) 
__________________________________________________________________________ 
6 Invention 
&lt;0.01 2.0 0 0 0 2.9 21.7 10.1 27.5 
7 " &lt;0.01 2.4 0 0 0 3.0 26.0 11.9 25.0 
8 Comparative 
&lt;0.01 3.1 0 0 0 3.6 21.5 9.8 27.2 
9 " &lt;0.01 3.5 0 0 0 3.7 26.1 12.3 25.9 
10 Invention 
&lt;0.01 2.3 0 0 0 3.0 26.5 12.4 24.8 
__________________________________________________________________________ 
As is clear from Tables 3 and 4, the samples according to the present 
invention (Nos. 6, 7 and 10) all satisfy the requirement that the maximum 
surface defect should be smaller than 3.0 .mu.m in length, irrespective of 
the rolling conditions subsequent to the continuous casting. However, in 
the comparative samples (Nos. 8 and 9) which have a cast thickness in 
excess of 15 mm, the intermetallic compound refining effect is 
insufficient since there exist defects of a length greater than 3 .mu.m on 
the finished surface, which, accordingly, fail to meet the requirements 
for magnetic discs of higher density and capacity. 
Having now fully described the invention, it will be apparent to one or 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.