Ultimate tensile strength is not likely to change much with processing.  It is dependent upon chemistry, impurities, defects, etc.  Yield strength is much more dependent upon processing.  Within the limits you have set, the ways to improve strength are:

1) increase strain hardening by increasing the area reduction (start with a higher cross-sectional area to achieve a given final area)

2) decrease grain size (prior processing, or this may occur because of item 1 above)

The tensile strength for this type of aluminium wire should be around 305-315 MPa minimum, so it seems that something in your process is not correct.  Do you know what the solution temperature is?  It should be 510 degrees Celsius.  What type of quench is used after the solution treatment?  Quench rate and delays in quenching can have significant effects on strength.  What is the aging temperature and time?  The aging temperature should be 150 degrees Celsius for approximately 4 hours.

If the heat treating process is not at fault, then introducing more strain hardening or reducing grain size (or both) as mentioned by CoryPad may be the only options for you to investigate.  There is considerable information available on aluminium alloys, heat treating, etc.  I recommend you investigate some of the references available from ASM Interntational (http://www.asminternational.org).  The following books are all very good:

ASM HANDBOOK Volume 2 Properties and Selection: Nonferrous Alloys and Special-Purpose Materials

ASM HANDBOOK Volume 4 Heat Treating

ASM SPECIALTY HANDBOOK Aluminum and Aluminum Alloys - this book is a compilation of the information contained in all 20 ASM HANDBOOKS, including Volumes 2 and 4

Heat Treater's Guide: Practices and Procedures for Nonferrous Alloys

We have purchased the alloy rod with T4 temper and are not aware of the process of production including solution temp. and type of quenching. Average tensile of alloy rod after Natural Aging for about 50 days from production and before drawing process was around 190Mpa.We have implemented different Degree/time combination for aging and the best results achieved (283Mpa) were at 150 D.C/ 4 hours as TVP indicated exactly. We are almost sure that we can reach the required tensile if we produce the wires from a dia. 9mm alloy rod as you and CoryPad indicated, for which I thank you very much, but using this method will minimize the efficiency of the machine and also we may not be able to draw smaller diameters in one phase.Please advise.

I am not familiar enough with the strain hardening characterisitics of this alloy to know if the reduction in area that you have (from 7.62 to 3.23 mm) is adequate to produce the required strength properties.  I recommend you do the following:

1. Discuss this problem with your rod supplier.  Ask them the questions I asked you regarding solution time, quench rate, quench delay, etc.  Perhaps they can improve the strength in the T4 condition.

2. Discuss this problem with someone with direct experience with this alloy.  Perhaps your rod supplier has a contact at Alcoa, Pechiney, etc. that would be more helpful.